Role of Structural Dynamics at the Receptor G Protein Interface for Signal Transduction

Abstract

GPCRs catalyze GDP/GTP exchange in the α-subunit of heterotrimeric G proteins (Gαßγ) through displacement of the Gα C-terminal α5 helix, which directly connects the interface of the active receptor (R ) to the nucleotide binding pocket of G. Hydrogen-deuterium exchange mass spectrometry and kinetic analysis of R catalysed G protein activation have suggested that displacement of α5 starts from an intermediate GDP bound complex (R •GGDP). To elucidate the structural basis of receptor-catalysed displacement of α5, we modelled the structure of R •GGDP. A flexible docking protocol yielded an intermediate R •GGDP complex, with a similar overall arrangement as in the X-ray structure of the nucleotide free complex (R •Gempty), however with the α5 C-terminus (GαCT) forming different polar contacts with R . Starting molecular dynamics simulations of GαCT bound to R in the intermediate position, we observe a screw-like motion, which restores the specific interactions of α5 with R in R •Gempty. The observed rotation of α5 by 60° is in line with experimental data. Reformation of hydrogen bonds, water expulsion and formation of hydrophobic interactions are driving forces of the α5 displacement. We conclude that the identified interactions between R and G protein define a structural framework in which the α5 displacement promotes direct transmission of the signal from R to the GDP binding pocket.