Permeation dynamics of chloride ions in the ClC-0 and ClC-1 channels Ben Corry * ,1 , Megan OÕMara, Shin-Ho Chung * Department of Theoretical Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia Received 16 December 2003; in final form 19 January 2004 Published online: 11 February 2004 Abstract We examine the mechanisms underlying the transport of ions across ClC-0 and ClC-1, the two best characterized members of the ubiquitous ClC chloride channel family. Using molecular and Brownian dynamics techniques we create an open channel structure, deduce the permeation characteristics, and highlight the amino acid residues responsible for the different conductance properties of these channels. Ó 2003 Elsevier B.V. All rights reserved. The ClC family of chloride channels is widely dis- tributed in the membranes of prokaryotic and eukary- otic cells and performs diverse physiological roles, from the control of cellular excitability, acidification of intracellular vesicles, to the regulation of cell volume [1–3]. However, the dynamics of Cl  permeation through these channels are not fully understood. We examine the mechanisms underlying the transport of ions across ClC-0, found in the electric organ of Torpedo [4], and ClC-1, the muscle chloride channel, mutations of which cause myotonia in mice [5] and humans [6]. Biophysical studies reveal that these channels undergo voltage-dependent transitions between open and closed states [7–9], gating being strongly facilitated by Cl  ions in the extracellular solution. It is believed that an ex- tracellular Cl  ion entering the pore displaces the side- chain of a glutamate residue (Glu 148) that is blocking the ion pathway, creating an open state and thus en- abling conduction to occur [10,11]. Analysis of the conduction processes in ClC-0 in a mixture of Cl  /NO  3 indicates that the permeation of ions takes place in a pore that is normally occupied by two or more ions [8]. Also, the conduction properties differ among the iso- forms. For example, the current-voltage relationship measured from ClC-0 is linear [4], whereas it is inwardly rectifying in ClC-1 [9,12]. To understand these proper- ties, we create open state structures of ClC-0 and ClC-1 using the crystal structure of the prokaryotic EcClC Cl  channel as a basis. We then investigate the steps in- volved in ion permeation and the characteristics of the current–voltage curves using electrostatic calculations and three-dimensional Brownian dynamics simulations. Our results highlight the amino acid residues responsible for the different conductance properties of ClC-0 and ClC-1 and explain many experimental observations. The crystal structures of the Escherichia coli ClC channel reported by Dutzler et al. [10,11] represent the wild-type channel in a closed state and the E148A mu- tant channel in a postulated open state. The ClC protein is a dimer, forming two identical pores, but we deal with only one of these throughout this study. A crosssection of this channel, with the front half of the protein re- moved, is shown in Fig. 1a. The protein has no con- tinuous conduit from one side of the membrane to the other that is wide enough to accommodate ions. How- ever a clear depression is apparent on each side of the protein, indicating the likely entrances to the pore. In the E148A mutant channel these two entrances are * Corresponding authors. Fax: +61-893-801-005. (B. Corry). E-mail addresses: ben@theochem.uwa.edu.au (B. Corry), shin-ho. chung@anu.edu.au (S.-H. Chung). 1 Department of Chemistry, University of Western Australia, Crawley, WA, Perth 6009, Australia. 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.01.072 Chemical Physics Letters 386 (2004) 233–238 www.elsevier.com/locate/cplett