Gβ5 prevents the RGS7-Gαo interaction through binding to a distinct Gγ-like domain found in RGS7 and other RGS proteins

The G protein beta subunit G beta 5 deviates significantly from the other four members of G beta-subunit family in amino acid sequence and subcellular localization. To detect the protein targets of G beta 5 in vivo, we have isolated a native G beta 5 protein complex from the retinal cytosolic fraction and identified the protein tightly associated with G beta 5 as the regulator of G protein signaling (RGS) protein, RGS7. Here we show that complexes of G beta 5 with RGS proteins can be formed in vitro from the recombinant proteins. The reconstituted G beta 5-RGS dimers are similar to the native retinal complex in their behavior on gel-filtration and cation-exchange chromatographies and can be immunoprecipitated with either anti-G beta 5 or anti-RGS7 antibodies. The specific G beta 5-RGS7 interaction is determined by a distinct domain in RGS that has a striking homology to G gamma subunits. Deletion of this domain prevents the RGS7-G beta 5 binding, although the interaction with G alpha is retained. Substitution of the G gamma-like domain of RGS7 with a portion of G gamma 1 changes its binding specificity from G beta 5 to G beta 1. The interaction of G beta 5 with RGS7 blocked the binding of RGS7 to the G alpha subunit G alpha o indicating that G beta 5 is a specific RGS inhibitor.