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. The aim of this research is to define the formation of distinct Gβγ dimers, their functional differences in activating downstream effectors such as phospholipase Cβ (PLCβ) isoforms, the molecular determinant of Gβγ formation and the dissociation of different G protein family members upon activation. Co-immunoprecipitation assays using COS7 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 Gβ...[ Read more ]

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. The aim of this research is to define the formation of distinct Gβγ dimers, their functional differences in activating downstream effectors such as phospholipase Cβ (PLCβ) isoforms, the molecular determinant of Gβγ formation and the dissociation of different G protein family members upon activation. Co-immunoprecipitation assays using COS7 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 Gβ_1γ and Gβ_2γ dimers significantly stimulated PLCβ isoforms (PLCβ₂ ≥ PLCβ₃ > PLCβ₁), but Gβ_3γ and Gβ_4γ dimers were poor activators of PLCβ₁ and exhibited preference for PLCβ₃ and PLCβ₂, respectively. Mutation of certain residues in Gβ₃ enhanced PLCβ₂-activating functions when co-expressed with different Gγ subunits, while the corresponding stimulation on PLCβ₃ was not altered. All these 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. Apart from the N-terminal α helix and blades 5 – 7 of Gβ subunits, the region of blades 1 – 4 also play a minor but significant role in Gβγ dimer formation. Furthermore, different from the classical model of G protein activation upon receptor-mediated activation, some activated Gα subunits, including Gα_q, Gα₁₁, Gα₁₄, Gα₁₆, Gα_z, Gα_t1, Gα_t2, Gα_s, Gα_olf, Gα₁₂ and Gα₁₃, may remain associate with Gβγ dimer upon G protein activation, indicating that a new model of G protein activation mechanism is required for replacement of the classical model in which G protein activation is always coupled with subunit dissociation.