Abstract
While conformational flexibility of proteins is widely recognized as one of their functionally crucial features and enjoys proper attention for this reason, their elastic properties are rarely discussed. In ion channel studies, where the voltage-induced or ligand-induced conformational transitions, gating, are the leading topic of research, the elastic structural deformation by the applied electric field has never been addressed at all. Here we examine elasticity using a model channel of known crystal structure—Staphylococcus aureus α-hemolysin. Working with single channels reconstituted into planar lipid bilayers, we first show that their ionic conductance is asymmetric with voltage even at the highest salt concentration used where the static charges in the channel interior are maximally shielded. Second, choosing 18-crown-6 as a molecular probe whose size is close to the size of the narrowest part of the α-hemolysin pore, we analyze the blockage of the channel by the crown/K+ complex. Analysis of the blockage within the framework of the Woodhull model in its generalized form demonstrates that the model is able to correctly describe the crown effect only if the parameters of the model are considered to be voltage-dependent. Specifically, one has to include either a voltage-dependent barrier for crown release to the cis side of the channel or voltage-dependent interactions between the binding site and the crown. We suggest that the voltage sensitivity of both the ionic conductance of the channel seen at the highest salt concentration and its blockage by the crown reflects a field-induced deformation of the pore.
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Acknowledgements
We are grateful to Genrih N. Berestovsky, Alexander M. Berezkovskii, Daniel Harries, and V. Adrian Parsegian for fruitful discussions. Our special thanks are due to Sergey M. Bezrukov, who made many useful suggestions and also helped with the writing of the manuscript. This study was supported by Conselho National de Desenvolvimento Cientifico e Tecnologico (Brazil).
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After this manuscript was accepted, Aksimentiev & Schulten published an article (Aksimentiev A. and Schulten K. Imaging a-Hemolysin with Molecular Dynamics: Ionic Conductance, Osmotic Permeability, and the Electrostatic Potential Map. Biophys. J. 88 June 2005 3745–3761) showing that residues, which are located at the outer surface of the beta-barrel could change their orientations without any large structural changes of the channel when altering the value of the transmembrane potential. Moreover, in the personal communication they revealed that switching the transmembrane potential from +120 to −120 mV (the sign is assigned to the cis opening of the channel) directs Lys147, on average, upward (to the cis vestibule of the channel), increasing the radius of the pore constriction by about 0.8 Å. Thus there is remarkable agreement between that theoretical prediction and our experimental data indicating the elastic deformation of the alpha-hemolysin channel under the electric field influence.
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Krasilnikov, O.V., Merzlyak, P.G., Yuldasheva, L.N. et al. Protein electrostriction: a possibility of elastic deformation of the α-hemolysin channel by the applied field. Eur Biophys J 34, 997–1006 (2005). https://doi.org/10.1007/s00249-005-0485-9
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DOI: https://doi.org/10.1007/s00249-005-0485-9