Receptor selectivity between the G proteins Ga12 and Ga13 is defined by a single leucine-to-isoleucine variation

Abstract

Despite recent advances in structural definition of GPCR–G protein complexes, the basis of receptor selectivity between G proteins remains unclear. The Ga12 and Ga13 subtypes together form the least studied group of heterotrimeric G proteins. G protein–coupled receptor 35 (GPR35) has been suggested to couple efficiently to Ga13 but weakly to Ga12. Using combinations of cells genome-edited to not express G proteins and bioluminescence resonance energy transfer–based sensors, we confirmed marked selectivity of GPR35 for Ga13. Incorporating Ga12/ Ga13 chimeras and individual residue swap mutations into these sensors defined that selectivity between Ga13 and Ga12 was imbued largely by a single leucine-to-isoleucine variation at position G.H5.23. Indeed, leucine could not be substituted by other amino acids in Ga13 without almost complete loss of GPR35 coupling. The critical importance of leucine at G.H5.23 for GPR35–G protein interaction was further demonstrated by introduction of this leucine into Gaq, resulting in the gain of coupling to GPR35. These studies demonstrate that Ga13 is markedly the most effective G protein for interaction with GPR35 and that selection between Ga13 and Ga12 is dictated largely by a single conservative amino acid variation.