viral rhodopsins and similar proteins, members of the seven-transmembrane GPCR superfamily
This subfamily is composed of viral homologs of proteorhodopsins (PRs), which are blue-light absorbing and green-light absorbing proteins acting as light-driven proton pumps that play a major role in supplying light energy for phototropic marine microorganisms, by a mechanism similar to that of bacteriorhodopsin. Viral proteorhodopsins are predicted to function as sensory rhodopsins that could affect signaling, for example, phototaxis in the infected protists, perhaps stimulating relocation of the infected protists to areas that are rich in nutrients required for virus reproduction. Viral proteorhodopsins are monophyletic and split into two distinct groups, I and II, represented by Phaeocystis globosa virus 12T VirRDTS and Organic Lake phycodnavirus OLPVRII, respectively. PRs belong to the microbial rhodopsin family, also known as type 1 rhodopsins, which also comprise the light-driven inward chloride pump halorhodopsin (HR), the light-gated cation channel channelrhodopsin (ChR), the light-sensor activating transmembrane transducer protein sensory rhodopsin II (SRII), the light-sensor activating soluble transducer protein Anabaena sensory rhodopsin (ASR), and the other light-driven proton pumps such as bacteriorhodopsin (BR). While microbial (type 1) and animal (type 2) rhodopsins have no sequence similarity with each other, they share a common architecture consisting of seven-transmembrane alpha-helices (TM) connected by extracellular loops and intracellular loops. Both types of rhodopsins consist of opsin and a covalently attached retinal (the aldehyde of vitamin A), a photoreactive chromophore, via a protonated Schiff base linkage to an amino group of lysine in the middle of the seventh transmembrane helix (TM7). Upon the absorption of light, microbial rhodopsins undergo light-induced photoisomerization of all-trans retinal into the 13-cis isomer, whereas the photoisomerization of 11-cis retinal to all-trans isomer occurs in the animal rhodopsins. While animal visual rhodopsins are activated by light to catalyze GDP/GTP exchange in the alpha subunit of the retinal G protein transducin (Gt), microbial rhodopsins do not activate G proteins, but instead can function as light-dependent ion pumps, cation channels, and sensors.