Photoactivation of rhodopsin: interplay between protein and chromophore

Data in the literature suggest a finely tuned interaction between ligand (11-cis-retinal) and protein (opsin) in order to allow very efficient photoactivation of the ligand and highly vectorial rhodopsin activation with a huge increase in receptor activity. We have further investigated this interaction using ligand homologues, 13C-ligand labelling or 15N-protein labelling, in combination with Fourier transform infrared (FT-IR) and solid-state magic angle spinning (ss-MAS)-NMR spectroscopy. Using 1D rotational resonance (RR) or double-quantum heteronuclear local field (2Q-HLF) ss-MAS-NMR we report the first structure refinement of the rhodopsin chromophore in situ. These measurements yield a specification of the torsional strain in the for isomerization essential C10-C13 segment of the chromophore. This strain is thought to contribute to the high rate and stereospecificity of the photoisomerization reaction. In agreement with previous data, the C10-C13 segment region reaches a relaxed all-trans configuration at the lumirhodopsin photointermediate. MAS-NMR analysis of [15N]lysine-labelled rhodopsin reveals the presence of a 'soft' counterion, requiring intermediate water molecules for stabilization. FT-IR studies on [2H]tyrosine-labelled rhodopsin demonstrate participation of several tyrosin(at)e residues in receptor activation. One of these, probably Tyr268, is already active at the bathorhodopsin stage. Finally, the effect of ligands with single additional methyl substituents in the C10-C12 region has been investigated. They do not affect the general activation pathway, but perturb the activation kinetics of rhodopsin, suggesting steric interference with protein residues. Possible implications of these results for a structural role of water residues will be discussed, as well.