Proteorhodopsin (also known as pRhodopsin) is a family of transmembrane proteins that use retinal as a chromophore for light-mediated functionality, in this case, a proton pump.

[3] Proteorhodopsin (PR or pRhodopsin) was first discovered in 2000 within a bacterial artificial chromosome from previously uncultivated marine Gammaproteobacteria, still only referred to by their ribotype metagenomic data, SAR86[2].

Oxyrrhis marina is a dinoflagellate protist with green-absorbing proteorhodopsin (a result of the L109 Group) that exists mostly in shallow tide pools and shores, where green light is still available.

Karlodinium micrum, another dinoflagelate, expresses a blue tuned proteorhodopsin (E109) which may be related to its deep water vertical migrations.

However, in proteorhodopsin, there are no carboxylic acid residues directly homologous to Glu194 or Glu204 of bacteriorhodopsin (or Glu 108 and 204 depending on the bacRhodopsin variant), which are thought to be involved in the proton release pathway at the extracellular surface.

[14][16][17][18] Complete proteorhodopsin based photosystems have been discovered and expressed in E. coli, giving them additional light mediated energy gradient capability for ATP generation without external need for retinal or precursors; with the PR, gene five other proteins code for the photopigment biosynthetic pathway.

[19] If the gene for proteorhodopsin is inserted into E. coli and retinal is given to these modified bacteria, then they will incorporate the pigment into their cell membrane and will pump H+ in the presence of light.