light-sensor activating transmembrane transducer protein sensory rhodopsin I and II; member of the seven-transmembrane GPCR superfamily
This subgroup includes the light-sensor activating transmembrane transducer proteins, sensory rhodopsin I (SRI) and II (SRII, also called phoborhodopsin). SRI and SRII are responsible for positive (attractive) and negative (repellent) phototaxis in halobacteria, respectively, thereby controlling the cell's directional movement in response to changes in light intensity by swimming either towards or away from the light. Both sensory rhodopsins belong to the family of microbial rhodopsins, also known as type I rhodopsins, consisting of the light-driven inward chloride pump halorhodopsin (HR), the outward proton pump bacteriorhodopsin (BR), the light-gated cation channel channelrhodopsin (ChR), and the other light-driven proton pumps such as blue-light absorbing and green-light absorbing proteorhodopsins, among others. Microbial rhodopsins have been found in various single-celled microorganisms from all three domains of life, including halophile archaea, gamma-proteobacteria, cyanobacteria, fungi, and green algae. 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.
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