text: Proteorhodopsin belongs to a family of similar retinylidene proteins, most similar to its archaeal homologues halorhodopsin and bacteriorhodopsin. Sensory rhodopsin was discovered by Franz Christian Boll in 1876. Bacteriorhodopsin was discovered in 1971 and named in 1973 and is currently only known to exist in archaea, not bacteria.  Halorhodopsin was first discovered and named in 1977.
   Bacteriorhodopsin and Halorhodopsin both only exist in archaea whereas proteorhodopsin spans bacteria, archaea, and eukaryotes.  Proteorhodopsin shares seven transmembrane α-helices retinal covalently linked by a Schiff base mechanism to a lysine residue in the seventh helix (helix G).  Bacteriorhodopsin, like proteorhodopsin, is a light-driven proton pump. Sensory rhodopsin is a G-coupled protein involved in sight.