The Cell Surface: Receptors, Membrane Recycling and Signal Transduction

Abstract

Whereas the previous chapter focused primarily on mass transport across the membrane, this chapter is largely concerned with the transfer of information across the bilayer. All cells need mechanisms to monitor their environment and to be able to respond to changes which occur. There is a myriad of specialized receptor molecules present in the cytoplasmic membranes of bacterial, plant, and animal cells which interact with extracytoplasmic components and elicit specific cellular responses. Some receptors bind to nutrients or metabolites, others to hormones or neurotransmitters, and still others are involved in cell-cell recognition and adhesion either to other cells or to insoluble matrix components in the environment. Most of the response systems involve a series of steps: (1) binding of a ligand or agonist to a receptor located outside the cell, (2) transmittal of the information concerning receptor occupancy to the inside of the cell, and (3) the cellular response, which can be further subdivided into primary and secondary responses. This is an extremely active area of research which, largely due to the increasing impact of molecular biology, is advancing at a startling rate. It is now becoming clear that numerous seemingly unrelated response systems are, in fact, quite closely related by underlying common features. Several families of sequence-related, homologous receptor proteins have been identified in which individual members each respond to different ligands and elicit unique cellular responses.