The complement receptors on macrophage are responsible for their binding and ingestion of opsonized targets. The two established receptors are CR1, which recognizes C3b, and CR3, which recognizes iC3b, the natural product of C3b from cleavage by the complement control protein factor I and its cofactors. CR1 belongs to a group of proteins that contain a structural element characterized by its size of 60-65 amino acids, and four conservatively positioned cysteines, which engage in a self-contained 1-3, 2-4 disulphide arrangement. This structural unit is called SCR (short consensus repeat) and is found in the complement proteins Clr, Cls, C2, factor B, factor H, C4BP, DAF, MCP and CR2, each of which interacts with some cleavage products of C3 and/or C4. CR1 has 30 SCR units accounting for its entire extracellular structure. It has a transmembrane segment and a small cytoplasmic domain. CR3 is a heterodimer containing an aand ß subunit held together by non-covalent forces. The ß subunit is also found in the two leukocyte antigens, LFA-1 and p150,95, which have α subunits distinct from that of CR3. The ß subunit contains 56 cysteine residues, 42 of which lie in a span of 256 residues immediately adjacent to the transmembrane segment. It shares extensive sequence homology with subunits of membrane protein complexes that bind fibronectin and vitronectin, implicating that they all belong to an extended set of surface adhesion molecules not restricted to the immune system. p150,95 is also expressed on macrophages and it has iC3b binding activity. It also shares some functional properties with CR3 as an ahesion surface molecule.

The complement proteins are the effector molecules of the humoral immune system. To date, more than 20 proteins have been identified and classified as members of this group. Apart from the nine classical components, Cl-C9, other proteins include the activation components of the alternative pathway and the control proteins, which keep the complement system finely tuned to mediate and coordinate various processes in host defence and inflammation. Of all the components, C3 occupies the pivotal position in the system, for its activation to yield the anaphylatoxin, C3a, and C3b, which binds covalently to target cells, is the point at which the classical and the alternative pathways converge. (For a review on the activation of complement, see Reid, 1986.) Cells coated with C3b can either be lysed by the terminal components, C5-C9, or be ingested by phagocytic cells, which have specific receptors for C3b and its cleavage products. Two of these receptors have been characterized on the plasma membrane of macrophages. They are the complement receptor type 1 (CR1), earlier known as the C3b receptor or the C3b/C4b complement receptor, and complement receptor type 3 (CR3, also referred to by its antigenic properties as the Mac-1 or Mol antigen), which recognizes iC3b, the cleavage product of C3b by factor I and its cofactors.

The interaction between surface-bound complement components and membrane molecules on blood cells was first formalized in 1953 by R. A. Nelson, who used the term immune adherence to describe the attachment of microorganisms sensitized with antibody and complement to human erythrocytes. The complement-dependent adherence was also recognized to have an enhancing effect on the phagocytosis of the target. Subsequent work by Nelson and colleagues extended the term immune adherence to describe the attachment of complement-treated targets to primate erythrocytes and non-primate platelets (Siqueira & Nelson, 1961). This adherence phenomenon was not observed with non-primate erythrocytes and primate platelets. (For review of earlier work, see D. S. Nelson, 1963.) Clearly, some factors in the complement system and corresponding ones on erythrocyte and platelet surfaces were responsible for the phenomenon of immune adherence. The factor in the complement system was identified first.

Nishioka & Linscott (1963) reported a subcomponent of C’3 from guinea-pig serum as being essential for the immune adherence reaction. They called it C’3c in order to distinguish it from the other three subcomponents, C’3a, C’3b and C’3d. Two more subcomponents of C’3 were found when Nelson et al. (1966) purified the nine components of guinea-pig complement and delineated the activation steps of the classical pathway. C’3c was renamed as C’3 (see Müller-Eberhard, 1968) and later as C3. In the same period, the third component in the human complement system was identified as the ß1C-glycoprotein (Müller-Eberhard & Nilsson, 1960; Müller-Eberhard et al. 1966).

Introduction of the resetting technique to study the adherence of complement-coated sheep erythrocytes to different cell types allowed Lay & Nussenzweig (1968) to determine the presence or absence of C3 receptors on the surface of these cells. It also enabled them to identify functionally two distinct types of C3 receptors. The adherence of C3-bearing sheep erythrocytes to lymphocytes could take place in the presence of EDTA, whereas their adherence to monocytes required the presence of Mg2+.

Progress towards the classification of C3 receptors was made in the early 1970s by studying their distribution on different classes of leukocytes and their roles in various physiological responses including phagocytosis (for review see Bianco & Nussenzweig, 1977). During this period there were parallel advances in the description of the molecular structure of C3 and its degradation products. The view that C3b was cleaved by factor I (then known as the C3b inactivator) into C3c and C3d was reevaluated by establishing the existence of a stable intermediate product, iC3b (see below). The covalent nature of the bond between the labile binding site of C3b and cell surface structures was also clarified (Law & Levine, 1977). It was only then that cells bearing structurally defined C3 fragments could be prepared (Law et al, 1979; Carlo et al. 1979) and it was thus possible to demonstrate that receptors for C3b, iC3b and C3d were distinct (Carlo et al. 1979).

The investigation of receptors at the molecular level began with the purification of CR1 by Fearon (1979). Subsequently, the polypeptide structures of the receptors for C3d (Barel et al. 1981) and iC3b (Wright et al. 1983A), now known as CR2 and CR3, respectively, were identified (see Table 1). Techniques involving the use of monoclonal antibodies and recombinant DNA, as well as the availability of cell lines expressing various complement receptors, contributed much to the purification and structural characterization of these molecules. Other membrane proteins having an affinity for C3 were also discovered (see below and also Ross & Medof, 1985; Sim & Walport, 1987). Before discussing the details of the structure of the receptor molecules, however, it is essential to have an appreciation for the molecular structure of their ligands C3, C4 and their cleavage products.

Table 1.

C3 receptors and other C3-binding membrane proteins

C3 receptors and other C3-binding membrane proteins
C3 receptors and other C3-binding membrane proteins

C3

Native C3 in plasma consists of two disulphide cross-linked polypeptides α and ß of molecular weights 115K and 75K (K = 103Mr) respectively (Bokisch et al. 1975; Nilsson et al. 1975). An internal thioester is located in the α chain between the cysteine and glutamine residues in a sequence Cys-Gly-Glu-Gln (Tack et al. 1980). Upon activation, a fragment of 77 amino acids, C3a, is removed from the N terminus of the α chain (Müller-Eberhard et al. 1966; Hugh, 1975). The cleavage event induces a conformational change in the remainder of the protein, C3b, resulting in the exposure of the thioester, which becomes extremely reactive with hydroxyl nucleophiles. Thus if C3 is activated by a surface-bound enzyme, known as C3-convertase, in the complement system, a portion of the C3b generated will become covalently bound to the cell surface by reacting with surface-bound hydroxyl groups to form acyl ester bonds. For the majority of the activated C3b molecules, their thioester will be hydrolysed, thus yielding fluid-phase C3b. (For review of the covalent binding reaction, see Law, 1983.) The ratio between bound and fluid-phase C3b depends upon the type of cell surface on which activation takes place; both the surface density of the acceptor molecules and the reactivity of the hydroxyl groups on them play a role in the overall binding efficiency of C3b. Under artificial, experimental conditions, for example the binding of C3 to sheep erythrocytes or zymosan, a value of 10% is generally accepted.

Fig. 1.

C3, C4 and their cleavage products. A. C3, the two chains of C3, α and ß, are shown with the interchain disulphide bonds as determined by Janatova (1986). Molecular weight of each fragment is expressed as Mr× 10-3 as derived from de Bruijn & Fey (1985). The thioester site is shown as an asterisk. The cleavage sites are marked: A, the convertase cleavage site to generate C3a and C3b; I1, 12, and 13, the factor I cleavage sites to generate iC3b’, iC3b and C3f, and C3c and C3dg; T, the trypsin cleavage site to produce C3d. The polypeptide chains of C3c are cross-hatched. B. C4, the three chains of C4, α, ß and γ, are shown with the interchain disulphide bonds according to Janatova (1986) and Seya et al. (1986b). Molecular weight of each fragment (Mr× 10-3) as derived from Belt et al. (1984). The thioester site is shown as an asterisk. The cleavage sites are marked: A, the Cl cleavage site to generate C4a and C4b; II, the factor I cleavage site to produce iC4b; 12, the factor I cleavage site to yield C4c and C4d. The polypeptide chains of C4c are cross-hatched.

Fig. 1.

C3, C4 and their cleavage products. A. C3, the two chains of C3, α and ß, are shown with the interchain disulphide bonds as determined by Janatova (1986). Molecular weight of each fragment is expressed as Mr× 10-3 as derived from de Bruijn & Fey (1985). The thioester site is shown as an asterisk. The cleavage sites are marked: A, the convertase cleavage site to generate C3a and C3b; I1, 12, and 13, the factor I cleavage sites to generate iC3b’, iC3b and C3f, and C3c and C3dg; T, the trypsin cleavage site to produce C3d. The polypeptide chains of C3c are cross-hatched. B. C4, the three chains of C4, α, ß and γ, are shown with the interchain disulphide bonds according to Janatova (1986) and Seya et al. (1986b). Molecular weight of each fragment (Mr× 10-3) as derived from Belt et al. (1984). The thioester site is shown as an asterisk. The cleavage sites are marked: A, the Cl cleavage site to generate C4a and C4b; II, the factor I cleavage site to produce iC4b; 12, the factor I cleavage site to yield C4c and C4d. The polypeptide chains of C4c are cross-hatched.

Both fluid-phase and surface-bound C3b are cleaved by factor I in the presence of cofactors. To date, three molecules are known to have the cofactor activity for this reaction. They are factor H (previously known as the IH-globulin) (Whaley & Ruddy, 1976a,b; Pangburn et al. 1977), membrane cofactor protein (MCP, previously referred to as gp45-70) (Seya et al. 1986a), and CR1 (Fearon, 1979). Two sites on the α’ chain of C3b, marked as I1and 12 in Fig. 1A, are cleaved sequentially to generate fragments of sizes 63K, 3K, and 40K in that order from the N terminus (Harrison & Lachmann, 1980; Sim et al. 1981). The 3K fragment (C3f) is free and what remains is a three-chain disulphide cross-linked structure composed of the 63K and 40K polypeptides of the α’ chain and the intact ß chain. This molecule is referred to as iC3b (previously also referred to as C3b’ or C3bi). If generated from surface-bound C3b, iC3b remains covalently linked to the cell surface via the 63K fragment (Law et al. 1979).

iC3b can be further degraded into smaller fragments by different proteolytic enzymes. In serum, where protease inhibitors are in abundance, iC3b is relatively stable. In the laboratory, proteases such as trypsin are used to assist its subsequent cleavages. Two major fragments are generally obtained and are loosely referred to as C3c and C3d. With most proteases, there are additional cleavage sites on C3c. Thus the structure of C3c, with a protease-dependent variable number of polypeptides held together as a.single molecule by disulphide bonds and non-covalent forces, has usually not been analysed in detail. C3d, however, is invariably found as a single polypeptide containing both the covalent binding site and the classical D antigen (West et al. 1966; Law et al. 1979). Most relevant for the purpose of this review is the fact that C3d contains the recognition site for CR2. When generated with trypsin, C3d has a size of about 35K.

The physiological breakdown products of iC3b have been studied by prolonged incubation of blood or serum in which the complement system has been artificially activated by immune aggregates or cobra venom factor (Davis et al. 1984; Janatova & Gobel, 1985). The larger product of C3 degradation is again referred to as C3c, but the smaller one is called C3dg in order to distinguish it from C3d generated by exogenous enzymes. It is larger than the trypsin-generated C3d by 48 residues in the N terminus and is electrophoretically different (Lachmann et al. 1982). Presumably, C3dg is identical to the α2D fragments described by West et al. (1966) (Lachmann et al. 1982; Davis et al. 1984). C3c generated this way contains three disulphide-linked polypeptides, the intact ß chain, the N-terminal 23K fragment and C-terminal 4OK fragment of the α’ chain (Davis et al. 1984; Janatova & Gobel, 1985). Analysis of C3 fragments from the serum of a patient with circulating C3 cleavage products showed fragment sizes different from the above, suggesting the natural degradation of iC3b is likely to be more complicated (Davis et al. 1984). The enzyme(s) responsible for the conversion of iC3b to C3c and C3dg has not been identified. However, the extent of conversion is different in activated serum and activated whole blood. Whereas variable amounts of residual iC3b are found in activated serum, the generation of C3c and C3dg in activated blood always appears to proceed to completion. These observations suggest that the enzyme(s) mediating this reaction could be of cellular origin (Davis et al. 1984).

The degradation of surface-bound C3b is complicated by the fact that these molecules are covalently linked to a variety of cell surface molecules. The overall cell surface properties, such as surface charge and the nature of the acceptor molecules to which C3b is attached, may affect the rate and degree of degradation of C3b. In fact, it is precisely this property that distinguishes activating and non-activating surfaces of the alternative pathway of complement. Activating surfaces retard the degradation of some of the surface-bound C3b to iC3b, thus allowing the surface-bound C3b to form the C3-convertase with factor B to initiate the positive feedback loop of C3 activation (Fearon & Austen, 1977a,b). Recently, Newman & Mikus (1985) studied the kinetics of the deposition of C3b and its subsequent degradation to iC3b on a number of cell surfaces when treated with serum. Whereas the conversion of C3b to iC3b was virtually instantaneous and complete on sheep erythrocytes, a non-activator of the alternative pathway, a substantial fraction of C3b, ranging from 10 % to 80 %, remained unconverted on activating surfaces such as rabbit erythrocytes, yeast and five different strains of bacteria.

The conversion of surface-bound iC3b to C3dg in serum is at best slow on a number of surfaces including the erythrocytes of sheep (Law et al. 1979; Ross et al. 1982; Medicus et al. 1983), rabbit and guinea-pig (Medicus et al. 1983) as well as yeast and various bacteria (Newman & Mikus, 1985). Although soluble iC3b was thought not to be degraded by factor I with any of the three known cofactors under physiological conditions, CR1 appeared to be able to act as a cofactor for a factor-I-mediated cleavage of bound iC3b (reaction 13 in Fig. 1A) to C3c and C3dg on immune aggregates (Medof et al. 1982a,b), and human and sheep erythrocytes (Ross etal. 1982; Medicus et al. 1983). Different results have also been reported (Malhotra & Sim, 1984). It is possible that the rate and extent of the breakdown of surfacebound iC3b are different from the breakdown of fluid-phase iC3b. However, a systematic study of this aspect has not been reported.

C4

The structure and function of C4 resemble those of C3 in a variety of ways (for review see Reid, 1986; Campbell et al. 1988). Both are synthesized as a single polypeptide (for review see Colten, 1986) with the individual chains found in the mature molecule arranged in the order of ß-α-γ for C4 and ß-α for C3 (Belt et al. 1984; de Bruijn & Fey, 1985). The inter-chain sites on the pro-molecules are marked by stretches of tetrabasic residues which are probably removed by a common set of enzymes. The pro-C4 and pro-C3 are of similar molecular sizes and their primary amino acid sequences, as derived from their cDNA sequences, show an identity of about 25 % after alignment. Both are activated similarly by the removal of the N-terminal 77 amino acid residues of their respective a chains, and they both bind to cell surfaces by an acyl-transfer reaction between the internal thioester and surfacebound nucleophilic groups. They play corresponding roles in the activation of the classical and alternative pathways, where C4b and C3b serve as the non-catalytic component of the C3-convertases of the classical and alternative pathways respectively, and they do so by interacting with the two homologous proteins, C2 and factor B. Both are under the similar regulatory control of factor I with an overlapping set of cofactors. The three cofactors for the degradation of C4b are C4BP (Shiraishi & Stroud, 1975; Fujita et al. 1978; Fujita & Nussenzweig, 1979), CR1 (Iida & Nussenzweig, 1981; Medof & Nussenzweig, 1984), and MCP (Seya et al. 1986a). Factor I cleaves C4b at sites on either side of the thioester (marked II and 12 in Fig. 1B) to yield C4c and C4d. The relative efficiency of the cofactors in the II and 12 cleavages has been studied and found to be different; CR1 is effective in mediating both II and 12 cleavages for both surface-bound and fluid-phase C4b (Medof & Nussenzweig, 1984), whereas MCP is only active in mediating the II cleavage (Seya et al. 1986a). The intermediate, iC4b, which exists in laboratory conditions (Nagasawa et al. 1980; Seya et al. 1986a), may not be a stable product in serum or blood. The affinity of C4b for CR1 is demonstrated by the immune adherence reaction between C4b-coated sheep erythrocytes and human erythrocytes (Cooper, 1969). Receptors for iC4b or C4d have not been described.

A list of membrane proteins having an affinity for various C3 fragments is found in Table 1. Three of them, CR1, CR2 and CR3, have been established as receptors, the numerical order given to them reflects the chronological order of establishment of the molecular structure of their respective major ligands, C3b, C3d and iC3b. Two proteins have been described as CR4. They are referred to in this article as CR4-1 (Vik & Fearon, 1985) and CR4-2 (also known as p150,95, see Ross & Medof, 1985), and they are distinct both in their ligand specificity and their divalent cation requirement. Whereas the binding of iC3b and C3dg to CR4-1 takes place in the presence of EDTA (Vik & Fearon, 1985), the interaction between iC3b and CR4-2 requires divalent cations (Micklem & Sim, 1985; Malhotra et al. 1986). It is to be hoped that the nomenclature will be standardized in the near future. Other C3-binding proteins are not called receptors, probably because their major known functions do not include the triggering of a cellular response upon interaction with ligand. The major ligands for these molecules, the apparent molecular weight of their component polypeptides, and the distribution of their expression among major cell types are also included in Table 1.

Except for two, the proteins listed in Table 1 can be divided into two groups according to their structural similarity to either CR1 or CR3. Proteins belonging to the CR1 group contain repeating structural units each of about 60 amino acid residues known as SCR. This structure was first described in some soluble components of complement and later in proteins outside the complement system. All proteins in the CR1 group that contain this structural unit interact with C3 and/or C4. CR2, which is found predominantly on B lymphocytes (Tedder et al. 1984), also belong to this group (Weis et al. 1986). The CR3 group includes CR3 and CR4-2. Together with the LFAT antigen found on lymphocytes, granulocytes, and activated macrophages, the three proteins form the leukocyte adhesion glycoprotein family of the immune system. They are probably a subgroup of a more extensive family of cell adhesion glycoproteins including the receptors for fibronectin and vitronectin.

The two proteins that do not belong to either the CR1 or the CR3 group are CR4-1 and p90. No structural data are available for CR4-1, and p90 has been reported as a protein in spleen extract that shows binding affinity for iC3b at low ionic strength (Micklem & Sim, 1985). Neither will be discussed further in this article. More detailed structural information regarding CR1, CR3 and related proteins, is presented below.

The SCR-containing proteins

Investigation of the activation of the alternative pathway (Fearon & Austen, 1977A,B) led Fearon (1979) to postulate a factor on the surface of human erythrocyte membranes that could inhibit the formation of the surface-bound C3-convertase, thus accounting for the finding that human erythrocytes fail to activate the alternative pathway even after the removal of surface sialic acid residues. The factor was shown to be a protein with the ability to accelerate the decay of cell-bound C3-convertase of the alternative pathway (reaction 3D, Table 2). Using this property as an assay, a protein was purified from human erythrocyte membranes; it was also found to have the cofactor activity for the factor-I-mediated cleavage of C3b. Its affinity for C3 was demonstrated in the purification procedure in which a key step was the affinity chromatography of the partially purified protein on a C3-Sepharose column. Subsequently, its identity as the C3b receptor was firmly established when antibodies against this protein were found to inhibit C3b receptor functions in both peripheral blood leukocytes and erythrocytes (Fearon, 1980).

Table 2.

Function of proteins in the complement system with the short consensus repeat (SCR) units

Function of proteins in the complement system with the short consensus repeat (SCR) units
Function of proteins in the complement system with the short consensus repeat (SCR) units

Data on the primary structure of CR1 were obtained by protein and cDNA sequencing (Wong et al. 1985; Klickstein et al. 1987a). The protein was found to contain an array of a structural element found in complement proteins that interact with C3b and C4b (for review see Reid et al. 1986; Kristensen et al. 1987a). This element is now known as SCR (for short consensus repeat, as distinct from LHR for long homologous repeat, see below).

SCRs were first observed in factor B (Morley & Campbell, 1984). Sequence analysis showed that the N-terminal residues of factor B could be arranged into three repeating units, each about 60 residues in length. The possible role of these units in binding C4b or C3b was suggested when eight similar repeats were found in the monomeric subunit of the C4-binding protein, C4BP, accounting for the major part of the primary structure of that protein (Chung et al. 1985). Subsequently, these units were found in other complement proteins as well as membrane proteins that interact with C3 and/or C4. A list of these proteins and their functions is shown in Table 2.

A problem in analysing tandem repeating structures is to determine the boundaries that mark the end of one unit and the beginning of the next. The first suggestive evidence in defining an SCR unit came from the study of the exon/intron organization of the factor B gene (Campbell et al. 1984). Each of the second, third, and fourth exons was found to code for a region that could be regarded as a repeating unit. This appears to be a general rule because data from the exon/intron structure of other SCR-containing proteins support this contention (see Reid et al. 1986; Kristensen et al. 1987a). However, exceptions are also found; the second SCRs of the C4BP (Barnum et al. 1987) and factor H (Vik et al. 1987) of the mouse appear to be coded in two exons.

Extensive analysis of the primary structure of the SCRs available to date clearly shows a consensus sequence built around the four cysteine residues. The consensus structure is shown in Fig. 2, along with the number and arrangement of SCRs for each protein.

Fig. 2.

The schematic structure of short consensus repeats (SCRs) and their arrangement in complement proteins. A. Three contiguous SCRs are shown with conserved residues marked in the middle SCR (see Reid et al. 1986; Kristensen et al. 1987A; Klickstein et al. 1987a). The two disulphide bonds within each unit are also shown. The consensus structure is generated from data obtained from the complement proteins as well as non-complement proteins including ft-glycoprotein I (Lozier et al. 1984), factor XIIIb (Ichinose et al. 1986), haptoglobin (Kurosky et al. 1980), and interleukin-2 receptor (Leonard et al. 1985). Amino acids are encircled and represented by their single letter codes: C, cysteine; F, phenylalanine; G, glycine; I, isoleucine; L, leucine; P, proline; S, serine; T, threonine; V, valine; W, tryptophan; Y, tyrosine. Circles enclosing more than one letter show the possible alternatives. B. The arrangement of SCRs, each represented by a circle, in ten complement proteins is shown. Clr, Cls, C2 and factor B also contain a serine protease (SP) domain at the C-terminal end of the protein (see Reid, 1986). C4BP contains seven identical subunits each with eight SCRs. They are disulphide bonded together via the non-SCR structure at the C terminus. The CR1 structure shown represents the most common allelic form, CR1-A, with 30 SCRs (see text). Two CR2 structures had been reported in the XIIth International Complement Workshop; the one from tonsil lymphocytes contains 15 SCRs (Weis et al. 1987) and one from Raji cells contains 16 SCRs (Moore et al. 1987). Both CR1 and CR2 have a hydrophobic transmembrane segment and a relatively small cytoplasmic domain. DAF anchors to the membrane by a lipid tail in the form of a phosphatidylinositol. MCP contains four SCR units (J. P. Atkinson, personal communication), but the remainder of its structure is not known

Fig. 2.

The schematic structure of short consensus repeats (SCRs) and their arrangement in complement proteins. A. Three contiguous SCRs are shown with conserved residues marked in the middle SCR (see Reid et al. 1986; Kristensen et al. 1987A; Klickstein et al. 1987a). The two disulphide bonds within each unit are also shown. The consensus structure is generated from data obtained from the complement proteins as well as non-complement proteins including ft-glycoprotein I (Lozier et al. 1984), factor XIIIb (Ichinose et al. 1986), haptoglobin (Kurosky et al. 1980), and interleukin-2 receptor (Leonard et al. 1985). Amino acids are encircled and represented by their single letter codes: C, cysteine; F, phenylalanine; G, glycine; I, isoleucine; L, leucine; P, proline; S, serine; T, threonine; V, valine; W, tryptophan; Y, tyrosine. Circles enclosing more than one letter show the possible alternatives. B. The arrangement of SCRs, each represented by a circle, in ten complement proteins is shown. Clr, Cls, C2 and factor B also contain a serine protease (SP) domain at the C-terminal end of the protein (see Reid, 1986). C4BP contains seven identical subunits each with eight SCRs. They are disulphide bonded together via the non-SCR structure at the C terminus. The CR1 structure shown represents the most common allelic form, CR1-A, with 30 SCRs (see text). Two CR2 structures had been reported in the XIIth International Complement Workshop; the one from tonsil lymphocytes contains 15 SCRs (Weis et al. 1987) and one from Raji cells contains 16 SCRs (Moore et al. 1987). Both CR1 and CR2 have a hydrophobic transmembrane segment and a relatively small cytoplasmic domain. DAF anchors to the membrane by a lipid tail in the form of a phosphatidylinositol. MCP contains four SCR units (J. P. Atkinson, personal communication), but the remainder of its structure is not known

The view that each SCR, as defined by primary and genomic structural analysis, represents an individual domain at the protein level, also found support from the limited information on the arrangement of the disulphide bonds in C4BP (Janatova et al. 1987) and factor H (Day et al. 1987), as well as in the non-complement protein β2-glycoprotein I (Lozier et al. 1984). The four cysteine residues in each SCR were shown to engage in disulphide bonds in a self-contained 1-3, 2-4 fashion. Inter-SCR disulphide bonds have not been found. The current model for the unit structure appears to be a ‘triple-loop’ hinged at two disulphide bonds (Klickstein et al. 1987a). The fourth cysteine residue and the first cysteine residue of the adjacent SCR is, on the average, separated by four residues. The engagement of the cysteine residues in different disulphide bonds suggests that adjacent SCRs are tightly packed against each other. This is in agreement with the appearance of the C4BP as a ‘spider-like’ structure in electron micrographs (Dahlback et al. 1983). The seven identical subunits form the ‘legs’ of the spider extending from a small central core, presumably formed by the non-SCR region of the subunits. Each ‘leg’ of the spider is interpreted to be a stack of the eight SCRs (Chung et al. 1985) with a dimension of 30×330À; thus each SCR is more or less globular with a diameter of about 35 À (Dahlback et al. 1983; Perkins et al. 1986).

The significance of the SCR in C3/C4 binding proteins is not certain. It is attractive to postulate that each SCR is a C3/C4 binding unit, with each unit contributing to the combined affinity of the protein for either C3 or C4. This conjecture, however, is not supported by experiments. C4BP and factor H have specificities for C4 and C3, respectively, and cross activities are weak if positive. Furthermore, the cofactor activities of both factor H (Alsenz et al. 1984) and C4BP (Chung & Reid, 1985; Fujita et al. 1985) are associated with particular proteolytic fragments of the respective proteins. Currently, the SCRs are thought of as the building blocks of the C3/C4 binding proteins with one or a few from each protein containing an active binding site.

Family studies of the genes specifying SCR-containing proteins, either by allotypic polymorphism at the protein level or by restriction fragment length polymorphism (RFLP) at the nucleic acid level, showed that those for CR1, factor H, C4BP and DAF are found to be closely linked (Rodriguez de Cordoba et al. 1985; Rey-Campos et al. 1987; Lublin et al. 1987). The gene cluster is referred to as the RCA (regulators for complement activation) linkage group and has been localized to the q32 region of chromosome 1 (Wongeí al. 1985; Lublin et al. 1987). Using pulsefield electrophoresis, a genomic fragment of 950 kb was shown to contain the genes for CR1, DAF, C4BP and CR2 (Carroll et al. 1987). The gene for factor H, however, is not in this genomic fragment. It is not known whether the remaining regulatory protein, MCP, belongs to this linkage group.

CR1

CR1 is the longest polypeptide of this family of proteins and it contains the greatest number of SCRs reported to date. Klicksteinet al. (1987a) described a partial cDNA clone coding for about 80% of CR1 inclusive of the C terminus, and the predicted structure included 23 SCRs, a transmembrane segment of 25 amino acid residues, and a cytoplasmic domain of 43 amino acid residues. A higher order of organization was found among the first 21 SCRs, which could be broken down into groups of seven SCRs to give three long homologous repeat (LHR) units. Anticipating that an extra LHR would account for the missing 5’ end of the clone, Klickstein et al. (1987a) called the three units LHR-B, LHR-C, and LHR-D. The LHRs are highly homologous, with the lowest pairwise post-alignment identity of 67 % between LHR-B and LHR-D. LHR-C appears to be a composite of LHR-B and LHR-D. Its first four SCRs can be aligned with the corresponding ones of LHR-B at 99 % and those of LHR-D at 61 % homology. The remaining three SCRs, however, showed 76% and 91% homology with the corresponding SCRs in LHR-B and LHR-D, respectively. It is interesting to note that the gene segment for LHR-C could not have been generated by unequal crossing over between pre-existing gene segments for LHR-B and LHR-D but could possibly have arisen by gene conversion. Two other partial cDNA clones were reported by Hourcade et al. (1987) and Klickstein et al. (1987b). Both clones contain a signal peptide at the 5’ end with a 3’ end extending into the clone previously described by Klickstein et al. (1987a). The derived amino acid sequence indicated that they contain the postulated LHR-A unit. Of the seven SCRs, the five C-terminal ones share 99% homology with the corresponding SCRs of LHR-B, whereas the two N terminal are at a lower level of about 60 % homology.

Another unusual structural feature of CR1 is its size polymorphism. CR1 is coded for by a single gene, and four allotypes of different sizes have been identified. They are designated as types A (190K), B (220K), C (160K) and D (250K) in the descending order of their gene frequencies of 0·83,0·16, 0·01 and 0·002 respectively (Holers et al. 1987). The two most abundant forms are also known as the F and S allotypes (Dykman et al. 1983; Wong et al. 1983). The structure described by Klickstein et al. (1987a,b) and Hourcade et al. (1987) is that of the A allotype. The polymorphism has been indicated to lie in the length of the polypeptide structure since the size differences between the allotypes is unlikely to be caused by differential glycosylation (Lublin et al. 1986), and the mRNA size differences are found to correlate with the size of the allotypes (Holers et al. 1987). The different sizes of the allotypes could be accounted for by the assumption that they differ from each other by the addition or the removal of discrete numbers of LHRs (Klickstein et al. 1987a). Supportive evidence is found in the analysis of genomic clones covering the gene encoding the B allotype (Wong et al. 1987), which appears to contain five LHRs, one more than the A allotype.

Based on the presumably rigid structure conferred by the linear array of 30 SCRs and the previous finding of Abrahamson & Fearon (1983), who observed that ferritin-conjugated anti-CRl antibodies bound to neutrophils were frequently 500 A away from the plasma membrane, Klickstein et al. (1987A) proposed that CR1 could be a structure whose extension from the membrane increases its efficiency to interact with C3b-coated targets. This proposal is in line with the postulate of Hourcade et al. (1987), who suggested that the active site of CR1 resides in the first two SCRs, based on the fact that these two SCRs are more divergent in their primary structure than the corresponding ones in other LHRs. The interaction of C3b (C4b) with CR1 mediates the known functions of CR1, all of which lead to the processing of the C3b bearing targets (for review see Fearon, 1985). The regulatory activity of CR1 in the complement cascade has been discussed extensively with respect to both its capacity to act as a cofactor for factor-I-mediated cleavage of C3b and in its decay-accelerating activity in dissociating, and hence inactivating, the C3bBb enzyme complex. In addition, CR1 on primate erythrocytes may have an important role in the traffic and clearance of immune complexes. Apparently, immune complexes bind to the erythrocytes via a C3b-CRI interaction and are removed from the erythrocytes during their passage through liver and spleen (Medof & Oger, 1982; Cornacoff et al. 1983; Waxman et al. 1984). CR1 on platelets of non-primates may have an equivalent function. CR1 can exist in two states on macrophages and other phagocytes. In its passive state it promotes adherence of the phagocytes to C3b-coated targets, thus enhancing the occurrence of ingestion mediated by other receptors, e.g. Fc receptor (Mantovani et al. 1972; Bianco etal. 1975; Griffin et al. 1975; Newman etal. 1980). Upon treatment of macrophages with T lymphokines (Griffin & Griffin, 1979), phorbol esters (Wright & Silverstein, 1982), or extracellular matrix proteins such as fibronectin (Wright etal. 1983b), CR1 is promoted to an active state and the C3b-CR1 interaction appears to be sufficient to trigger the phagocytic process. Changelian & Fearon (1986) studied the phosphorylation of cell surface receptors and demonstrated the phosphorylation of CR1 on phagocytic cells upon stimulation with phorbol myristate acetate (PMA). Phosphorylation did not occur without stimulation or on non-phagocytic cells with or without stimulation. Thus the activity state of the phagocytic cell correlates well with the phosphorylation of CR1. A possible site for protein kinase C mediated phosphorylation has been identified on the putative cytoplasmic domain (Klickstein et al. 1987a).

CR3

Before the polypeptide structures of C3 and its breakdown products were clarified, the only C3 receptor having an identifiable ligand was CR1. The major reason was that C3b-coated sheep erythrocytes could be easily prepared from purified complement components for immune adherence or rosetting assays. Although receptors distinct from CR1 had been described (Lay & Nussenzweig, 1968; Ross et al. 1973), their ligand specificities required a re-examination when the conversion of surface-bound C3b to C3d was found to have a stable intermediate, iC3b (Law et al. 1979). Receptor activity specific for iC3b was established by the demonstration that the rosetting pattern of iC3b-coated sheep erythrocytes with different leukocytes was different from those of C3b-or C3d-coated cells (Carlo et al. 1979; Ross & Lambris, 1982). However, the receptor for iC3b continued to escape identification because most leukocytes bear more than one type of C3 receptor on their surfaces, and these receptors cross-react with the three ligands, C3b, iC3b and C3d. CR3 was finally distinguished from CR1 and CR2, functionally by its requirement for divalent cations (Wright & Silverstein, 1982; Ross et al. 1983), and structurally by its identification with specific monoclonal antibodies (Beller et al. 1982; Wright et al. 1983A) . CR2 was found to have binding affinity for both C3d and iC3b in the absence of divalent cations (Ross et al. 1983). It was also found to be the receptor for the Epstein Barr virus on B lymphocytes (Fingeroth et al. 1984; Nemerow et al. 1985).

The function of CR3 on phagocytes was found to be similar to that of CR1 (Newman et al. 1980) except for its requirement for divalent cations (Wright & Silverstein, 1982; Ross et al. 1983). Both CR1 and CR3 appear to respond to similar stimulants for transition from passive to active mediators of phagocytosis (Wright & Silverstein, 1982; Wright et al. 1983b). Release of toxic oxygen metabolites is not coupled to phagocytosis mediated by active CR1 and CR3 in contrast to the IgG-FcR-mediated activity (Wright & Silverstein, 1983). CR3 is also found to possess divalent cation dependent binding to unopsonized zymosan (Ross et al. 1985a) and ß glucan (Ross et al. 1987). This interaction differs from that between CR3 and iC3b by its ability to trigger a phagocytic response and respiratory burst from unstimulated neutrophils and monocytes. Furthermore, two monoclonal anti-CR3 antibodies, anti-Leu-15 and OKMI, were found to block iC3b and ß-glucan binding to CR3, respectively, without reciprocal blocking activity (Ross et al. 1985A), suggesting that iC3b and -glucan bind to different sites on CR3.

The molecular structure of mouse CR3 was first established when Beller et al. (1982) observed that the binding of iC3b-coated erythrocytes to macrophages is inhibited by a monoclonal antibody against the macrophage surface marker, Mac-1, thus demonstrating that CR3 and Mac-1 are the same molecule. The molecular structure of Mac-1 was already known. It contains two non-covalently associated subunits with apparent molecular weights for the α and ß subunits at 160K and 95K respectively (Springer et al. 1979). The human CR3 was later shown to have a similar structure by Wright et al. (1983a) and Sanchez-Madrid et al. (1983).

Leukocyte adhesion glycoproteins

In characterizing surface molecules that mediate T lymphocyte functions, Davignon et al. (1981) described a set of antigens that is required for T cell adhesion to target cells. One of the antigens was named LFA-1 (lymphocyte-function-associated antigen-1). Like Mac-1, it is also a heterodimer with apparent molecular weights for its two subunits of 180K and 95K. Although the LFA-1 and Mac-1 antigens apparently have different α subunits, their ß subunits have a very similar, if not identical two-dimensional tryptic peptide map (Kurzinger et al. 1982). Using a panel of monoclonal antibodies, Sanchez-Madrid et al. (1983) showed that the ß subunits of LFA-1 and CR3 are identical with respect to their antigenicity, apparent molecular weights and isoelectric points. A third antigen with the same ß subunit was also identified by these authors. In the absence of a known function, it was referred to as p150,95 by the molecular weight of its subunits.

Later work by Micklem & Sim (1985) and Malhotra etal. (1986) demonstrated the affinity of p150,95 for iC3b. Iodinated membrane proteins were passed through an iC3b—Sepharose column at low ionic strength. The bound material eluted with EDTA was shown to contain CR3 and p150,95 by specific monoclonal antibodies as well as by SDS-polyacrylamide gel electrophoresis. The high level of p150,95 antigen on the surface of tissue macrophages led to the suggestion that it may be the iC3b receptor on these cells (Hogg et al. 1986). This is supported by recent results (Myones et al. 1987; Keizer et al. 1987) indicating that p150,95 may function as an iC3b phagocytic receptor.

Human LFA-1 was initially defined by monoclonal antibodies which inhibit cell killing by cytolytic T lymphocytes and natural killer cells (Sanchez-Madrid et al. 1982; Hildreth etal. 1983) at the Mg2+-dependent adherence stage prior to the delivery of the lethal hit (Springer et al. 1982). It was subsequently shown also to partake in a wide range of T-lymphocyte-mediated adherence activities including antigen presentation to both T and B cells as well as antigen-independent aggregation of leukocytes (for review see Springer et al. 1987). Springer et al. (1987) proposed a unified scheme for various LFA-1-mediated adherence reactions by suggesting that they require the promotion of LFA-1 to an active state. In the case of antigendependent adherence, the triggering signal comes from the antigen-receptor interaction, whereas in the case of antigen-independent adherence, the signal is provided for by non-specific agents such as phorbol esters.

In LFA-l-mediated aggregation of leucocytes, LFA-1-negative cells can coaggregate with LFA-1-positive cells, thus LFA-1-dependent cell adhesion is not mediated by a ‘like-like’ recognition between LFA-1 molecules on different cells (Rothlein & Springer, 1986), suggesting the existence of ligand molecules for LFA-1. One molecule that satisfies the functional criteria of a ligand for LFA-1 is ICAM-1 (intercellular adhesion molecule-1). Monoclonal antibodies against ICAM-1 inhibit LFA-1-dependent cell adhesion. The adherence of T lymphocytes (LFA-l-positive, low ICAM-1 expression) and fibroblasts (LFA-l-negative, I CAM-1-positive) can be inhibited by either the pre-treatment of T cells with antibodies against LFA-1 or by the pre-treatment of the fibroblasts with antibodies against ICAM-1. Furthermore, the inhibition is not additive since either treatment effectively inhibits aggregation to an extent significantly greater than 50%. These observations thus provide strong evidence that ICAM-1 and LFA-1 are receptor-ligand counterparts (Dustin et al. 1986). ICAM-1 may not be the only ligand for LFA-1, however, since the LFA-1-dependent homotypic aggregation of SKW-3, the T lymphoma line with low ICAM-1 expression, is not inhibited by an anti-ICAM-1 antibody (Rothlein et al. 1986).

Although CR3, LFA-1 and possibly p150,95 have their respective distinct functions, they also have a common adherence activity with unopsonized microorganisms. Wright & Jong (1986) demonstrated the divalent cation dependent binding of the rough strains of Escherichia coli and Salmonella typhimurium to macrophages via the interaction between the bacterial lipopolysaccharide and each of the three adhesion proteins. The binding was not observed with the smooth strains of these bacteria, which have the additional O antigen on their lipopolysaccharide. The significance of this binding is not known, but it may be a general property of macrophages with respect to a selected spectrum of microorganisms. This binding may be related to that of Histoplasma capsulatum to macrophages (Bullock & Wright, 1987) and of Staphylococcus epidermidis to neutrophils (Ross et al. 1985b). Since all three proteins can mediate this activity, it is likely that they do so vra their common ß subunit. It should be pointed out, however, that this binding is distinct from that of CR3 to unopsonized yeast and zymosan, which is not mediated by LFA-1 (Ross et al. 1985a).

Patients who lack CR3, LFA-1 and p150,95 on their leukocyte surfaces suffer from recurrent infections (for review see Anderson & Springer, 1987). The failure to express these antigens is associated with the failure to produce a mature form of the ß subunit (Springer et al. 1984; Dana et al. 1987; Kishimoto etal. 1987a) and consequently the maturation of the α subunits is affected. The deficiency is inherited as an autosomal recessive trait and the gene encoding the ß subunit had been mapped to region q22.l-qter of chromosome 21 (Marlin et al. 1986; Solomon et al. 1988).

The primary structure of the ß subunit derived from cDNA was obtained by a combination of protein and cDNA sequencing (Law et al. 1987; Kishimoto et al. 1987b). It contains 747 amino acid residues, six possible β-glycosylation sites, a transmembrane segment of 23 residues, and a cytoplasmic domain of 47 residues. The most striking feature is an abundance of cysteine residues, 56, accounting for 7·6 % of the total number of residues. Of the cysteine residues, 42 lie within a span of 256 residues immediately N-terminal to the transmembrane segment. Sequence analysis indicates that the cysteine-rich region may be grouped into three or four repeating structures, each containing eight cysteine residues. However, the boundaries of these units are not known. By comparing their N-terminal sequences, the three a subunits appear to share a significant amount of homology (Pierce et al. 1986; Miller et al. 1987a). Furthermore, homology between the N-terminal sequences of the a subunit of LFA-1 and a interferon has been observed, the significance of which is not clear (Springer et al. 1985).

High cysteine content is a characteristic of a number of receptors including the epidermal growth factor (EGF) (Ullrich et al. 1984), low-density lipoprotein (LDL) (Yamamoto et al. 1984) and insulin receptors (Ullrich et al. 1985). No significant homology was found between the ß subunit of the leukocyte adhesion glycoproteins and these proteins by the ALIGN program (Dayoff et al. 1983), although a high degree of homology was found between the ß subunit and the subunits of two other receptors, both having affinity for the active site tripeptide, Arg-Gly-Asp, of fibronectin. They are the IIIa component of the glycoprotein IIb/lIIa on human platelets (Pytela et al. 1986; Fitzgerald et al. 1987) and a subunit of the integrin complex from chicken fibroblasts (Horwitz et al. 1985; Tamkun et al. 1986). An Arg-Gly-Asp tripeptide is located in the primary structure of C3 (de Bruijn & Fey, 1985) and the region around this tripeptide is found to share significant sequence homology with the Arg-Gly-Asp region of fibronectin and vitronectin (Wright et al. 1987). Indeed, a synthetic peptide covering the C3 sequence in this region inhibited the iC3b-mediated binding of erythrocyte to macrophages (Wright et al. 1987). However, the specificity for Arg-Gly-Asp is apparently lost in CR3. A hexapeptide, Gly-Arg-Gly-Asp-Ser-Pro, highly reactive with the fibronectin receptors (Pierschbacher & Ruoslahti, 1984; Pytela et al. 1986), was not inhibitory to the iC3b-CR3 interaction. Furthermore, mouse C3, which mediates equivalent functions to those of human C3, has a leucine in place of arginine in the tripeptide (Wetsel et al. 1984). It is possible that the iC3b-CR3 interactive site has evolved along a course different from that between fibronectin and its receptors, although some of their features remain identifiably related.

When the amino acid sequences of the ß subunit of the leukocyte adhesion glycoproteins and the related subunits of the fibronectin-binding proteins are compared, the aligned structures show a pairwise identity of over 40 % including all 56 cysteine residues (Tamkun et al. 1986; Law et al. 1987; Kishimoto et al. 1987b`, Fitzgerald et al. 1987). An analysis of their homology, based in part on the combined strategy used by the ALIGN (Dayhoff et al. 1983) and DIAGON (Staden, 1982) programs, is shown in Fig. 3. The pairwise aligned sequences were adjusted to generate consensus alignment of the three proteins. Alignment scores for each position similar to the DIAGON program were generated for the three sequences in the pairwise fashion. Three values were obtained for each position and their geometric mean was taken. The final score as a function of sequence position is shown as the upper curve in Fig. 3 (for details see Fig. 3 legend). This manipulation is useful in comparing sequences in a semi-quantitative fashion. The composite comparison tends to exaggerate the similarity common to all three sequences. Four homologous regions, I, II, III and IV are identified, in which III is the cysteine-rich region and IV is the transmembrane region extending to the cytoplasmic domain. The high degree of homology in the transmembrane-cytoplasmic region may emphasize the similarity between these proteins in their interaction with cytoskeletal and cytoplasmic proteins. The two N-terminal regions may be responsible for their interaction with the presumably homologous partner subunits.

Fig. 3.

Composite analysis on the primary structure homology of the ß subunit of the human leukocyte adhesion glycoproteins, the IlIa subunit of the human IIb/IIIa glycoprotein on platelets and the corresponding subunit of the integrin complex from chicken fibroblasts. This analysis is based in part on the strategies employed by the sequence analysis programs ALIGN and DIAGON. The three sequences were first aligned pairwise by ALIGN and a consensus alignment was generated by minor adjustments. Three scores were obtained at each position by pairwise comparison of the three aligned sequences using the alogarithm of DIAGON, using a bias of +10 to the Mutation Data Matrix. An unmatched gap was given a score of 6 and a matched gap a score of 12. A window of 25, with 12 on each side of the residue of interest, was used and the score was normalized for each residue. The value of 12 would be the default cut-off value in DIAGON. The geometric mean of the three comparison scores is plotted against the sequence and is shown as the upper curve. The lower curve was generated in the same way except the contributions from matching cysteine residues were nullified. The differential between the two curves is shaded. The position of the transmembrane segment (TM) is marked.

Fig. 3.

Composite analysis on the primary structure homology of the ß subunit of the human leukocyte adhesion glycoproteins, the IlIa subunit of the human IIb/IIIa glycoprotein on platelets and the corresponding subunit of the integrin complex from chicken fibroblasts. This analysis is based in part on the strategies employed by the sequence analysis programs ALIGN and DIAGON. The three sequences were first aligned pairwise by ALIGN and a consensus alignment was generated by minor adjustments. Three scores were obtained at each position by pairwise comparison of the three aligned sequences using the alogarithm of DIAGON, using a bias of +10 to the Mutation Data Matrix. An unmatched gap was given a score of 6 and a matched gap a score of 12. A window of 25, with 12 on each side of the residue of interest, was used and the score was normalized for each residue. The value of 12 would be the default cut-off value in DIAGON. The geometric mean of the three comparison scores is plotted against the sequence and is shown as the upper curve. The lower curve was generated in the same way except the contributions from matching cysteine residues were nullified. The differential between the two curves is shaded. The position of the transmembrane segment (TM) is marked.

Since cysteine matches in sequence comparison are given a high score in the Mutation Data Matrix (Dayhoff et al. 1983), scores obtained from sequences with high cysteine content could be biased and require adjustment. Assuming that the cysteine residues in the extracellular domain are engaged in disulphide bonds and they are conserved solely for that reason, their contribution is accounted for by the alignment of the sequences. Hence, further analysis of the homology between the sequences could be made without the score from the cysteine matches. By this criteria, a new score was obtained for each position and the result is shown as the lower curve in Fig. 3. It is clear that the scores for regions I, II and IV remain high but those for region III drop to below the level of significance. Thus, the cysteine-rich region may only be important in providing a rigid structure so that other domains of the molecule may be held and presented in the desired spatial configuration.

In the past two years a substantial amount of information along with an abundance of questions have been obtained for the structure of CR1 and CR3. The complete primary structure of CR1 can be deduced from the combined work of two research groups (Klickstein et al. 1987a,b; Hourcadeel al. 1987), and its overall structure on the cell surface has been interpreted as a proteinaceous ‘skyscraper’ with the active site on top. The protein is anchored to the membrane with a hydrophobic segment linked to a cytoplasmic domain of 43 residues inclusive of a possible phosphorylation site. Except for the first two SCRs, where the binding site presumably resides, the remaining 28 SCRs may simply be playing a structural role to extend the active site from the plasma membrane so as to enhance its effectiveness. The binding signal has to be communicated to the cell to initiate functional activities. Two transmission mechanisms are possible. First, the linear array of SCRs may act as a conduit by which the signal is transmitted to the cytoplasmic domain. Since the 28 SCR units are not simply structural elements in this case, they must also possess the capacity to transmit information from one SCR to another along the rod-like structure. (If the LHRs confer a higher order of tertiary structure, they may be the unit signal transmission element.) Second, the signal may be transmitted by mobilizing CR1 laterally on the plasma membrane. The aggregation of the cytoplasmic domains could be the signal to initiate a cellular response. If this is correct, the 28 structural SCRs could be replaced by any other 28, or, to within certain limits, by a different number of SCR units. Michl et al. (1979) and Griffin & Mullinax (1981) observed that the lateral mobility of CR1 (and CR3) on the plasma membrane is correlated with the ability of the macrophages to bind and ingest C3b-(and iC3b-) coated particles, thus lending support to the proposed second mechanism. How the mobility of the receptor correlates with its phosphorylation and ultimately with its ability to mediate phagocytosis, is not known.

The CR1 structure described here is for the most common allotype, CR1-A. The next frequent allotype, CR1-B, is larger by about 30K and presumably by one LHR unit of seven SCRs (Wongeí al. 1987). Extrapolation from the estimated length of CR1-A of 1140A, the active site of CR1-B would be about 1350Å from the cell surface. However, whether this structural extension has a functional advantage is not clear. The efficiency of CR1-B in mediating decay acceleration and cofactor activity is not appreciably different from that of CR1-A (Seyaet al. 1985) and its efficiency in mediating receptor function has not been evaluated quantitatively.

Though we lack a knowledge of the structure of the a subunit of CR3, our knowledge of the structure of the ß subunit allows us to speculate on the structure and function of CR3 as a whole, as well as that of some of its related molecules. The region adjacent to the transmembrane segment contains 20 % cysteine and predictably has a very tightly knotted tertiary structure. The three or four repeating units have not been defined in terms of their boundaries; the exon/intron organization of the gene may, however, provide indicative information in this regard. Because of the high density of cysteines, determination of disulphide bonds is at best difficult. Phosphorylation of CR3 has not been observed (Changelian & Fearon, 1986). However, the activation of CR3 by PMA treatment of phagocytes led to the speculation that a protein kinase C type of phosphorylation may be involved and that the reason why receptor-bound phosphates have not been detected is because of dephosphorylation. In line with this argument are the observations of Wright & Meyer (1986), who showed that the activated state of CR3 in polymorphonuclear leukocytes was transient in nature. In addition, by introducing to their experiments the phosphate analogue, thio-phosphate, which can be incorporated into proteins but is resistant to subsequent hydrolysis by phosphatases, they were able to prolong the activated state of CR3. The activated state of CR1 has also been shown to be reversible. However, its deactivation appeares to follow a slower kinetics (Wright & Meyer, 1986), thus providing a plausible explanation for the detection of CR1 but not CR3 phosphorylation on PMA-treated phagocytes (Changelian & Fearon, 1986).

Comparison of the N-terminal sequences for the a subunits of CR3, p150,95 and LFA-1 reveal significant homology between the three subunits (Pierce et al. 1986; Miller et al. 1987A). They are also found to be related to the corresponding subunits of fibronectin receptors and platelet gpIIb/IIIa. This latter finding suggests that the leukocyte adhesion glycoproteins belong to the immune system branch of a more extended family of proteins involved in cell adhesion and cell migration whose members are characterized, at least in part, by their heterodimeric structure (Hynes, 1987; Kishimoto et al. 1987c).

CR1 and CR3, although structurally very different, are surprisingly similar in their role and regulation in phagocytosis. They are passive receptors until the phagocytes are stimulated. Their lateral mobility in the plasma membrane and phosphorylation may be important in the elevation to an active state. Furthermore, unlike Fc-receptor-mediated phagocytosis, phagocytosis via active CR1 and CR3 is not coupled to the release of toxic oxygen metabolites. The expression of CR1 and CR3 on polymorphonuclear leukocytes and monocytes is also under another type of regulation. A latent pool of receptors could be mobilized to the surface upon stimulation with chemotactic factors such as C5a and N-formyl-methionyl-leucyl-phenylalanine (fMLP), resulting in an increase by several fold of receptor molecules on the cell surface (Fearon & Collins, 1983; Miller et al. 1987b). Since CR1 and CR3 recognize different cleavage products of C3 and have different divalent cation requirements, it is not surprising that their extracellular structures are different. Their cytoplasmic domains, however, should show some resemblance to each other because either directly or indirectly, they have to link to the cytoskeleton to initiate the ingestion process. Treatment of phagocytes with drugs that disrupt actin filaments, such as cytochalasin B, prevents ingestion of targets but not binding (Axline & Reaven, 1974). Similar features can be found in the activities mediated by other cell adhesion proteins: functions of LFA-1 are also inhibited by cytochalasin B (Rothlein et al. 1986), and integrin, the fibronectin binding protein on chicken fibroblasts, has been shown to have affinity for the cytoskeletal protein talin (Horowitz et al. 1986). However, CR1 and the ß subunit of CR3 do not share any structural homology inclusive of the cytoplasmic domain. The key may lie, of course, in the a subunit of CR3. Work is in progress to study the a subunit of CR3 as well as those of LFA-1 and p150,95. Their structures will shed light on the functions of the leukocyte adhesion glycoproteins and will contribute to our general understanding of the functions of the extended group of adhesion molecules, their interaction with structures on cells, microorganisms and extracellular matrix, and their communication with cytoskeletal and cytoplasmic proteins during cell mobility and differentiation.

I thank Dr K. B. M. Reid and Professor R. P. Levine for critical comments and Ms C. Brooks for preparation of the manuscript.

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