Part A. Identification of G-Proteins The past decade has seen the emergence of a rapidly expanding superfamily of regulatory proteins, the G-proteins, that transduce as diverse a range of biological functions as protein synthesis, transmembrane signaling, intracellular trafficking, and cell proliferation (reviewed in 1 ). The application of biochemical and molecular biological techniques has substantially increased our understanding of the structure and function of G-proteins and has revealed a highly conserved primary structure and molecular mechanism throughout evolution. The central mechanistic concept is that G-proteins can exist in two interconvertible conformational states, one inactive (GDP-bound) and one active (GTP-bound). This basic cycle of GTP binding and hydrolysis (by an intrinsic GTPase) can confer both directionality and amplification to G-protein-mediated events. Although novel individual species have recently been identified (e.g., 2 ), there are presently three major classes of G-proteins
