light-driven inward chloride pump halorhodopsin, member of the seven-transmembrane GPCR superfamily
Halorhodopsin (HR) acts as a light-driven inward-directed chloride pump. When activated by yellow light, HR pumps chloride ions into the cell cytoplasm, generating a negative-inside membrane potential which drives proton uptake. The resulting electrochemical ion gradient provides an energy source to the cell and contributes to pH homeostasis. HR is found in phylogenetically ancient archaea, known as halobacteria which live in high salty environments. HR belongs to the microbial rhodopsin family, also known as type I rhodopsins, comprising light-driven retinal-binding outward pump bacteriorhodopsin (BR), light-gated cation channel channelrhodopsin (ChR), light-sensor activating transmembrane transducer protein sensory rhodopsin II (SRII), light-sensor activating soluble transducer protein Anabaena sensory rhodopsin (ASR), and other light-driven proton pumps such as blue-light absorbing and green-light absorbing proteorhodopsins, among others. They 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.