A CLC-type F-/H+ antiporter in ion-swapped conformations

Fluoride/proton antiporters of the CLC F family combat F – toxicity in bacteria by exporting this halide from the cytoplasm. These transporters belong to the widespread CLC superfamily but display transport properties different from those of the well-studied Cl – /H + antiporters. Here, we report a structural and functional investigation of these F – -transport proteins. Crystal structures of a CLC F homolog from Enterococcus casseliflavus are captured in two conformations with simultaneous accessibility of F – and H + ions via separate pathways on opposite sides of the membrane. Manipulation of a key glutamate residue critical for H + and F – transport reverses the anion selectivity of transport; replacement of the glutamate with glutamine or alanine completely inhibits F – and H + transport while allowing for rapid uncoupled flux of Cl – . The structural and functional results lead to a ‘windmill’ model of CLC antiport wherein F – and H + simultaneously move through separate ion-specific pathways that switch sidedness during the transport cycle. Crystal structures of the CLC F proton-coupled fluoride antiporter Eca in two conformations capture two rotamers of the gating glutamate and reveal simultaneous accessibility of F – and H + ions via separate pathways on opposite sides of the membrane.