Archaerhodopsin-2, from Halobacterium sp. aus-2 further reveals essential amino acid residues for light-driven proton pumps.

We have isolated a retinal protein which differs from bacteriorhodopsin and archaerhodopsin and pumps out as many protons in the light as those proton pumps. We tentatively named it archaerhodopsin-2. We have cloned and sequenced the gene that encodes archaerhodopsin-2. The gene consists of 780-bp nucleotides for 259 amino acids with a molecular mass of 27,937 Da. The amino acid sequence of archaerhodopsin-2 is 56% identical to bacteriorhodopsin and 88% to archaerhodopsin, with a few gaps of a few amino acids in both cases. Although the amino acid sequence of archaerhodopsin has revealed 157 conserved residues common to bacteriorhodopsin, the sequence of archaerhodopsin-2 reduces that number to 133. Of these, 38 amino acids are also common to chloride pumps and 24 to all bacterial retinal proteins known to date.