Gβ3 forms distinct dimers with specific Gγ subunits and preferentially activates the β3 isoform of phospholipase C

Heterotrimeric G proteins regulate multiple effectors of which some are mediated via the Gβγ dimers. There is evidence to suggest that the functions of Gβγ dimers are not shared by all possible permutations of Gβγ complexes. Here, we report our efforts in defining the formation of distinct Gβγ dimers and their functional differences in activating phospholipase Cβ (PLCβ) isoforms. Co-immunoprecipitation assays using Cos-7 cells transiently expressing 48 different combinations of Gβ(1–4) and Gγ(1–5, 7–13) subunits showed that Gβ1 and Gβ4 could form dimers with all known Gγ subunits, whereas several dimers could not be observed for Gβ2 and Gβ3. All Gβ1γ and Gβ2γ dimers significantly stimulated PLCβ isoforms (PLCβ2 ≥ PLCβ3 > PLCβ1), but Gβ3γ and Gβ4γ dimers were poor activators of PLCβ1 and exhibited preference for PLCβ3 and PLCβ2, respectively. All Gβ subunits revealed to date contain the previously identified PLCβ2-interacting residues, but their neighboring residues in the proposed 3-D structures are different. To test if differences in these neighboring residues affect the interactions with PLCβ isoforms, we generated several Gβ3 mutants by replacing one or more of these residues with their Gβ1 counterparts. One of these mutants (M120I, S140A and A141G triple mutant) acquired enhanced PLCβ2-activating functions when co-expressed with different Gγ subunits, while the corresponding stimulation on PLCβ3 was not altered. Taken together, our results show that the exact composition of a Gβγ dimer can determine its selectivity for activating PLCβ isoforms, and certain residues in Gβ3 may account for the preferential stimulation of PLCβ3 by Gβ3γ dimers.