Central‐pore conduction of chaotropic ions through oligomeric membrane proteins

Abstract

Ionic permeabilities in biomembranes can usually be assigned to channels or carriers evolved for a specific class or species of ion. In the past decade, however, chloride permeability has been shown to reside also in proteins specialized for other purposes: cation transport or cation-coupled amine or amino-acid transport. Many of these proteins cluster as membrane oligomers, within which protein-packing interstices are available as potential primitive ion channels, susceptible to modulation by metabolic messengers (e.g., Ca++, polyamines), by physical forces such as membrane stretch, and by bulk cytoplasmic composition, incl. ionic strength and chaotropic ions. In particular, the so-called TRK proteins (K+ transporters) in yeast plasma membranes conduct inward currents of chaotropic anions (ion efflux) in direct proportion to the intracellular anion concentration, in rough proportion to the cube root of extracellular proton concentration, and with exponential dependence on membrane voltage progressing negative from ca. −100 mV. These properties are described, economically and quantitatively, by a steady-state kinetic model comprising two activation barriers in series within the membrane dielectric: one near the cytoplasmic face, and one near the extracellular face. This kinetic model, in turn, fits well with the central-pore structure in a theoretical TRK tetramer previously drawn on the basis of sequence homology with bacterial K+ channels and sequence conservation among fungal TRK proteins (Biophys. J. 77:789, 1999). Supported by N.I.H. Grant GM-60696.