G beta 5 prevents the RGS7-G alpha o interaction through binding to a distinct G gamma-like domain found in RGS7 and other RGS proteins

The G protein beta subunit G beta 5 deviates significantly from the other f our 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 i solated a native G beta 5 protein complex from the retinal cytosolic fracti on and identified the protein tightly associated with G beta 5 as the regul ator of G protein signaling (RGS) protein, RGS7. Here we show that complexe s 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 chr omatographies and can be immunoprecipitated with either anti-G beta 5 or an ti-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 i nteraction with G alpha is retained. Substitution of the G gamma-like domai n 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 bin ding of RGS7 to the G alpha subunit G alpha o indicating that G beta 5 is a specific RGS inhibitor.