TCB Publications - Abstract

Chandra M Khantwal, Sherwin J Abraham, Wei Han, Tao Jiang, Tanmay S Chavan, Ricky C Cheng, Shelley M Elvington, Corey W Liu, Irimpan I Mathews, Richard A Stein, Hassane S Mchaourab, Emad Tajkhorshid, and Merritt Maduke. Revealing an outward-facing open conformational state in a CLC Cl/H exchange transporter. eLife, pp. e11189, 2016. (PMC: PMC4769167)

KHAN2016-ET 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 $^{19}$F NMR, we show that as [H$^+$] is increased to protonate Glu$_{ex}$ and enrich the outward-facing state, a residue $\sim$20 Å away from Glu$_{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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