FLASH-INDUCED VOLTAGE CHANGES IN HALORHODOPSIN FROM NATRONOBACTERIUM-PHARAONIS

The flash-induced voltage response of halorhodopsin at high NaCl conce ntration comprises two main kinetic components. The first component wi th tau approximate to 1 mu s does not exceed 4% of the overall respons e amplitude and is probably associated with the formation of the L (hR 520) intermediate, The second main component with tau approximate to 1 -2.5 ms which is independent of Cl- concentration can be ascribed to t he transmembrane Cl- translocation during the L intermediate decay, Th e photoelectric response in the absence of Cl- has the opposite polari ty and does not exceed 6% of the overall response amplitude at high Na Cl concentration. A pH decrease results in substitution of the Cl--dep endent components by the photoresponse which is similar to that in the absence of Cl-. Thus, the difference between photoresponses of chlori de-binding and chloride-free halorhodopsin forms resembles that of bac teriorhodopsin purple neutral and blue acid forms, respectively. The p hotovoltage data obtained can hardly be explained within the framework of the photocycle scheme suggested by Varo et al. [Biochemistry 34 (1 995), 14490-14499], We suppose that the O-type intermediate belongs to some form of halorhodopsin incapable of Cl- transport. (C) 1998 Feder ation of European Biochemical Societies.