Chloride Concentration Dependency of the Electrogenic Activity of Halorhodopsin

The capacitive photoelectric current responses of the halorhodopsins from Halobacterium salinarum (shR) and from Natronobacterium pharaonis (phR) were studied using membrane fragments adsorbed onto a thin polyester film. The electric current of shR was not much affected by ionic strength or cations...

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Published inBiochemistry (Easton) Vol. 38; no. 17; pp. 5422 - 5429
Main Authors Okuno, Daichi, Asaumi, Makoto, Muneyuki, Eiro
Format Journal Article
LanguageEnglish
Published United States American Chemical Society 27.04.1999
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Summary:The capacitive photoelectric current responses of the halorhodopsins from Halobacterium salinarum (shR) and from Natronobacterium pharaonis (phR) were studied using membrane fragments adsorbed onto a thin polyester film. The electric current of shR was not much affected by ionic strength or cations present in the medium (Na+, K+, Li+, Mg2+, or Ca2+), but was greatly influenced by the Cl- concentration. It increased biphasically as the Cl- concentration increased from 0 to 5 M, then decreased and almost vanished at around 10 or 12 M. Apparent K d's of about 0.1 and 6 M were deduced for the K d of Cl- uptake sites. We had to assume a sigmoidal increase of Cl- binding with a Hill coefficient of about 8 at the cytoplasmic, Cl- release site(s). The half-maximum Cl- concentration for the sigmoidal binding was about 7.5 M. The electric current of phR had a maximum around 30 mM Cl- and biphasically decreased at higher Cl- concentrations. The apparent K d for the Cl- uptake site was 5 mM. The biphasic decrease in the transport activity was explained by assuming a sum of simple hyperbolic type binding (K d = 0.2 M) and sigmoidally increasing binding with a Hill coefficient of 10 on the cytoplasmic side. The half-maximum concentration of the latter cooperative binding was 5.6 M. This great difference between the apparent affinity of the release site of shR and that of phR can explain the previously reported difference between the Cl- dependency of their photocycles. These results also suggest that there may be multiple Cl- binding sites in the Cl- transport pathway. A simple sequence of Cl- transport steps based on a multiion channel model is proposed.
Bibliography:istex:7FC55268F0F693ED1219D686ED2B67427027B483
ark:/67375/TPS-CGF37GQ1-2
This work was supported in part by Grant-in-Aid for Scientific Research (C) 09833001 (to E.M.) from the Ministry of Education, Science, Sports and Culture of Japan.
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ISSN:0006-2960
1520-4995
DOI:10.1021/bi9826456