Unique Cl^- pump rhodopsin with close similarity to H⁺ pump rhodopsin

Microbial rhodopsin is a ubiquitous membrane protein in unicellular microorganisms. Similar to animal rhodopsin, this protein consists of seven transmembrane helices and the chromophore retinal. However, unlike animal rhodopsin, microbial rhodopsin acts as not only a photosignal receptor but also a light-activated ion transporter and light-switchable enzyme. In this article, the third Cl^- pump microbial rhodopsin will be introduced. The physiological importance of Cl^- pumps has not been clarified. Despite this, their mechanisms, especially that of the first Cl^- pump halorhodopsin (HR), have been studied to characterize them as model proteins for membrane anion transporters. The third Cl^- pump defines a phylogenetic cluster distinct from other microbial rhodopsins. However, this Cl^- pump conserves characteristic residues for not only the Cl^- pump HR but also the H⁺ pump bacteriorhodopsin (BR). Reflecting close similarity to BR, the third Cl^- pump begins to pump H⁺ outwardly after single amino acid replacement. This mutation activates several residues that have no roles in the original Cl^- pump function but act as important H⁺ relay residues in the H⁺ pump mutant. Thus, the third Cl^- pump might be the model protein for functional differentiation because this rhodopsin seems to be the Cl^- pump occurring immediately after functional differentiation from the BR-type H⁺ pump.