Regulator of G protein signaling Z1 (RGSZ1) interacts with Galpha i subunits and regulates Galpha i-mediated cell signaling

Regulator of G protein signaling (RGS) proteins constitute a family of over 20 proteins that negatively regulate heterotrimeric G protein-coupled receptor signaling pathways by enhancing endogenous GTPase activities of G protein alpha subunits. RGSZ1, one of the RGS proteins specifically localized to the brain, has been cloned previously and described as a selective GTPase accelerating protein for Galpha(z) subunit. Here, we employed several methods to provide new evidence that RGSZ1 interacts not only with Galpha(z,) but also with Galpha(i), as supported by in vitro binding assays and functional studies. Using glutathione S-transferase fusion protein pull-down assays, glutathione S-transferase-RGSZ1 protein was shown to bind (35)S-labeled Galpha(i1) protein in an AlF(4)(-)dependent manner. The interaction between RGSZ1 and Galpha(i) was confirmed further by co-immunoprecipitation studies and yeast two-hybrid experiments using a quantitative luciferase reporter gene. Extending these observations to functional studies, RGSZ1 accelerated endogenous GTPase activity of Galpha(i1) in single-turnover GTPase assays. Human RGSZ1 functionally regulated GPA1 (a yeast Galpha(i)-like protein)-mediated yeast pheromone response when expressed in a SST2 (yeast RGS Protein) knockout strain. In PC12 cells, transfected RGSZ1 blocked mitogen-activated protein kinase activity induced by UK14304, an alpha(2)-adrenergic receptor agonist. Furthermore, RGSZ1 attenuated D2 Dopamine Receptor agonist-induced serum response element reporter gene activity in Chinese hamster ovary cells. In summary, these data suggest that RGSZ1 serves as a GTPase accelerating protein for Galpha(i) and regulates Galpha(i)-mediated signaling, thus expanding the potential role of RGSZ1 in G protein-mediated cellular activities.