Heterotrimeric G Proteins

Abstract

Aconsiderable number of proteins incorporate GTP in a regulatory cycle of their activity. These proteins are able to bind GTP, possess intrinsic GTPase activity and undergo GTP-dependent conformational changes critical for their functioning. This superfamily of GTP-binding proteins consists of several families including translational factors, heterotrimeric GTP-binding proteins (G proteins), protooncogenic Ras and Ras-related proteins, tubulins and some others.¹⁹⁷ G proteins stand apart from other members of the superfamily of GTP-binding proteins due to their heterotrimeric structure and their function in coupling of specific cell surface receptors to effector molecules. Initially they were discovered as critical intermediates in visual transduction in photoreceptor cells of the retina and in hormonal stimulation of cAMP production in the liver. However in subsequent studies G proteins became recognized as signal transducers in a vast number of signaling pathways.^35,354