The G protein β5subunit differs from other β subunits in having divergent sequence and subcellular localization patterns. Although β5γ2modulates effectors, β5associates with R7 family regulators of G protein signaling (RGS) proteins when purified from tissues. To investigate β5complex formation in vivo, we used multicolor bimolecular fluorescence complementation in human embryonic kidney 293 cells to compare the abilities of 7 γ subunits and RGS7 to compete for interaction with β5. Among the γ subunits, β5interacted preferentially with γ2, followed by γ7, and efficacy of phospholipase C-β2 activation correlated with amount of β5γ complex formation. β5also slightly preferred γ2over RGS7. In the presence of coexpressed R7 family binding protein (R7BP), β5interacted similarly with γ2and RGS7. Moreover, γ2interacted preferentially with β1rather than β5. These results suggest that multiple coexpressed proteins influence β5complex formation. Fluorescent β5γ2labeled discrete intracellular structures including the endoplasmic reticulum and Golgi apparatus, whereas β5RGS7 stained the cytoplasm diffusely. Coexpression of αotargeted both β5complexes to the plasma membrane, and αqalso targeted β5γ2to the plasma membrane. The constitutively activated αomutant, αoR179C, produced greater targeting of β5RGS7 and less of β5γ2than did αo. These results suggest that αomay cycle between interactions with β5γ2or other βγ complexes when inactive, and β5RGS7 when active. Moreover, the ability of β5γ2to be targeted to the plasma membrane by α subunits suggests that functional β5γ2complexes can form in intact cells and mediate signaling by G protein-coupled receptors.
