The bacterial opsins are retinal-binding proteins that provide light- dependent ion transport and sensory functions to a family of halophilic bacteria [<cite idref="PUB00005349"/>, <cite idref="PUB00001180"/>]. They are integral membrane proteins believed to contain seven transmembrane (TM) domains, the last of which contains the attachment point for retinal (a conserved lysine). <p>There are several classes of these bacterial proteins: they include bacteriorhodopsin and archaerhodopsin, which are light-driven proton pumps; halorhodopsin, a light-driven chloride pump; and sensory rhodopsin, which mediates both photoattractant (in the red) and photophobic (in the UV) responses.</p><p>Fungi also contain proteins with similarities to opsin. In the <taxon tax_id="5141">Neurospora crassa</taxon> opsin NOP-1 the chromophore is buried in a pocket within a 7TM structure, and bound by a protonated Schiff base to a lysine. The absorption of green light leads to an all-trans isomerisation of retinal, followed by the deprotonation of the Schiff base, resulting in a near-UV-absorbing intermediate. Archaeal rhodopsins employ this mechanism in order to pump protons over the plasma membrane and act predominantly as light-driven ion transporters the reaction cycle of NOP-1 is far too long (up to seconds) to operate as an effective ion pump, suggesting rather that it has signaling functions [<cite idref="PUB00044876"/>]. Deletion of nop-1 does not cause any discernible phenotype [<cite idref="PUB00044877"/>, <cite idref="PUB00044878"/>].</p><p>This entry contains two conserved patterns: the first pattern (BACTERIAL_OPSIN_1) corresponds to the third transmembrane region (called helix C) and includes an arginine residue which seems involved in the release of a proton from the Schiff's base to the extracellular medium, the second pattern (BACTERIAL_OPSIN_RET) includes the retinal binding lysine [<cite idref="PUB00044796"/>]. </p> Rhodopsin, retinal binding site