Abstract

Several domains of G protein α subunits are implicated in the control of receptor-G protein coupling specificity. Among these are the extreme N-and C-termini, the α4/β6-loops, and the loop linking the N-terminal α-helix to the β1-strand of the ras-like domain. In this study, we illustrate that single-point mutations of a highly conserved glycine residue within the linker I region of the Gα_q subunit confers upon the mutant Gα_q the ability to be activated by Gα_i- and Gα_s -coupled receptors, as evidenced by guanosine 5′-O-(3-[³⁵S]thio)triphosphate binding and inositol phosphate turnover assays. The mutations did not affect expression of Gα_q proteins nor their ability to stimulate phospholipase Cβ. It is noteworthy that both mutant and wild-type Gα_q proteins are indistinguishable in their ability to reconstitute a functional Gq-PLCβ-calcium signaling pathway when cotransfected with the Gα_q-coupled neurokinin 1 or muscarinic M3 receptor into mouse embryonic fibroblasts derived from Gα_q/11 knockout mice. On a three-dimensional model of the receptor-G protein complex, the highly conserved linker I region connecting the helical and the GTPase domain of the Gα protein is inaccessible to the intracellular surface of the receptors. Our data indicate that receptor-G protein coupling specificity is not exclusively governed by direct receptor-G protein interaction and that it even bypasses the requirement of the extreme C terminus of Gα, a well accepted receptor recognition domain, suggesting a novel allosteric mechanism for G protein-coupled receptor-G protein selectivity.