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

Abstract A novel protein class, termed regulators of G protein signaling (RGS), negatively regulates G protein pathways through a direct interaction with Gα subunits and stimulation of GTP hydrolysis. An RGS subfamily including RGS6, -7, -9, and -11, which contain a characteristic Gγ -like domain, also has the unique ability to interact with the G protein β subunit Gβ5. Here, we examined the behavior of Gβ5, RGS7, RGS9, and Gα in tissue extracts using immunoprecipitation and conventional chromatography. Native Gβ5 and RGS7 from brain, as well as photoreceptor-specific Gβ5L and RGS9, always co-purified as tightly associated dimers, and neither RGS-free Gβ5 nor Gβ5-free RGS could be detected. Co-expression in COS-7 cells of Gβ5 dramatically increased the protein level of RGS7 and vice versa, indicating that cells maintain Gβ5:RGS stoichiometry in a manner similar to Gβγ complexes. This mechanism is non-transcriptional and is based on increased protein stability upon dimerization. Thus, analysis of native Gβ5-RGS and their coupled expression argue that in vivo, Gβ5and Gγ-like domain-containing RGSs only exist as heterodimers. Native Gβ5-RGS7 did not co-precipitate or co-purify with Gαo or Gαq; nor did Gβ5 L-RGS9 with Gαt. However, in transfected cells, RGS7 and Gβ5-RGS7 inhibited Gαq-mediated Ca2+ response to muscarinic M3 receptor activation. Thus, Gβ5-RGS dimers differ from other RGS proteins in that they do not bind to Gα with high affinity, but they can still inhibit G protein signaling.