Regulators of G-protein signalling: A novel protein family involved in timely deactivation and desensitization of signalling via heterotrimeric G proteins

In a variety of signalling pathways heterotrimeric guanine-nucleotide- binding proteins (G proteins) trigger physiological responses elicited by hormones, neurotransmitters and sensory stimuli. Receptor-induced GDP/GTP exchange activates G proteins by dissociating G-protein α-subunits from the βγ-dimers. Both α-subunits and βγ-dimers are involved in effector regulation. The deactivation of these active forms is controlled by the hydrolysis of GTP bound to α-subunits, allowing the inactive heterotrimer to reform. Termination of G-protein-mediated signalling in vivo is 10- to 100- fold faster than the in vitro rate of GTP hydrolysis by α-subunits, suggesting that in analogy to the GTPases of the Ras-superfamily, GTPase- activating proteins (GAPs) are required to achieve timely deactivation. Recently, members of a novel protein super- family, known as 'regulators of G-protein signalling' (RGS), were identified as potent GAPs for at least one subset of heterotrimeric G-protein α-subunits. In this re- view, we intend to discuss the proposed mechanism by which RGS proteins exert GAP activity for G-protein α- subunits as well as their specificities. The role of RGS proteins in desensitization and temporal resolution in certain signalling pathways will also be addressed.