THE N-TERMINAL EXTENSION OF G-ALPHA(Q) IS CRITICAL FOR CONSTRAINING THE SELECTIVITY OF RECEPTOR COUPLING

Characteristically, an individual member of the superfamily of G prote in coupled receptors can interact only with a limited number of the ma ny structurally closely related G protein heterotrimers that are expre ssed within a cell. Interestingly, the N termini of two G protein a su bunits, G alpha(q) and G alpha(11), differ from those of other or subu nits in that they display a unique, highly conserved six-amino acid ex tension, To test the hypothesis that this sequence element is critical for proper receptor recognition, we prepared a G alpha(q) deletion mu tant (-6q) lacking these first six amino acids. The -6q construct (or wild type G alpha(q) as a control) was coexpressed (in COS-7 cells) wi th several different G(i/o)- or G(s)-coupled receptors, and ligand-ind uced increases in inositol phosphate production were determined as a m easure of G protein activation. Whereas these receptors did not effici ently interact with wild type G alpha(q), most of them gained the abil ity to productively couple to -6q. Additional experiments indicated th at the observed functional promiscuity of -6q is not due to overexpres sion (as compared with wild type G alpha q) or to a lack of palmitoyla tion. We conclude that the N-terminal extension characteristic for G a lpha(q/11) proteins is critical. for constraining the receptor couplin g selectivity of these subunits, indicative of a novel mechanism by wh ich the fidelity of receptor-G protein interactions can be regulated.