Australian Halobacteria and their retinal-protein ion pumps

Halophiles collected in Western Australia have been found to be examples of extremely halophilic rod-shaped archaebacteria, members of the genus Halobacterium. Most of them contain retinal proteins, and these proteins differ from one another and also from both bacteriorhodopsin (bR) and halorhodopsin [and sensory rhodopsins (sR)] isolated from Halobacterium salinarium (halobium), as revealed by their peptide maps and amino acid sequences. However, these retinal proteins still have the ability to pump protons or chloride ions in the light. These new ion pumps, designated archaerhodopsins (aR) [Mukohata et al. (1988) Biochem. Biophys. Res. Commun. 151, 1339-1345], are almost identical in terms of their molecular sizes and transient photochemical properties to the ion pumps identified previously. Differences are found in the: (1) apparent extinction coefficient of dark/light-adapted aR-2; (2) titration profiles at acidic pH of the absorption spectra of all aRs; and (3) circular dichroism spectra, which are influenced by the coexistent isoprenoid bacterioruberin. The amino acid sequences of two proton pumps from the Australian halobacteria, namely aR and aR-2, are approximately 90% homologous and both sequences are about 60% homologous with that of bR. Hydropathy plots suggest that these pumps also have a seven-helical structure similar to that of bR. The amino acid residues are highly conserved in the helical regions, in particular in the case of helices C and G (91 and 84%, respectively), among the three proton pumps.