Bacteriorhodopsin

Bacteriorhodopsin is a photosynthetic pigment used by archaea, most notably halobacteria. It is an integral membrane protein that is composed of seven transmembrane alpha helices and acts as a proton pump.

Its three-dimensional tertiary structure structure resembles that of vertebrate rhodopsins, the pigments that sense light in the retina. However, their function is different and there is no homology of their amino acid sequences. Bacteriorhodopsin is therefore not a member of the G protein coupled receptor family. The crystal structure of bacteriorhodopsin had been used as a template to build models of G protein-coupled receptors before crystallographic structures were also available for these proteins. Many molecules have homology to bacteriorhodopsin, including some directly light-activated channels like channelrhodopsin.