Abstract
The regulator of G-protein signaling type 8 (RGS8) has a high affinity for Gαi and only a low affinity for Gαq. We previously reported that RGS8 decreased the amplitude of Gq-mediated response in a receptor type-specific manner and that RGS8S, a splice variant of N-terminus region, induced less inhibition. Although molecular mechanisms underlying receptor type-specific attenuation by RGS8 still remains unclear, recent evidences have raised a possibility that RGS may interact with certain GPCRs. Here we show by co-immunoprecipitation experiments that RGS8 directly binds to the third intracellular (i3) loop of M1- and M3-muscarinic AChR but not of M2, and that binding of RGS8S is weaker. We observed that a deletion of N-terminal 9 aa of RGS8 or substitutions of Arg-8 and Arg-9 of RGS8 for Ala reduced binding with M1i3, suggesting the importance of N-terminal region. To examine whether or not the interaction between RGS8 and M1 may occur in living cells, we performed BRET analysis. The results showed that RGS8 actually interacts with M1 and that the interaction of RGS8S is less clear. We next analyzed electrophysiologically the inhibitory effects of RGS8 w.t. and R8A/R9A mutant on Gq-mediated responses using Xenopus oocytes, and observed that the inhibitory effect of RGS8 was decreased by the mutations. These biochemical and electrophysiological results show that RGS8 inhibits M1-muscarinic AChR-mediated responses by a mechanism which involves direct interaction between N-terminus of RGS8 and i3 loop of M1. [J Physiol Sci. 2006;56 Suppl:S159]
