Evidence for a halide-binding site in halorhodopsin.

In attempting to describe a halide-binding site in halorhodopsin (P580), a light-driven chloride pump in halobacterial membranes, we have investigated the effects of chloride and bromide on flash-induced absorption changes in this pigment, and studied the effects of a diuretic drug, MK-473, on the photochemistry and the transport. We find that at high sulfate or phosphate concentrations, but in the absence of halide, the principal photointermediate is P660, whose half-life is about 1.5 ms. When chloride or bromide are added, the production of P660 is depressed and its half-life becomes longer (up to approximately 10 ms). With increasing halide concentration, the cycle proceeds more and more via the alternative photointermediate, P520, whose half-life varies with the halide concentration in a fashion similar to that of P660. Transport activity, measured during sustained illumination, increases in a manner parallel to the accumulation of P520 up to about 400 mM halide, but declines at concentrations above this value. The transport is inhibited by MK-473 with competitive kinetics, and the effects of this inhibitor on the photocycle are also consistent with displacement of halide ions from their binding site. The observations reported here suggest that chloride and bromide bind to P580, P660, and P520, and that this binding is to a distinct site on the protein.