Differential activity of the G protein β5γ2 subunit at receptors and effectors

The G protein beta(5) subunit differs substantially in amino acid sequence from the other known beta subunits suggesting that beta gamma dimers containing this protein may play specialized roles in cell signaling. To examine the functional properties of the beta(5) subunit, recombinant beta(5)gamma(2) dimers were purified from baculovirus-infected Sf9 insect cells using a strategy based on two affinity tags (hexahistidine and FLAG) engineered into the N terminus of the gamma(2) subunit (gamma(2HF)). The function of the pure beta(5)gamma(2HF) dimers was examined in three assays: activation of pure phospholipase C-beta in lipid vesicles; activation of recombinant, type II adenylyl cyclase expressed in Sf9 cell membranes; and coupling of a subunits to the endothelin B (ETB) and M-1 muscarinic receptors. In each case, the efficacy of the beta(5)gamma(2HF) dimer was compared with that of the beta(1)gamma(2HF) dimer, which has demonstrated activity in these assays. The beta(5)gamma(2HF) dimer activated phospholipase C-beta with a potency and efficacy similar to that of beta(1)gamma(2) or beta(1)gamma(2HF); however, it was markedly less effective than the beta(1)gamma(2HF) or beta(1)gamma(2) dimer in its ability to activate type II adenylyl cyclase (EC50 of similar to 700 nM versus 25 nM). Both the beta(5)gamma(2HF) and the beta(1)gamma(2HF) dimers supported coupling of M-1 muscarinic receptors to the G(q) alpha subunit. The ET, receptor coupled effectively to both the G(i) and G(q) alpha subunits in the presence of the beta(1)gamma(2HF) dimer. In contrast, the beta(5)gamma(2HF) dimer only supported coupling of the G(q) alpha subunits to the ETB receptor and did not support coupling of the G(i) alpha subunit. These results suggest that the beta(5)gamma(2HF) dimer binds selectively to G(q) alpha subunits and does not activate the same set of effecters as dimers containing the beta(1) subunit. Overall, the data support a specialized role for the beta(5) subunit in cell signaling.