A C-terminal mutant of the G protein beta subunit deficient in the activation of phospholipase C-beta.

Abstract The molecular mechanism by which the G protein βγ complex modulates multiple mammalian effector pathways is unknown. Homolog-scanning mutagenesis of the G protein β subunit was employed to identify residues critical for the activation of phospholipase C-β2 (PLC-β2). A series of chimeras was made by introducing small segments of the Dictyostelium β subunit into a background of mammalian β1 and tested in COS cell cotransfection assays for their ability to activate PLC-β2 and assemble with mammalian γ2. A chimera that contained four Dictyostelium β substitutions within the C-terminal 14 residues was unable to activate PLC-β2 when cotransfected with γ, despite its demonstrable expression in a γ-dependent manner. Cotransfection of the mutant blocked m2 muscarinic receptor activation of PLC by a pertussis toxin-sensitive pathway. This C-terminal mutant retained the ability, however, to stimulate the mitogen-activated protein kinase pathway. These results imply that activation of different βγ-responsive effectors is mediated by distinct domains.