The N-terminal extension of Galphaq is critical for constraining the selectivity of receptor coupling

Characteristically, an individual member of the superfamily of G protein-coupled receptors can interact only with a limited number of the many structurally closely related G protein heterotrimers that are expressed within a cell. Interestingly, the N termini of two G protein alpha subunits, Galphaq and Galpha11, differ from those of other alpha subunits in that they display a unique, highly conserved six-amino acid extension. To test the hypothesis that this sequence element is critical for proper receptor recognition, we prepared a Galphaq deletion mutant (-6q) lacking these first six amino acids. The -6q construct (or wild type Galphaq as a control) was coexpressed (in COS-7 cells) with several different Gi/o- or Gs-coupled receptors, and ligand-induced increases in inositol phosphate production were determined as a measure of G protein activation. Whereas these receptors did not efficiently interact with wild type Galphaq, most of them gained the ability to productively couple to -6q. Additional experiments indicated that the observed functional promiscuity of -6q is not due to overexpression (as compared with wild type Galphaq) or to a lack of palmitoylation. We conclude that the N-terminal extension characteristic for Galphaq/11 proteins is critical for constraining the receptor coupling selectivity of these subunits, indicative of a novel mechanism by which the fidelity of receptor-G protein interactions can be regulated.