Functional role of protonation and hydration at the lipid protein interface in membrane receptors

The visual photoreceptor rhodopsin is a proto¬typical 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 trans¬membrane helix-3 promotes the formation of the G-protein-activating state. Using trans¬mem¬brane segments derived from helix-3 of bovine rhodopsin, we show that lipid protein interactions play a key role in this cytosolic "proton switch". Infra¬red- and fluo¬res¬cence-spectroscopic pKa deter¬minations reveal that the D(E)RY motif is an auto¬no¬mous functional module coupling side chain neu¬trali¬za¬tion to conformation and helix positio¬ning as evidenced by side chain to lipid headgroup Foerster-re¬so¬nance-energy-transfer. Using rapid scan FTIR spectroscopy, we show that the motif is also a local protonein hydration site. This dual function renders the proton acceptor group a key residue in conformational control by positioning helix-3 at the water lipid protein interface.