Coupling as a Way of Life

We have progressed a very long way in the past 35 years, to the point where we know that all of these processes are mediated rather specifically by proteins, and where we know the primary structures (amino acid sequences) of tens to hundreds of proteins in each category. One of the major generalities that has emerged during this progress is that the great functional diversity of active transport processes existing among living systems is based mainly on mechanisms in the fourth category: protein-mediated coupling between different species of transiting ions and molecules. That is the central subject of this volume, which has been assembled to provide a comprehensive, contemporary review of coupled transport and the responsible transporter proteins.

The volume is directed to scientists working in membrane transport and related areas, to graduate students and advanced undergraduate students seeking a broad purview of the subject, and to other investigators and would-be investigators seeking a view across the new frontier of Molecular Physiology. Contributers were chosen by the Editorial Board of the Journal of Experimental Biology and were selected for their accomplishments in the field, for their clarity of presentation and for their breadth of coverage in ensemble. The articles are arranged into seven chapters, each with a short introduction to put the chapter topics into a general perspective; moreover, most of the articles themselves begin with cogent background to their specific material. All of the articles were written in the period January–June, 1994.

A major difficulty in reading the literature of transport coupling proteins, particularly on first appproach, is a somewhat skeltered and redundant terminology, which arose as the field grew up among two quite separated groups of investigators. First, medically oriented researchers – exploiting such traditional model systems as the frog skin or toad bladder, and clinically related tissues such as red blood cells, kidney, gut and brain – adopted ‘exchange diffusion’ (Ussing, 1947), ‘ion exchange’ and later the term ‘countertransport’ for coupling in which the separate ionic (or molecular) species move in opposite directions through the biomembrane; they used the term ‘cotransport’ (Crane, 1965) for coupled movement of two species in the same direction. Later, bioenergeticists and microbial physiologists – inspired by Peter Mitchell’s insightful work on mitochondria and chloroplasts (e.g. Mitchell, 1972) and by the rapid development of molecular studies on bacterial transporters, particularly the lactose permease (Kaback, 1974; Newman and Wilson, 1980) – adopted the more compact terms ‘antiport’ and ‘symport’ for countertransport and cotransport; and adopted the related term ‘uniport’ (in favor of ‘facilitated diffusion’) for protein-mediated transfer of a single species of ion or molecule.

Multiple terminologies have likewise been used in reference to membrane electrical variables: for example, ‘P.D.’, ‘membrane potential’, ‘membrane voltage’, V_m, E_m, and ΔΨ, for the transmembrane difference of electrical potential. Since most coupled transporters move ionic charges through biological membranes and therefore must interact with the membrane’s electric field, this multiplicity too is cumbersome.

In order to minimize the reader’s confusion due to these and other purely semantic problems, we have set down a list of preferred terms in Tables 1 and 2 and have asked contributing authors to adhere to them as closely as possible and, when variant terms are deemed necessary, to indicate the corresponding form in the table upon first use of each particular variant. A table of these terms has been compiled previously (Harold, 1986) and they are discussed more fully in Wolfersberger (1994).