Glucose activates the carboxyl methylation of gamma subunits of trimeric GTP-binding proteins in pancreatic beta cells - Modulation in vivo by calcium, GTP, and pertussis toxin

The gamma subunits of trimeric G-proteins (gamma 1, gamma 1, gamma(5), and gamma(7) isoforms) were found to be methylated at their carboxyl termini in normal rat islets, human islets and pure beta [HIT-T15] cells. Of these, GTP gamma S significantly stimulated the carboxyl methylation selectively of gamma(2) and gamma(5) isoforms. Exposure of intact HIT cells to either of two receptor-independent agonists-a stimulatory concentration of glucose or a depolarizing concentration of K+-resulted in a rapid (within 30 s) and sustained (at least up to 60 min) stimulation of gamma subunit carboxyl methylation. Mastoparan, which directly activates G-proteins (and insulin secretion from beta cells), also stimulated the carboxyl methylation of gamma subunits in intact HIT cells. Stimulatory effects of glucose or K+ were not demonstrable after removal of extracellular Ca2+ or depletion of intracellular GTP, implying regulatory roles for calcium fluxes and GTP; however, the methyl transferase itself was not directly activated by either, The stimulatory effects of mastoparan were resistant to removal of extracellular Ca2+, implying a mechanism of action that is different from glucose or K+ but also suggesting that dissociation of the alpha beta gamma trimer is conducive to gamma subunit carboxyl methylation. Indeed, pertussis toxin also markedly attenuated the stimulatory effects of glucose, K+ or mastoparan without altering the rise in intracellular calcium induced by glucose or K+, Glucose-induced carboxyl methylation of gamma(2) and gamma(5) isoforms was vitiated by coprovision of any of three structurally different cyclooxygenase inhibitors. Conversely, exogenous PGE(2), which activates G(i) and G(o) in HIT cells and which thereby would dissociate alpha from beta(gamma), stimulated the carboxyl methylation of gamma(2) and gamma(5) isoforms and reversed the inhibition of glucose-stimulated carboxyl methylation of gamma subunits elicited by cyclooxygenase inhibitors. These data indicate that gamma subunits of trimeric G-proteins undergo a glucose-and calcium-regulated methylation-demethylation cycle in insulin-secreting cells, findings that may imply an important role in beta cell function. Furthermore, this is the first example of the regulation of the posttranslational modification of G-protein gamma subunits vi a nonreceptor-mediated activation mechanisms, which are apparently dependent on calcium influx and the consequent activation of phospholipases releasing arachidonic acid.