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Mchaourab Lab

Revealing an outward-facing open conformational state in a CLC Cl(-)/H(+) exchange transporter.

AUTHORS

Khantwal CMChandra M , Abraham SJ Sherwin J , Han W Wei , Jiang T Tao , Chavan TS Tanmay S , Cheng RC Ricky C , Elvington SM Shelley M , Liu CW Corey W , Mathews II Irimpan I , Stein RA Richard A , Mchaourab HS Hassane S , Tajkhorshid E Emad , Maduke M Merritt . eLife. 2016 ; 5().

ABSTRACT

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 (Gluex) upon its protonation. Using (19)F NMR, we show that as [H(+)] is increased to protonate Gluex and enrich the outward-facing state, a residue ~20 Å away from Gluex, 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.