CLC channels and transporters: Proteins with borderline personalities

Abstract

Controlled chloride movement across membranes is essential for a variety of physiological processes ranging from salt homeostasis in the kidneys to acidification of cellular compartments. The CLC family is formed by two, not so distinct, sub-classes of membrane transport proteins: Cl− channels and H+/Cl− exchangers. All CLC’s are homodimers with each monomer forming an individual Cl− permeation pathway which appears to be largely unaltered in the two CLC sub-classes. Key residues for ion binding and selectivity are also highly conserved. Most CLC’s have large cytosolic carboxy-terminal domains containing two cystathionine β-synthetase (CBS) domains. The C-termini are critical regulators of protein trafficking and directly modulate Cl− by binding intracellular ATP, H+ or oxidizing compounds. This review focuses on the recent mechanistic insights on the how the structural similarities between CLC channels and transporters translate in unexpected mechanistic analogies between these two sub-classes.

A Single-Pore Residue Renders the Arabidopsis Root Anion Channel SLAH2 Highly Nitrate Selective

“…Here, we used Xenopus laevis oocytes to study the anion selectivity of SLAH2 and compared it to that of SLAC1. In this way, we were able to establish SLAH2, in contrast to the NO 3 2 and Cl 2 permeable SLAC1, and homologs from other kingdoms, as an anion channel that transports nitrate exclusively (Chen and Hwang, 2008;Jentsch, 2008;Picollo et al, 2009;Accardi and Picollo, 2010;Hedrich, 2012;Stauber et al, 2012). Based on homology models for SLAC1 and SLAH2, structure-guided sitedirected mutations were used to explore the molecular basis of the extraordinarily high nitrate selectivity of SLAH2.…”

Water access points and hydration pathways in CLC H + /Cl − transporters

“…The latter, also known as H + /Cl − exchangers, drive uphill movement of H + by coupling the process to downhill movement of Cl − or vice versa, thereby exchanging the two types of ions across the membrane at fixed stoichiometry (9). ClC-ec1, a CLC from Escherichia coli, has served as the prototype CLC for biophysical studies because of its known crystal structures (10,11), its tractable biochemical behavior, and its structural and mechanistic similarities to mammalian CLC transporters (3)(4)(5)(6)(7)(8)(12)(13)(14)(15)(16)(17). Detailed structural and functional studies of ClC-ec1 (9,11,(18)(19)(20)(21)(22)(23)(24)(25)(26)(27) have shed light on some of its key mechanistic aspects.…”

“…The results, by providing a detailed microscopic view of the dynamics of water wire formation and confirming the involvement of specific protein residues, offer a mechanism for the coupled transport of H + and Cl − ions in CLC transporters. membrane transporters | membrane proteins | membrane exchangers | antiporters | coupling mechanism T he chloride channel (CLC) family (1, 2) includes both passive Cl − channels and secondary active H + -coupled Cl − transporters (3)(4)(5)(6)(7)(8). The latter, also known as H + /Cl − exchangers, drive uphill movement of H + by coupling the process to downhill movement of Cl − or vice versa, thereby exchanging the two types of ions across the membrane at fixed stoichiometry (9).…”

Fluoride resistance and transport by riboswitch-controlled CLC antiporters

“…This family turned out to be split into proteins of two mechanistically disparate subtypes: anion channels and Cl − ∕H þ antiporters (3)(4)(5)(6). CLCs participate in diverse biological tasks requiring transmembrane anion conductance, such as acidification of lysosomes, control of skeletal muscle excitability, renal regulation of blood pressure, and extreme acid resistance in enteric bacteria (7). Most CLCs thus far studied use Cl − for their physiological purposes, but NO 3 − has been identified as the substrate anion in a plant vacuolar CLC (8).…”