Chloride- and pH-dependent proton transport by BR mutant D85N.

Photocurrents from purple membrane suspensions of D85N BR mutant adsorbed to planar lipid membranes (BLM) were recorded under yellow (lambda > 515 nm), blue (360 nm < lambda < 420 nm) and white (lambda > 360 nm) light. The pH dependence of the transient and stationary currents was studied in the range from 4.5 to 10.5. The outwardly directed stationary currents in yellow and blue light indicate the presence of a proton pumping activity, dependent on the pH of the sample, in the same direction as in the wild-type. The inwardly directed currents in white light, due to an inverse proton translocation, in a two-photon process, show a pH dependence as well. The stationary currents in blue and white light are drastically increased in the presence of azide, but not in yellow light. The concentration dependence of the currents on azide indicates binding of azide to the protein. In the presence of 1 M sodium chloride, the stationary proton currents in yellow light show an increase by a factor of 25 at pH 5.5. On addition of 50 mM azide, the stationary current in yellow light decreases again, possibly by competition between azide and chloride for a common binding site. The observed transport modes are discussed in the framework of the recently published IST model for ion translocation by retinal proteins [U. Haupts et al., Biochemistry 36 (1997) 2-7].