Mechanisms of Receptor-coupled Signal Transduction

Abstract

The mechanisms by which specific cell surface receptor proteins transmit signals to enzymes, transport proteins or other biological effectors has been the subject of extensive investigation. Among the most studied of these receptor-coupled signal transduction systems have been those involved in the adrenergic regulation of adenylate cyclase activity. Most of the primary components of the catecholamine-mediated stimulatory and inhibitory pathways of adenylate cyclase have now been purified. Both stimulation and inhibition of the enzyme activity is initiated by the binding of hormones to specific receptors; the β-adrenergic receptor being involved in stimulation while the α₂-adrenergic receptor is responsible for triggering inhibition of the enzyme activity. Each of these receptors is known to be comprised of a single type of polypeptide with an apparent Mr ≅ 65,000 daltons (Benovic et al., 1984; Regan et al., 1986). Following their interaction with hormones, these receptors go on to interact with, and promote the activation of, distinct GTP binding proteins which are typically designated as N_s or G_s (the stimulatory GTP binding protein) and N_i or G_i (the innibitory GTP binding protein). Both N_s and N_i are heterotrimeric in structure.