proton pumping rhodopsins in fungi and algae, member of the seven-transmembrane GPCR superfamily
This subgroup represents uncharacterized proton pumping rhodopsins found in fungi and algae. They belong to the microbial rhodopsin family, 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), the light-sensor activating transmembrane transducer protein sensory rhodopsin II (SRII), 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.