Lipid Protein Interactions Couple Protonation to Conformation in a Conserved Cytosolic Domain of G-Protein-Coupled Receptors

The visual photoreceptor rhodopsin is a prototypical class-I (rhodopsin-like) G protein-coupled receptor (GPCR). Photoisomerization of the covalently bound ligand 11-cis-retinal leads to restructuring of the cytosolic face of rhodopsin. The ensuing protonation of Glu-134 in the class-conserved D(E)RY motif at the C-terminal end of transmembrane helix-3 promotes the formation of the G-protein-activating state. Using transmembrane segments derived from helix-3 of bovine rhodopsin, we show that lipid protein interactions play a key role in this cytosolic "proton switch". Infrared- and fluorescence-spectroscopic pKa determinations reveal that the D(E)RY motif is an autonomous functional module coupling side chain neutralization to conformation and helix positioning as evidenced by side chain to lipid headgroup Förster-resonance-energy-transfer. The free enthalpies of helix stabilization and hydrophobic burial of the neutral carboxyl shift the side chain pKa into the range typical of Glu-134 in photoactivated rhodopsin. The lipid-mediated coupling mechanism is independent of interhelical contacts allowing its conservation without interference with the diversity of ligand-specific interactions in class-I GPCRs.