Activation of the alpha subunit of Gs in intact cells alters its abundance, rate of degradation, and membrane avidity

Abstract

Binding of GTP induces alpha subunits of heterotrimeric G proteins to take on an active conformation, capable of regulating effector molecules. We expressed epitope-tagged versions of the alpha subunit (alpha s) of Gs in genetically alpha s-deficient S49 cyc- cells. Addition of a hemagglutinin (HA) epitope did not alter the ability of wild type alpha s to mediate hormonal stimulation of adenylyl cyclase or to attach to cell membranes. The HA epitope did, however, allow a mAb to immunoprecipitate the recombinant protein (HA-alpha s) quantitatively from cell extracts. We activated the epitope-tagged alpha s in intact cells by: (a) exposure of cells to cholera toxin, which activates alpha s by covalent modification; (b) mutational replacement of arginine-201 in HA-alpha s by a cysteine residue, to create HA-alpha s-R201C; like the cholera toxin-catalyzed modification, this mutation activates alpha s by slowing its intrinsic GTPase activity; and (c) treatment of cells with the beta-adrenoceptor agonist, isoproterenol, which promotes binding of GTP to alpha s, thereby activating adenylyl cyclase. Both cholera toxin and the R201C mutation accelerated the rate of degradation of alpha s (0.03 h-1) by three- to fourfold and induced a partial shift of the protein from a membrane bound to a soluble compartment. At steady state, 80% of HA-alpha s- R201C was found in the soluble fraction, as compared to 10% of wild type HA-alpha s. Isoproterenol rapidly (in < 2 min) caused 20% of HA-alpha s to shift from the membrane-bound to the soluble compartment. Cholera toxin induced a 3.5-fold increase in the rate of degradation of a second mutant, HA-alpha s-G226A, but did not cause it to move into the soluble fraction; this observation shows that loss of membrane attachment is not responsible for the accelerated degradation of alpha s in response to activation. Taken together, these findings show that activation of alpha s induces a conformational change that loosens its attachment to membranes and increases its degradation rate.