Nature's toolkit for microbial rhodopsin ion pumps.

Abstract

Microbial rhodopsins function as light-driven (retinal-based) ion pumps, cation channels, or light sensors in various microorganisms [see Heberle et al.’s Retinal Proteins—You can teach an old dog new tricks (1)], are found in all three domains of life (2⇓–4), and were recently reported in viruses as well (5, 6). The first discovered microbial rhodopsin, the light-driven proton pump bacteriorhodopsin (BR), was identified in halophilic (high-salt loving) archaea more than 40 y ago (in 1971). Later on, the light-driven chloride pump halorhodopsin (HR) was discovered in the same archaeal microorganisms (for a historical perspective, see ref. 2). Both pumps were discovered using biophysical and physiological techniques. The widespread bacterial light-driven proton pump proteorhodopsin (PR), however, was discovered using metagenomics (7), and was initially detected based on a weak homology to BR (less than 30% identity on the protein level). Although diverse PRs are found in most marine microorganisms …