Department of Molecular & Cellular Physiology, Stanford University School of Medicine, Stanford, United States
Tao Jiang
NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, United States
Irimpan I Mathews
Stanford Synchrotron Radiation Lightsource, Stanford University, Menlo Park, United States
Richard A Stein
Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, United States
Antoine Koehl
Department of Molecular & Cellular Physiology, Stanford University School of Medicine, Stanford, United States
Hassane S Mchaourab
Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, United States
NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, United States
Among coupled exchangers, CLCs uniquely catalyze the exchange of oppositely charged ions (Cl– for H+). Transport-cycle models to describe and explain this unusual mechanism have been proposed based on known CLC structures. While the proposed models harmonize with many experimental findings, gaps and inconsistencies in our understanding have remained. One limitation has been that global conformational change – which occurs in all conventional transporter mechanisms – has not been observed in any high-resolution structure. Here, we describe the 2.6 Å structure of a CLC mutant designed to mimic the fully H+-loaded transporter. This structure reveals a global conformational change to improve accessibility for the Cl– substrate from the extracellular side and new conformations for two key glutamate residues. Together with DEER measurements, MD simulations, and functional studies, this new structure provides evidence for a unified model of H+/Cl– transport that reconciles existing data on all CLC-type proteins.
