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 Gαq subunit confers upon the mutant Gαq the ability to be activated by Gαi- and Gαs -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 Gαq proteins nor their ability to stimulate phospholipase Cβ. It is noteworthy that both mutant and wild-type Gαq proteins are indistinguishable in their ability to reconstitute a functional Gq-PLCβ-calcium signaling pathway when cotransfected with the Gαq-coupled neurokinin 1 or muscarinic M3 receptor into mouse embryonic fibroblasts derived from Gαq/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.
- Received January 14, 2004.
- Accepted May 11, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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