1. Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
2. Howard Hughes Medical Institute, Laboratory of Mass Spectrometry and Gaseous Ion Chemistry, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA

Correspondence to: Roderick MacKinnon¹ Correspondence and requests for materials should be addressed to R.M. (e-mail: Email: mackinn@rockvax.rockefeller.edu). Coordinates have been deposited with the Protein Data Bank under accession codes 1KPK and 1KPL.

Abstract

The ClC chloride channels catalyse the selective flow of Cl^- ions across cell membranes, thereby regulating electrical excitation in skeletal muscle and the flow of salt and water across epithelial barriers. Genetic defects in ClC Cl^- channels underlie several familial muscle and kidney diseases. Here we present the X-ray structures of two prokaryotic ClC Cl^- channels from Salmonella enterica serovar typhimurium and Escherichia coli at 3.0 and 3.5 Å, respectively. Both structures reveal two identical pores, each pore being formed by a separate subunit contained within a homodimeric membrane protein. Individual subunits are composed of two roughly repeated halves that span the membrane with opposite orientations. This antiparallel architecture defines a selectivity filter in which a Cl^- ion is stabilized by electrostatic interactions with -helix dipoles and by chemical coordination with nitrogen atoms and hydroxyl groups. These findings provide a structural basis for further understanding the function of ClC Cl^- channels, and establish the physical and chemical basis of their anion selectivity.