Heterotrimeric G-protein Effectors

Preface Transduction of signals through G-protein pathways is achieved largely by production of intracellular messengers that either directly or through protein kinases regulate cellular biochemical and physiological processes. Several enzymes, including adenylyl cyclase, phospholipase C, and the cGMP phosphodiesterase, are well established as effectors directly regulated by G proteins. These enzymes are covered in this volume. Others such as phospholipase A2 and phospholipase D are often described as G-protein effectors. Though receptors that couple to G proteins undoubtedly regulate the activity of these enzymes, there is no compelling evidence at this time to indicate these enzymes are directly regulated by G-protein subunits. Hence these enzymes are not included. Over the past two years it has become increasingly obvious that there is considerable molecular and functional diversity of the effector enzymes as well. Eight mammalian Gs-sensitive adenylyl cyclases and four Gqstimulated phospholipases C-fl have been cloned, and many of these have been characterized as having distinct capabilities for signal input. The varied functional characteristics and the tissue-specific distribution of the effector isoforms allow the various cell types and tissues to develop customized response systems by altering the mix of the effector isoforms. Several approaches to characterize the molecular and functional identities of the effector isoforms are presented. Several mitogens use G-protein pathways to communicate proliferative signals. At