[Structure of adenylate cyclase and the coupling with the receptor-G protein system]

Adenylate cyclase is a key enzyme that couples with both the stimulatory and inhibitory G proteins (Gs and Gi). The cyclase has been purified and shown to be a glycoprotein of molecular weight 115,000-180,000. Cloning of cDNAs for adenylate cyclase showed that the cyclase is a member of a large family consisting of a variety of subtypes of the enzyme. These subtypes show different responses to calmodulin and G protein beta gamma subunits, and their distributions in tissues and organs are also different. This suggests that each subtype is involved in a particular physiological function. The general structure of adenylate cyclase is composed of two cytoplasmic domains and two membrane-spanning domains, each of which contains 6 transmembrane spans (12 spans in a molecule). The amino acid sequence of each cytoplasmic domain, which is thought to contain a nucleotide (ATP) binding site, is well-conserved among the various subtypes. This review also focuses on the regulation of adenylate cyclase activity by G protein subunits, particularly on several models for adenylate cyclase inhibition by Gi. As one of these mechanisms, direct inhibition of adenylate cyclase by the beta gamma subunits recently demonstrated by us will be discussed.