Functional interaction structures of the photochromic retinal protein rhodopsin.

We studied functional interaction structures of the vertebrate membrane photoreceptor rhodopsin containing retinal as a chromophore. Using time-resolved fluorescence depolarization we analyzed real-time dynamics and conformational changes of the cytoplasmic helix 8 (H8) preceding the long C-terminal tail of rhodopsin. H8 runs parallel to the membrane surface and extends from transmembrane helix 7 whose highly conserved NPxxY(x)F motif connects that region of rhodopsin with the retinal binding pocket. Our measurements indicate that photo-induced retinal isomerization from 11-cis to all-trans provokes conformational changes of H8, including slower motion and reduced flexibility, that are specific for the active metarhodopsin-II photo-intermediate. These conformational changes are absent in the retinal-devoid state opsin and in the phosphorylated metarhodopsin-II state upon receptor deactivation. Furthermore we show that membrane rim effects can influence interfacial reactions at the cytoplasmic rhodopsin surface such as proton transfer reactions between surface and aqueous bulk phase or binding of the signaling protein transducin visualized with single-molecule widefield microscopy. These findings are important for an understanding of the effects of membrane structure on the photo-transduction mechanism.