Fig. 1. CLC structure and conformational change compared to conformational change detected in other membrane transporters.

a X-ray crystallographic structure of ClC-ec1 (pdb ID: 1OTS). Each identical subunit catalyzes the exchange of 2 Cl^- for 1 H⁺. The H⁺ pathway bifurcates from the Cl^- pathway, passing along Glu_ex and Glu_in. Glu_ex is also a “gate” that occludes Cl^- from the extracellular solution. An intracellular Cl^- gate is formed by a Ser/Tyr pair (shown in spacefill), the latter residing on Helix R (dark green), a subject of study here.

b Structural comparison of WT ClC-ec1 with the Glu_ex → Gln mutant (E148Q). Left: Close-up of the Cl^--binding region in WT and E148Q. In E148Q, the glutamine side chain is flipped up out of the anion-binding site and replaced by a Cl^- ion (pdb ID: 1OTU).

c Structural comparison of active transporters crystallized in different conformational states. Top left: Backbone overlay of ClC-ec1 WT (orange) and E148Q (blue) structures, 0.6 Å RMSD, illustrates the lack of global conformational change in the putative outward-facing conformational state. Bottom left: GltPh, a glutamate transporter homolog (Focke et al. 2013) in outward-facing (orange) and inward-facing (blue) states (pdb 4OYE and 4P19) (Verdon et al. 2014). Bottom middle: LacY, a member of the Major Facilitator Superfamily of transporters (Yan 2013) in occluded (orange) and inward-facing (blue) conformational states (pdb 4OAA and 2V8N) (Guan et al. 2007; Kumar et al. 2014). Right: Maltose transporter, a member of the ATP-binding cassette (ABC) transporter superfamily (Chen 2013) in inward-facing (orange) and occluded (blue) states (pdb 3FH6 and 2R6G) (Khare et al. 2009; Oldham et al. 2007).