Polypeptide and Amine Hormone Regulation of Adenylate Cyclase

Introduction The biological effects of many polypeptide and amine hormones are trans­ mitted, upon interaction with specific receptors at the cell surface, through activation of the membrane-bound enzyme adenylate cyclase and genera­ tion thereby of cyclic 3',5'-AMP (cAMP), the intracellular regulator of enzyme and transport systems. The adenylate cyclase complex is composed of at least three distinct protein components-the receptor (R) itself; the catalytic (C) unit (cyclizes ATP to cAMP); and the guanine nucleotide regulatory unit (G unit). Functions of the G unit include interaction with the receptor, binding to guanine nucleotide, interaction with the catalytic unit, and possibly a GTPase activity. Work in several laboratories has elucidated further the function of the G unit, clarified its role in hormone and fluoride activation of adenylate cyclase, and isolated it in pure form. I review here characteristics of hormone receptors coupled through this mechanism, particularly systems linked through the guanine nucleotide regulatory protein.