SELECTIVITY AND PRODUCTIVITY OF GPCR-G PROTEIN INTERACTIONS

Abstract

Hundreds of human G protein-coupled receptors (GPCRs) converge on activation of four families of heterotrimeric G proteins. Individual receptors select a subset of G proteins in order to produce appropriate cellular responses. While the precise mechanisms of coupling selectivity are uncertain, the G alpha subunit carboxy (C) terminus is believed to be the primary region recognized by GPCRs. We directly assessed coupling between 14 representative GPCRs and 16 G alpha subunits, including one wild-type G alpha subunit from each of the four families and 12 chimeras with exchanged C termini. We found that Gi-coupled receptors were relatively selective for Gi1 heterotrimers, while Gs-, Gq-, and G12- coupled receptors were more promiscuous and always coupled in some measure to Gi1 heterotrimers. Our tests with G alpha subunit chimeras show that the G alpha subunit core and C terminus both play important roles in selectivity. This suggests that the key G protein determinants of selectivity vary widely, even for different receptors that couple to the same G protein. While promiscuous GPCR-G protein coupling is often observed. These interactions behave as expected with receptor-G protein coupling and activation being almost synonymous. Agonist bound GPCRs activate the G protein heterotrimers they interact with, while ignoring G protein subtypes that they cannot activate. However, we have shown that GPCRs can form unproductive complexes with G12 heterotrimers. Vasopressin 2 receptor (V2R) forms agonist-dependent complexes with G12 heterotrimers. Unlike V2R complexes with cognate Gs heterotrimers, V2R-G12 complexes do not dissociate when GDP or GTP is present. Stimulating V2R with arginine vasopressin (AVP) does not activate signaling responses downstream of G12 activation. Evaluation of several G12-coupled receptors demonstrated that agonist induced GPCR-G12 complexes have a wide range resistance to GDP. Like V2R receptors, formyl peptide 2 receptors (FPR2) and smoothened receptors (SMOR) formed complexes with G12 heterotrimers that were relatively resistant to GDP. Our results indicate that several GPCRs can form agonist-dependent unproductive complexes with G12 heterotrimers that are relatively resistant to GDP. Suggesting that for some GPCRs agonist-dependent association with G12 heterotrimers is weakly coupled to nucleotide exchange