Elusive Cell Membrane Transporter Gives Up Key Secrets

Structural rearrangements allow pH-dependent activation of CLC-e1. The video depicts the molecular movements of the CLC transporter from the inactive state to its two active conformations.

The molecular workings of a family of important but hard-to-study proteins called CLCs have been illuminated as never before in a study led by researchers at Weill Cornell Medicine.

CLCs play fundamental roles in biology by regulating the levels of chloride and other ions in cells and by maintaining the acidity of small waste-disposal compartments called lysosomes inside cells. Mutations in the genes for human CLCs cause disorders that affect muscle, brain, bone, the immune system, and other organs.

Despite their clear importance, CLCs have been challenging targets for scientists because of the difficulty of studying their complex, membrane-bound structures and, in particular, determining how these structures open and close to enable the flow of ions.

In the study, which appeared Jan. 26 in Nature Structural and Molecular Biology, the team used single-particle cryo-electron microscopy and an advanced fluorescence imaging technique to detail the molecular movements of a bacterial CLC (CLC-ec1)— the prototypical model for human CLCs—under conditions known to keep it open or closed. They showed how the double-barreled CLC structure undergoes a surprisingly extensive rearrangement under activating conditions, thinning part of the divide between its two halves to allow protons and chloride ion flows.

The mechanisms by which human CLC mutations lead to disease have never been well understood. However, the researchers now show how counterparts of those mutations in CLC-ec1 make the channel harder or easier to open. Supporting the relevance of CLC-ec1 as a model, the researchers demonstrated that gate-opening CLC-ec1 mutations dramatically accelerate gate-opening in a human CLC called CLC-7.

The next step is to investigate how CLCs’ gating mechanisms are normally regulated and “tuned” in cells, according to the investigators.

Senior authors: Dr. Alessio Accardi (Anesthesiology), Dr. Olga Boudker (Physiology & Biophysics)

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