Complexes of the G protein subunit G beta(5) 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), nega tively regulates G protein pathways through a direct interaction with G alp ha subunits and stimulation of GTP hydrolysis. An RGS subfamily including R GS6, -7, -9, and -11, which contain a characteristic G gamma-like domain, a lso has the unique ability to interact with the G protein beta subunit G be ta(5). Here, we examined the behavior of G beta(5), RGS7, RGS9, and G alpha in tissue extracts using immunoprecipitation and conventional chromatograp hy. 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-exp ression in COS-7 cells of GP, dramatically increased the protein level of R GS7 and vice versa, indicating that cells maintain GP,:RGS stoichiometry in a manner similar to GP gamma complexes. This mechanism is non-transcriptio nal and is based on increased protein stability upon dimerization. Thus, an alysis of native G beta(5)-RGS and their coupled expression argue that in v ivo, G beta(5) and G gamma-like domain-containing RGSs only exist as hetero dimers. Native G beta(5)-RGS7 did not co-precipitate or co-purify with G al pha(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 C a2+ response to muscarinic M3 receptor activation. Thus, G beta(5)-RGS dime rs 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.