Abstract
ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue (“fast gate”) known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule (“slow gating”). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1–related diseases.
Highlights
CLC proteins comprise a large family of chloride (Cl−)-transporting integral membrane proteins with diverse physiological functions [1,2,3]
Chloride transporting CLC proteins are expressed in a wide range of organisms, and the family encompasses several members with numerous roles in human health and disease
Physiologically essential because intense muscle exercise leads to acidosis, resulting in an increased nucleotide sensitivity of ClC-1 and consequent reduction of gCl, thereby assisting in preventing muscle fatigue [20, 21]
Summary
CLC proteins comprise a large family of chloride (Cl−)-transporting integral membrane proteins with diverse physiological functions [1,2,3]. The molecular mechanisms of ion transport in CLC antiporters have been extensively studied functionally and structurally [8, 10,11,12,13,14,15]. Activity of ClC-1 is modulated by cellular cues such as phosphorylation [17], pH, and nucleotides [18, 19] in an unknown manner Such regulation is, physiologically essential because intense muscle exercise leads to acidosis, resulting in an increased nucleotide sensitivity of ClC-1 and consequent reduction of gCl, thereby assisting in preventing muscle fatigue [20, 21]
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