Water repartitioning at the water lipid protein interface controls receptor activation in G-Protein coupled receptors

G-Protein coupled receptors (GPCRs) play a fundamental role in many physiological processes. High ligand specifity of rhodopsin-like GPCRs in contrast to the highly conserved and class defining D(E)RY motif undergoing a protonation upon receptor activation suggests a general local activation mechanism acting as an autonomous functional module.
It has been shown that the D(E)RY motif positioned at the phase boundary acts as a pH-dependent switch which is governed by side chain partitioning between the aqueous and lipidic phase [1].
Here we have addressed the putative reverse effect, i.e. restructuring of the water lipid interface upon side chain protonation to elucidate the functional implication of water lipid protein interactions in the control of protein conformation. We have studied a TM3 derived transmembrane segment where a fluorescence reporter resides below the D(E)RY motif inside the helix. By fluorescence spectroscopy, FRET studies and FTIR-Fluorescence cross correlation experiments we show the pH dependent hydration site N-terminally of the D(E)RY motif. Thus the ionized D(E) side chain attracts water that dissolves the TM-sequence even beyond the two preceding hydrophobic residues.
These results argue for a key role of the rearrangement of the water lipid protein microstructure upon GPCR activation as it effects not only the phase boundary but even more hydrophobic environment inside the lipidic phase.
[1] S. Madathil, K.Fahmy, J Biol Chem 284, 28801-28809 (2009)