Divergent Cl-and H + pathways underlie transport coupling and gating in CLC exchangers and channels
The CLC family of anion transporting proteins is comprised of secondary active H+-coupled exchangers and of Cl- channels. Both functional subtypes play key roles in human physiology, and mutations causing their dysfunction lead to numerous genetic disorders. Current models suggest that the CLC exchangers do not utilize a classical ping-pong mechanism of antiport, where the transporter sequentially interacts with one substrate at a time. Rather, in the CLC exchangers both substrates bind and translocate simultaneously while moving through partially congruent pathways. How ions of opposite electrical charge bypass each other while moving in opposite directions through a shared permeation pathway remains unknown. Here, we use MD simulations in combination with biochemical and electrophysiological measurements to identify a pair of highly conserved phenylalanine residues that form an aromatic pathway, separate from the Cl- pore, whose dynamic rearrangements enable H+ movement. Mutations of these aromatic residues impair H+ transport and voltage-dependent gating in the CLC exchangers. Remarkably, the role of the aromatic pathway is evolutionarily conserved in CLC channels. Using atomic-scale mutagenesis we show that the electrostatic properties and conformational flexibility of these aromatic residues are essential determinants of channel gating. Our results suggest that Cl- and H+ move through physically distinct and evolutionarily conserved routes through the CLC channels and transporters. We propose a unifying mechanism that describes the gating mechanism of CLC exchangers and channels.