Structural basis of effector regulation and signal termination in heterotrimeric Galpha proteins.

This chapter addresses, from a molecular structural perspective gained from examination of x-ray crystallographic and biochemical data, the mechanisms by which GTP-bound Galpha subunits of heterotrimeric G proteins recognize and regulate effectors. The mechanism of GTP hydrolysis by Galpha and rate acceleration by GAPs are also considered. The effector recognition site in all Galpha homologues is formed almost entirely of the residues extending from the C-terminal half of alpha2 (Switch II) together with the alpha3 helix and its junction with the beta5 strand. Effector binding does not induce substantial changes in the structure of Galpha*GTP. Effectors are structurally diverse. Different effectors may recognize distinct subsets of effector-binding residues of the same Galpha protein. Specificity may also be conferred by differences in the main chain conformation of effector-binding regions of Galpha subunits. Several Galpha regulatory mechanisms are operative. In the regulation of GMP phospodiesterase, Galphat sequesters an inhibitory subunit. Galphas is an allosteric activator and inhibitor of adenylyl cyclase, and Galphai is an allosteric inhibitor. Galphaq does not appear to regulate GRK, but is rather sequestered by it. GTP hydrolysis terminates the signaling state of Galpha. The binding energy of GTP that is used to stabilize the Galpha:effector complex is dissipated in this reaction. Chemical steps of GTP hydrolysis, specifically, formation of a dissociative transition state, is rate limiting in Ras, a model G protein GTPase, even in the presence of a GAP; however, the energy of enzyme reorganization to produce a catalytically active conformation appears to be substantial. It is possible that the collapse of the switch regions, associated with Galpha deactivation, also encounters a kinetic barrier, and is coupled to product (Pi) release or an event preceding formation of the GDP*Pi complex. Evidence for a catalytic intermediate, possibly metaphosphate, is discussed. Galpha GAPs, whether exogenous proteins or effector-linked domains, bind to a discrete locus of Galpha that is composed of Switch I and the N-terminus of Switch II. This site is immediately adjacent to, but does not substantially overlap, the Galpha effector binding site. Interactions of effectors and exogenous GAPs with Galpha proteins can be synergistic or antagonistic, mediated by allosteric interactions among the three molecules. Unlike GAPs for small GTPases, Galpha GAPs supply no catalytic residues, but rather appear to reduce the activation energy for catalytic activation of the Galpha catalytic site.