Structure and Function of Archaeal Rhodopsin

Halobacteria contain four rhodopsins bacteriorhodopsin, halorhodopsin, sensor-yrhodopsin, and phoborhodopsin (Fig. 4.2A) [11-17]. Bacteriorhodopsin and halorhodopsin are light-driven ion pumps, which act as an outward proton pump and an inward Ch pump, respectively. Sensoryrhodopsin and phoborhodopsin are photoreceptors that act to produce attractant and repellent responses in phototaxis, respectively. These four archaeal rhodopsins have similar structures seven helices constitute the transmembrane portion of the protein, and a retinal chromophore is bound to a lysine residue of the seventh helix via a protonated SchifF base linkage (Fig. 4.1). A negatively charged counterion is present to stabilize the positive charge inside the protein the counterion is an aspartate except for in halorhodopsin, which possesses a chloride ion. In sensoryrhodopsin, interaction with a transmembrane transducer protein raises the pKa of the aspartate, so that the aspartate is protonated at neutral pH. 

Green spheres represent oxygen atoms of water. Dotted lines are putative hydrogen bonds, whose distances are shown in angstroms. 

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