Structure and pH sensitivity of the transmembrane segment 3 of rhodopsin

Activation of G protein-coupled receptors (GPCRs) originates in ligand-induced protein conformational changes that are transmitted to the cytosolic receptor surface. In the photoreceptor rhodopsin, and possibly other rhodopsin-like GPCRs, protonation of a carboxylic acid in the conserved E(D)RY motif at the cytosolic end of transmembrane helix 3 (TM3) is coupled to receptor activation. Here, we have investigated the structure of synthetic peptides derived from rhodopsin TM3. Polarized FTIR spectroscopy reveals a helical structure of a 31-mer TM3 peptide reconstituted into PC vesicles with a large tilt of 40-50 degrees of the helical axis relative to the membrane normal. Helical structure is also observed for the TM3 peptide in detergent micelles and depends on pH, especially in the C-terminal sequence. In addition, the fluorescence emission of the single tyrosine of the D(E)RY motif in the TM3 peptide exhibits a pronounced pH sensitivity that is abolished when Glu is replaced by Gln, demonstrating that protonation of the conserved Glu side chain affects the structure in the environment of the D(E)RY motif of TM3. The pH regulation of the C-terminal TM3 structure may be an intrinsic feature of the E(D)RY motif in other class I receptors, allowing the coupling of protonation and conformation of membrane-exposed residues in full-length GPCRs.