Falke, Joseph J. and Chan, Sunney I. and Steiner, Michael and Oesterhelt, Dieter and Towner, Paul and Lanyi, Janos K. (1984) Halide binding by the purified halorhodopsin chromoprotein. II. New chloride-binding sites revealed by 35Cl NMR. Journal of Biological Chemistry, 259 (4). pp. 2185-2189. ISSN 0021-9258 http://resolver.caltech.edu/CaltechAUTHORS:FALjbc84a
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Halorhodopsin is a light-driven chloride pump in the cell membrane of Halobacterium halobium. Recently, a polypeptide of apparent Mr = 20,000 has been purified that contains the halorhodopsin chromophore. Here we use 35Cl NMR to show that the purified chromoprotein possesses two previously unknown classes of chloride-binding sites. One class exhibits a low affinity (KD much greater than 1 M) for chloride and bromide. The second class exhibits a higher affinity (KD = 110 ± 50 mM) for chloride and also binds other anions according to the affinity series I-, SCN- greater than Br-, NO-3 greater than Cl- greater than F- , citrate. Both classes of NMR site remain intact at pH 11, indicating that the essential positive charges are provided by arginine. Also, both classes are unaffected by bleaching, suggesting that the sites are not in the immediate vicinity of the halorhodopsin chromophore. Although the chromoprotein also appears to contain the chloride- transport site (Steiner, M., Oesterhelt, D., Ariki, M., and Lanyi, J. K. (1984) J. Biol. Chem. 259, 2179-2184), this site was not detected by 35Cl NMR, suggesting that the transport site is in the interior of the protein where it is sampled slowly by chloride in the medium. It is proposed that the purified chromoprotein possesses a channel leading from the medium to the transport site and that the channel contains the high affinity NMR site which facilitates the migration of chloride between the medium and the transport site. We have also used 35Cl NMR to study chloride binding to purified monomeric bacteriorhodopsin; however, this protein contains no detectable chloride-binding sites.
|Additional Information:||Copyright © 1984 by the American Society for Biochemistry and Molecular Biology. (Received for publication, September 26, 1983) This work is Contribution 6908 from the Arthur Amos Noyes Laboratory of Chemical Physics. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. [J.J.F. was] [s]upported by a National Science Foundation predoctoral fellowship. [S.I.C. was] [s]upported by National Institute of General Medical Sciences Grant GM-22432|
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