Structure-activity relationships for transmembrane signaling: the receptor's turn

The cloning, sequencing, and functional expression in host cells of a variety of receptors has led to a focus on the structural determinants of pharmacologic receptors involved in the complex processes of ligand binding and cell activation. The three basic mechanisms of receptor-mediated transmembrane signaling (ligand-regulated ion flux; ligand-regulated receptor-enzymes; ligand-regulated receptor-G protein activation) can now be placed in the structural context of at least three receptor superfamilies: 1) ligand-regulated oligomeric ion channels, 2) ligand-regulated tyrosine kinases, and 3) G protein-linked rhodopsin-related receptors. For each of these receptor superfamilies, structure-activity studies that use 1) site-directed mutagenesis, 2) cassette switching to form receptor chimeras, and 3) sequence-specific antireceptor antibodies are beginning to delineate the domains responsible for specific receptor functions. Analyses of such receptor domains related to: 1) ligand binding, 2) membrane insertion, 3) catalytic activity (in the case of receptor-enzymes), 4) internalization and interaction with other membrane constituents, 5) substrate or G protein binding, and 6) regulatory sites of receptor phosphorylation are discussed, using as principal examples the nicotinic receptor for acetylcholine, the epidermal growth factor-urogastrone receptor, and the beta-adrenergic receptor. These studies illustrate that in terms of structure-activity studies, which have traditionally emphasized the ligand, it is now the receptor's turn for intense attention.