Khantwal, Chandra M.; Abraham, Sherwin J.; Han, Wei; Jiang, Tao; Chavan, Tanmay S.; Cheng, Ricky C.; Elvington, Shelley M.; Liu, Corey W.; Mathews, Irimpan I.; Steins, Richard A.; Mchaourab, Hassane S.; Tajkhorshid, Emad; Maduke, Merritt
Revealing an outward-facing open conformational state in a CLC Cl-/H+ exchange transporter
ELIFE, 5 Art. No. e11189, JAN 22 2016

CLC secondary active transporters exchange CL- for H+. Crystal structures have suggested that the conformational change from occluded to outward-facing states is unusually simple, involving only the rotation of a conserved glutamate (Glu(ex)) upon its protonation. Using F-19 NMR, we show that as [H+] is increased to protonate Gluex and enrich the outward-facing state, a residue similar to 20 angstrom away from Glu(ex), near the subunit interface, moves from buried to solvent-exposed. Consistent with functional relevance of this motion, constriction via inter-subunit cross-linking reduces transport. Molecular dynamics simulations indicate that the cross-link dampens extracellular gate-opening motions. In support of this model, mutations that decrease steric contact between Helix N (part of the extracellular gate) and Helix P (at the subunit interface) remove the inhibitory effect of the cross-link. Together, these results demonstrate the formation of a previously uncharacterized 'outward-facing open' state, and highlight the relevance of global structural changes in CLC function.


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