Ion Channels in Epithelial Tissue: Single-Channel Properties

Abstract

A major function of a cell membrane is to form a barrier between the internal environment of the cell and the external world. However, a complete barrier to the outside world is unacceptable since to survive cells require some ability to communicate with their external environment. Communication consists of a variety of different processes mediated by membrane proteins which serve as signal transduction pathways or as mediators to maintain the intracellular milieu. One subset of these proteins allows movement of ions across the cell membrane. An even more restrictive class of ion transport proteins are those which involve transmembrane movement of ions through “channels.” Ion channels form an interesting class of membrane proteins which have specifically evolved to provide an essentially aqueous pathway through the otherwise highly hydrophobic barrier of the cell membrane. As such, they are capable of carrying relatively large numbers of ions per unit time. In general, to regulate the number of ions which cross the cell membrane, a channel protein undergoes conformational changes which produce states of the molecule which either contain an aqueous pathway across the protein or which have no such pathway for ions. Typically, there may be several “open” states which allow ion movement as well as several “closed” states in which there is no ionic current across the protein. Since the primary function of channel proteins is the transport of ions across the membrane, the conformational changes which lead to alterations in current flow have been the subject of significant investigation.