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J. Biol. Chem., Vol. 278, Issue 37, 35542-35551, September 12, 2003

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Probing the Orientation of Reconstituted Maltoporin Channels at the Single-protein Level^*

Christophe Danelon , Thérèse Brando and Mathias Winterhalter

From the Institut de Pharmacologie et de Biologie Structurale, Université Paul Sabatier, 31077 Toulouse, France

Recently we have shown that maltoporin channels reconstituted into black lipid membranes have pronounced asymmetric properties in both ion conduction and sugar binding. This asymmetry revealed also that maltoporin insertion is directional. However, the orientation in the lipid bilayer remained an open question. To elucidate the orientation, we performed point mutations at each side of the channel and analyzed the ion current fluctuation caused by an asymmetric maltohexaose addition. In a second series we used a chemically modified maltohexaose sugar molecule with inhibited entry possibility from the periplasmic side. In contrast to the natural outer cell wall of bacteria, we found that the maltoporin inserts in artificial lipid bilayer in such a way that the long extracellular loops are exposed to the same side of the membrane than protein addition. Based on this orientation, the directional properties of sugar binding were correlated to physiological conditions. We found that nature has optimized maltoporin channels by lowering the activation barriers at each extremity of the pore to trap sugar molecules from the external medium and eject them most efficiently to the periplasmic side.

Received for publication, May 23, 2003 , and in revised form, June 26, 2003.

^* 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.

To whom correspondence should be addressed: Institut de Pharmacologie et de Biologie Structurale, CNRS-UMR 5089, 205 Rte. de Narbonne, 31077 Toulouse, France. Tel.: 33-5-61-17-54-50; Fax: 33-5-61-17-59-94; E-mail: danelon{at}ipbs.fr.

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