Characterization of the azide-dependent bacteriorhodopsin-like photocycle of salinarum halorhodopsin

The photocycle of salinarum halorhodopsin was investigated in the presence of azide. The azide binds to the halorhodopsin with 150 mM binding constant in the absence of chloride and with 250 mM binding constant in the presence of 1 M chloride. We demonstrate that the azide-binding site is different from that of chloride, and the influence of chloride on the binding constant is indirect. The analysis of the absorption kinetic signals indicates the existence of two parallel photocycles. One belongs to the 13-cis retinal containing protein and contains a single red shifted intermediate. The other photocycle, of the all-trans retinal containing halorhodopsin, resembles the cycle of bacteriorhodopsin and contains a long-living M intermediate. With time-resolved spectroscopy, the spectra of intermediates were determined. Intermediates L, N, and O were not detected. The multiexponential rise and decay of the M intermediate could be explained by the introduction of the "spectrally silent" intermediates M-1, M-2, and HR', HR, respectively. The electric signal measurements revealed the existence of a component equivalent with a proton motion toward the extracellular side of the membrane, which appears during the M-1 to M-2 transition. The differences between the azide-dependent photocycle of salinarum halorhodopsin and pharaonis halorhodopsin are discussed.