Structure of the CLC-1 chloride channel from Homo sapiens.

Eunyong Park,Roderick Mackinnon

doi:10.7554/elife.36629

Abstract

CLC channels mediate passive Cl- conduction, while CLC transporters mediate active Cl- transport coupled to H+ transport in the opposite direction. The distinction between CLC-0/1/2 channels and CLC transporters seems undetectable by amino acid sequence. To understand why they are different functionally we determined the structure of the human CLC-1 channel. Its 'glutamate gate' residue, known to mediate proton transfer in CLC transporters, adopts a location in the structure that appears to preclude it from its transport function. Furthermore, smaller side chains produce a wider pore near the intracellular surface, potentially reducing a kinetic barrier for Cl- conduction. When the corresponding residues are mutated in a transporter, it is converted to a channel. Finally, Cl- at key sites in the pore appear to interact with reduced affinity compared to transporters. Thus, subtle differences in glutamate gate conformation, internal pore diameter and Cl- affinity distinguish CLC channels and transporters.

Highlights

Transporters – known as pumps – and channels both mediate the transfer of ions and molecules across biological membranes
Because the CLC-1 structure suggests that T475 and G483 likely contribute to lowering of the kinetic barrier, we compared amino acids lining this region among both CLC channels and transporters (Figure 6—figure supplement 1)
The outer gate of the channel remains open because the carboxylic side-chain Glutamate gate (Glugate) is located off to the side, away from the ClÀ transport pathway (Figure 4)

Summary

Introduction

Transporters – known as pumps – and channels both mediate the transfer of ions and molecules across biological membranes. The transfer would naturally give rise to the 2:1 ClÀ: H+ exchange stoichiometry characteristic of CLC transporters because 2 ClÀ ions must be displaced when the glutamate gate moves between the extracellular solution and Scen This mechanism is consistent with the demonstrated conversion of a CLC transporter into a passive (but slow) ClÀ channel upon mutation of the glutamate, as well as the demonstrated ability of small carboxylate-containing organic acids to compete with ClÀ inside the pore (Accardi et al, 2004; Accardi and Miller, 2004; Feng et al, 2012). Data for CLC transporters seem consistent with this mechanism: they have a channel-like pore, an external ‘glutamate gate’ that competes with ClÀ binding and (presumably) transfers H+ across the membrane, and structurally what appears to be a relatively high resistance (i.e., a large kinetic barrier) to ClÀ flow near the intracellular aspect of the pore (i.e., the pore there is very narrow.). We are interested in the CLC-1 channel because it plays an important role in membrane repolarization of skeletal muscle cells following muscular contraction, and its mutation in humans causes hereditary muscle disorders known as myotonia congenita (George et al, 1993; Koch et al, 1992; Lorenz et al, 1994; Steinmeyer et al, 1991)

Results

D CmCLC transporter

Discussion

Materials and methods