The organism was first identified in a salt lake in Tibet. The gene encoding AR4 was identified by H Wang and colleagues in 2000. In most bacteriorhodopsin homologs, H+ release to the extracellular medium takes place before a replacement ion is taken up from the cytosolic side of the membrane, however under the acidic conditions found in the organism’s native habitat, the order of these stages in the AR4 photocycle is reversed. AR-BD1 is expressed by "Halorubrum xinjiangense". The organism was first isolated from Xiao-Er-Kule Lake in Xinjiang, China. HeAR is expressed by "Halorubrum ejinorense". The organism was first isolated from Lake Ejinor in Inner Mongolia, China. AR-TP009 is expressed by "Halorubrum sp. TP009". Its ability to act as a neural silencer has been investigated in mouse cortical pyramidal neurons. Like other members of the microbial rhodopsin family, archaerhodopsins are expressed in specialised, protein-rich domains of the cell surface membrane, commonly called the claret membrane. In addition to ether lipids, the claret membrane contains bacterioruberin, (a 50-carbon carotenoid pigment) which is thought to protect against photobleaching. Atomic force microscope images of the claret membranes of several archaerhodopsins, show that the proteins are trimeric and are arranged in a hexagonal lattice. Bacterioruberin has also been implicated in oligomerisation and may facilitate protein-protein interactions in the native membrane. Archaerhodopsins are active transporters, using the energy from sunlight to pump H+ ions out of the cell to generate a proton motive force that is used for ATP synthesis. Removal of the retinal cofactor (e.g. by treatment with hydroxylamine) abolishes the transporter function and dramatically alters the absorption spectra of the proteins. The proton pumping ability of AR3 has been demonstrated in recombinant "E. coli" cells and of AR4 in liposomes. In the resting or ground state of archaerhodopsin, the bound retinal is in the all-"trans" form, but on absorption of a photon of light, it isomerizes to 13-"cis". The protein surrounding the chromophore reacts to the change of shape and undergoes an ordered sequence of conformational changes, which are collectively known as the photocycle.