Heterotrimeric G proteins mediate cell growth and differentiation by coupling cell surface receptors to intracellular effector enzymes. The G-protein alpha subunit, Galpha(16), and its murine homologue Galpha(15), are expressed specifically in hematopoietic cells and their expression is highly regulated during differentiation of normal and leukemic cells. In this study, we examined the phosphorylation of Galpha(15)/Galpha(16) and its role in receptor and effector coupling. We observed a PMA-stimulated intact cell phosphorylation of Galpha(15) in COS7 cells transfected with Galpha(15) and protein kinase Calpha (PKCalpha), and phosphorylation of endogenous Galpha(16) in HL60 cells. We also showed that peptides derived from the two G-proteins were phosphorylated in vitro using purified brain PKC. Furthermore, we identified the putative phosphorylation site and showed that mutation or deletion of this PKC phosphorylation site inhibited phospholipase C (PLC) activation. The behavior of double mutants with the constitutively active G-protein mutation (QL-mutant) and mutation in the putative phosphorylation site suggests that the phosphorylation site of Galpha(15/16) is essential for receptor-coupled activation of PLC, but not for direct interaction of the G-protein with PLC-beta.
Copyright 2003 Wiley-Liss, Inc.