p300/CBP proteins (p. 2363) Signal Transduction and Cellular Organization
Members of the p300/CBP family are transcriptional activators that integrate signals controlling a wide variety of cellular processes, including cell proliferation, differentiation and apoptosis. In one of two Signal Transduction and Cellular Organization Commentaries in this issue, Ho Man Chan and Nicholas La Thangue review our understanding of the structure, function and regulation of this interesting family of proteins. The p300/CBP proteins interact with general transcription factors (e.g. TBP and TFIIB) and transcriptional regulators (e.g. JUN and CREB), forming ‘bridges’ that connect sequence-specific, regulatable transcription factors to the basal transcriptional apparatus. Furthermore, they underpin multicomponent transcriptional regulatory complexes by acting as scaffolds for recruitment of transcription factors and architectural proteins such as HMG proteins. An additional feature of p300/CBP proteins is their histone acetyl transferase (HAT) activity, which can regulate both chromatin organization (by targeting histones) and the activity of transcription factors such as p53. Since p300/CBP genes are mutated in several human tumours, and Cbp+/− mice develop haematological malignancies, Chan and La Thangue suggest that p300 and CBP can be viewed as classical tumour suppressors.
The PTEN tumour suppressor (p. 2375) Signal Transduction and Cellular Organization
PTEN is one of the most frequently mutated tumour suppressor genes in human cancer and appears to function in embryonic development, cell migration and apoptosis - as well as tumour suppression. The protein product is an unusual phosphatase: its primary target is PtdIns(3,4,5)P3, which it dephosphorylates to generate PtdIns(4,5)P2. Kenneth Yamada and Masaru Araki discuss recent work that has shed light on the structure, function and regulation of PTEN. The phosphatase appears to antagonize PI 3-kinase signalling by reducing PtdIns(3,4,5)P3 levels. This in turn can reduce Akt/PKB activation and, consequently, cause PTEN-dependent cell cycle arrest, anoikis (apoptosis after loss of contact with the extracellular matrix) or inhibition of cell migration. Interestingly, PTEN might also dephosphorylate tyrosine-phosphorylated proteins such as FAK and Shc, which could enhance its effects on PI 3-kinase signalling. Yamada and Araki conclude that PTEN serves as a hub/switchpoint that links several complex signalling pathways, likening the effects of its mutation to the air traffic disruption caused by problems at a single hub airport.
Immune recognition of bacterial LPS (p. 2535)
To combat bacterial infection, cells of the innate immune system (e.g. macrophages) recognize lipopolysaccharide (LPS) or lipoteichoic acid (LTA) in the bacterial cell wall and release inflammatory mediators such as interleukin 6 (IL-6). CD14 appears to function as the LPS receptor; however, its mechanisms of recognition and signal transduction have remained a mystery. Kathy Triantafilou and co-workers have used fluorescence recovery after photobleaching (FRAP) to monitor the mobility of CD14 in the cell membrane and the dynamics of LPS and LTA binding. They show that LPS and LTA rapidly become immobile when bound to cells but that their binding has no effect on the mobility of CD14 (which is relatively high). Interestingly, unlike CD14, the heat-shock proteins HSP70 and HSP90 are immobile in the cell membrane. Furthermore, the authors demonstrate that antibodies against HSP70/HSP90 block immobilization of LPS and inhibit LPS-induced IL-6 production. They therefore conclude that LPS/LTA briefly associates with CD14 and is then passed on to an immobile, lipid-raft-based signalling complex that contains HSP70 and HSP90.
Dictyostelium cAMP wave dynamics and development (p. 2513)
Dictyostelium development is controlled by propagating waves of cyclic AMP (cAMP). During aggregation, cAMP is released and binds to receptors on surrounding cells, causing them to release further cAMP and move chemotactically towards the source of the signal. cAMP also controls movement during later stages of development and directs specific developmental patterns of gene expression. Since Dictyostelium expresses cAMP receptors of varying affinities (cAR1-cAR4), Cornelius Weijer and co-workers have analysed how changes in cAR affinity affect cAMP wave dynamics and morphogenesis. Using dark-field-wave videomicroscopy, they find that cAR affinity does not influence wave velocity but dramatically affects the frequency of wave initiation and wave geometry. For example, Dictyostelium cells that normally express the high-affinity receptor cAR1 fail to aggregate if they instead express only the low-affinity cAR2 receptor, because waves initiate but do not form stable centres (foci for aggregation). Similarly, at the mound stage, expression of low-affinity cARs produces slow concentric waves rather than normal, multi-armed spiral waves and, consequently, the hemispherical mounds cannot transform into slugs.
Sorting Fas ligand (p. 2405)
Fas ligand (FasL) is a TNF-family protein that induces apoptosis in target cells expressing its receptor (Fas). In T cells and NK cells, FasL is first sorted to secretory lysosomes and then transported to the cell surface upon exocytosis/degranulation. By contrast, in cells that have conventional lysosomes, it is instead targeted to the surface directly. Gillian Griffiths and co-workers have defined the sequences responsible for targeting of FasL to secretory lysosomes: a proline-rich domain (PRD) in the FasL tail, together with positively charged sequences that flank this motif. In cells that lack secretory lysosomes, the motif is not recognised. Since PRDs often interact with SH3-domain proteins, the authors hypothesize that trafficking of FasL from the Golgi to secretory lysosomes requires a cytosolic SH3-domain protein. They have modelled binding of a motif in the FasL PRD to the FYN SH3 domain and predict a critical interaction between charged residues. Significantly, mutation of these residues of FasL results in its mis-sorting to the plasma membrane in cells that contain secretory lysosomes.
Cell Science at a Glance - Polo-like kinases (p. 2357)
The Polo-like kinases play a variety of roles during the passage of cells through M phase - for example, activating Cdc25 at mitotic entry and stimulating maturation and separation of the centrosomes. In this issue’s Cell Science at a Glance, David Glover, Erich Nigg and co-workers integrate information obtained from studies of polo-like kinases in several species to generate an overall picture of the functions of these important mitotic enzymes.