Functional regulation of G alpha(16) by protein kinase C

Recent evidence demonstrates that the a subunits of some heterotrimeric GTP -binding proteins (G proteins) are subject to modification by protein kinas e C (PKC). For the family of G proteins that activate the phospholipase C/i nositol trisphosphate/calcium/PKC pathway, such modification could result i n G protein autoregulation. To examine the potential regulation of members of the G alpha(q) family by PKC phosphorylation, we expressed the thyrotrop in-releasing hormone (TRH) receptor in combination with G alpha(q), G alpha (11), G alpha(14), G alpha(15), or G alpha(16) in Xenopus oocytes and exami ned the regulation of signaling by PKC activators and inhibitors, For G alp ha(16) and G alpha(15), the two family members of hematopoietic lineage, PK C activators reduce both the magnitude and the time course of TRH-mediated responses; PKC inhibitors have the opposite effect. The PKC-mediated effect s are evident in measurements of GTPase activity, suggesting that the regul ation is occurring early in the signaling pathway. In vivo phosphorylation experiments demonstrate that G alpha(16) is a substrate for PHC modificatio n. By comparison, G alpha(q) is not phosphorylated by PHC in vivo and oocyt es expressing G alpha(q) are not functionally modulated by PKC. Repeated TR H stimulation of oocytes expressing G alpha(16) mimics the effects of PKC a ctivators, and this functional regulation is correlated with an increase in G alpha(16) phosphorylation. A mutant G alpha(16) with four consensus PKC phosphorylation sites removed is not phosphorylated in vivo, and TRH respon ses mediated through the mutant are not regulated by PKC. These results dem onstrate that signaling involving hematopoietic G proteins is subject to PK C-mediated autoregulation, at least in part, by phosphorylation of the G pr otein a subunit.