Complexes of the G protein subunit Gβ5 with the regulators of G protein signaling RGS7 and RGS9 -: Characterization in native tissues and in transfected cells

A novel protein class, termed regulators of G protein signaling (RGS), negatively regulates G protein pathways through a direct interaction with G alpha subunits and stimulation of GTP hydrolysis. An RGS subfamily including RGS6, -7, -9, and -11, which contain a characteristic G gamma-like domain, also has the unique ability to interact with the G protein beta subunit G beta(5). Here, we examined the behavior of G beta(5), RGS7, RGS9, and G alpha in tissue extracts using immunoprecipitation and conventional chromatography. Native G beta(5) and RGS7 from brain, as well as photoreceptor-specific G beta(5)L and RGS9, always co-purified as tightly associated dimers, and neither RGS-free G beta(5) nor G beta(5)-free RGS could be detected, Co-expression in COS-7 cells of G beta(5), dramatically increased the protein level of RGS7 and vice versa, indicating that cells maintain G beta(5) : RGS stoichiometry in a manner similar to G beta gamma complexes. This mechanism is non-transcriptional and is based on increased protein stability upon dimerization. Thus, analysis of native G beta(5)-RGS and their coupled expression argue that in vivo, G beta(5) and G gamma-like domain-containing RGSs only exist as heterodimers. Native G beta(5)-RGS7 did not co-precipitate or co-purify with G alpha(o) or G alpha(q); nor did G beta(5)-L-RGS9 with G alpha(t). However, in transfected cells, RGS7 and G beta(5)-RGS7 inhibited G alpha(q)-mediated Ca2+ response to muscarinic M3 receptor activation. Thus, G beta(5)-RGS dimers differ from other RGS proteins in that they do not bind to G alpha with high affinity, but they can still inhibit G protein signaling.