MICHAEL FREEMANTLE
High-resolution x-ray analysis of proteins that catalyze the selective flow of chloride ions across cell membranes shows that each anion channel has two pores.
Roderick MacKinnon, Howard Hughes Medical Institute investigator and professor of molecular neurobiology and biophysics at Rockefeller University, and coworkers identified the peptide helices, amino acid side chains, and chloride ions inside the pores of two bacterial proteins with a resolution of 3 Å [Nature, 415, 287 (2002)].
They showed that both proteins are dimers with two subunits containing identical pores. Each subunit is composed of two roughly repeated halves that span the membrane with opposite orientations. The two halves of this “antiparallel” architecture fit together to form a symmetrical hourglass shape with positively charged internal surfaces that attract the chloride ions.
“The paper presents the first atomic structure of a chloride-selective ion channel,” MacKinnon tells C&EN. “The only other ion-selective channel solved to date is the K+ channel, a cation-selective channel.”
Chloride channels belong to a large family of anion channels that are found in both prokaryotic and eukaryotic cells.
“The channels underlie many important cellular processes such as regulation of electrical activity in muscle-cell membranes and epithelial salt and water transport in cells of the kidney,” MacKinnon explains. “Genetic abnormalities in human chloride channels underlie muscle diseases, such as familial myotonia, and kidney diseases, such as Bartter’s syndrome and Dent’s disease.”
“The channels solved in our study, one from E. coli and another from Salmonella typhimurium, are bacterial but are related in sequence and structure to the human versions of chloride channels,” he adds. “However, the importance of the work has much to do with the intellectual question of how nature designed an ion pathway across a membrane that is selective for the chloride anion.”
One such question is, How does the channel open and shut to regulate the flow of chloride ions? The structure obtained by MacKinnon’s group reveals a glutamate amino acid residue with a negatively charged side chain at the narrowest point of the hourglass. The group speculates that the side chain acts as a gate. When a chloride ion enters a pore, electrostatic repulsion causes the gate to swing out and thereby open the channel.
The long-awaited crystal structure of a chloride channel is a spectacular breakthrough, notes Thomas J. Jentsch, director of the Institute for Molecular Neurobiology at Hamburg University, Germany, in the same issue of Nature (page 276).
“These new structures are very welcome, in part because surprisingly little is known about how chloride channels work,” he writes. “We do know that they come from several gene families and that many are gated by voltage.”
Further work will be required to understand how the chloride channels gate and how the gating processes are coupled to ion conduction through the pore, MacKinnon and colleagues point out.
TWINS Chloride channel has two identical subunits (shown here in green and blue), each containing an anion pathway (red).
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