Nucleotide-decoupled G proteins reveal the role of G protein conformation in receptor-G protein selectivity

Abstract

G protein-coupled receptors (GPCRs) selectively activate at least one of the four families of heterotrimeric G proteins to transduce environmental cues, but the mechanistic basis of coupling selectivity remains unclear. Structural studies have emphasized structural complementarity of GPCR complexes with nucleotide-free G proteins, but it has also been suggested that selectivity may be determined by intermediate activation processes that occur prior to nucleotide release. To test these ideas we have studied coupling to nucleotide- decoupled G protein variants, which can adopt conformations similar to receptor-bound G proteins without the need for nucleotide release. We find that selectivity is significantly degraded when nucleotide release is not required for GPCR-G protein complex formation, to the extent that most GPCRs interact with most nucleotide-decoupled G proteins. These findings demonstrate the absence of absolute structural incompatibility between most GPCRs and G proteins, and are consistent with the hypothesis that high-energy intermediate state complexes are involved in coupling selectivity.