G protein βγ dimer formation:: Gβ and Gγ differentially determine efficiency of in vitro dimer formation

The G beta and G gamma subunit of the heterotrimeric G proteins form a functional dimer that is stable once assembled in vivo or in vitro. The requirements, mechanism, and specificity of dimer formation are still incompletely understood, but represent important biochemical processes involved in the specificity of cellular signaling through G proteins. Here, seven G beta and 12 FLAG-epitope-tagged G gamma subunits were separately synthesized in vitro using a rabbit reticulocyte lysate expression system. The translation products were combined and dimers isolated by immunoprecipitation. G beta 1 and G beta 4 formed dimers with all G gamma subunit isoforms, generally with G beta/G gamma stoichiometries between 0.2:1 and 0.5:1. G beta 5, G beta 5L, and G beta 3s did not form significant amounts of dimer with any of the gamma subunit isoforms. G beta 2 and G beta 3 formed dimers with selected G gamma isoforms to levels intermediate between that of G beta 1/G beta 4 and G beta 3s/G beta 5/G beta 5L. We also expressed selected G beta y in HEK293 cells and measured PLC beta 2 activity. G beta gamma dimer-dependent increases in IP3 production were seen with most G beta 1, G beta 2, and G beta 5 combinations, indicating functional dimer expression in intact cells. These results define the complete set of G protein beta gamma dimers that are formed using a single biochemical assay method and suggest that there are G beta isoform-specific factors in rabbit reticulocyte lysates that determine the efficacy of G beta gamma dimer formation.