Molecular mechanism of proton transport in CLC Cl-/H+ exchange transporters

CLC proteins underlie muscle, kidney, bone, and other organ system function by catalyzing the transport of Cl- ions across cell and organellar membranes. Some CLC proteins are ion channels while others are pumps that exchange Cl- for H+. The pathway through which Cl- ions cross the membrane has been characterized, but the transport of H+ and the principle by which their movement is coupled to Cl- movement is not well understood. Here we show that H+ transport depends not only on the presence of a specific glutamate residue but also the presence of Cl- ions. H+ transport, however, can be isolated and analyzed in the absence of Cl- by mutating the glutamate to alanine and adding carboxylate-containing molecules to solution, consistent with the notion that H+ transfer is mediated through the entry of a carboxylate group into the anion pathway. Cl- ions and carboxylate interact with each other strongly. These data support a mechanism in which the glutamate carboxylate functions as a surrogate Cl- ion, but it can accept a H+ and transfer it between the external solution and the central Cl- binding site, coupled to the movement of 2 Cl- ions.