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Author(s): Alina Pushkarev [sup.1] , Keiichi Inoue [sup.2] [sup.3] [sup.4] [sup.5] , Shirley Larom [sup.1] , José Flores-Uribe [sup.1] , Manish Singh [sup.2] , Masae Konno [sup.2] , Sahoko Tomida [...], Many organisms capture or sense sunlight using rhodopsin pigments.sup.1,2, which are integral membrane proteins that bind retinal chromophores. Rhodopsins comprise two distinct protein families.sup.1, type-1 (microbial rhodopsins) and type-2 (animal rhodopsins). The two families share similar topologies and contain seven transmembrane helices that form a pocket in which retinal is linked covalently as a protonated Schiff base to a lysine at the seventh transmembrane helix.sup.2,3. Type-1 and type-2 rhodopsins show little or no sequence similarity to each other, as a consequence of extensive divergence from a common ancestor or convergent evolution of similar structures.sup.1. Here we report a previously unknown and diverse family of rhodopsins--which we term the heliorhodopsins--that we identified using functional metagenomics and that are distantly related to type-1 rhodopsins. Heliorhodopsins are embedded in the membrane with their N termini facing the cell cytoplasm, an orientation that is opposite to that of type-1 or type-2 rhodopsins. Heliorhodopsins show photocycles that are longer than one second, which is suggestive of light-sensory activity. Heliorhodopsin photocycles accompany retinal isomerization and proton transfer, as in type-1 and type-2 rhodopsins, but protons are never released from the protein, even transiently. Heliorhodopsins are abundant and distributed globally; we detected them in Archaea, Bacteria, Eukarya and their viruses. Our findings reveal a previously unknown family of light-sensing rhodopsins that are widespread in the microbial world. An analysis based on functional metagenomics reveals a previously unknown group of microbial light-sensory rhodopsins that are widespread among a diverse range of microorganisms.

Wild-type rhodopsin is demonstrated to be targeted to the apical plasma membrane through the TGN on expression in polarized epithelial MDCK cells. Fusion of the cytopolasmic tail of rhodopsin to a cytosolic protein glutathione S-transferase (GST) targets the fusion protein GST-Rho39Tr to the apical membrane.

Halorhodopsin (HR), the bacterial retinal protein of halobacterium salinarium acts as a light-driven chloride pump. Resonance Raman (RR) spectroscopy was used to study HR from natronobacterium pharaonis loaded by different anions. Further insight in to the specific anion-chromophore interactions in HR was also gained with the extension of spectroscopic investigations to retinal Schiff base model compounds.

MC-SCF and multireference Moller-Plesset methods were used to compute the minimum energy path for photoisomerization of the minimal retinal protonated Shiff base model tZt-penta-3,5-dieniminium cation (cis-Csub4Hsub6NHsub2+). The molecule was shown to undergo a barrierless relaxation upon excitation to the spectroscopic state, where ground states are conically intersecting, providing an efficient nonadiabatic cis-trans isomerization. This indicates that cis-Csub5Hsub6NHsub2+ is a suitable ab initio model for the rationalization of the observed ultrafast isomerization of the retinal chromophore in rhodopsin.

A solid-state NMR technique was applied to a super13Csub2 labeled sample of the 41kD integral membrane protein rhodopsin, under magic-angle-spinning conditions. The technique allows good sensitivity and resolution, and enables a direct determination of molecular torsional angles. It was determined that the H-C10-C11-H torsional angle of the rentinylidene chromophore in rhodopsin was 160 plus or minus 10 deg.

Adenomatous polyposis coil is caused by variation of the tumor suppressor gene in humans. This causes a condition relating to colorectal cancer. The features of congenital hypertrophy of the retinal pigment epithelium (CHRPE) is associated with the syndrome. Ocular examination of such patients revealed that the CHRPE tends to cluster within specific groups. Approximately 42 unrelated patients exhibited APC mutation, which caused the formation of a truncated protein.