BPS2026 – Regulation of CLC transporters by dimerization

Structural studies of CLC ion channels and transporter membrane proteins reveal that subunits interact as dimer assemblies. However, functional studies show that only monomers are required for transport activity, raising the question—why are CLCs dimeric? Our lab has carried out an extensive set of studies combining experimental and computational approaches on the prokaryotic homologue CLC-ec1 and has found that its subunits participate in a slow but dynamic monomer-dimer equilibrium in lipid bilayer membranes. Dimerization is thermodynamically driven to minimize exposure of the binding interface to the lipid bilayer, which induces a membrane defect, and incurs an energetic penalty on dissociated state that comes from the membrane alone. Because dimerization is in a dynamic balance, there is the ability to link the dimerization equilibrium to other environmental factors, such as pH and lipid composition. Indeed, neutral pH inhibits CLC-ec1 transport activity, and this also leads to an apparent stabilization of dimerization, yielding a kinetically trapped form that likely constrains intrinsic protein motion. Alternatively, addition of short-chain lipids to the membrane at low levels destabilizes dimerization through the mechanism of preferential solvation, thereby reducing inhibition and increasing transport activity. With these observations, it is proposed that CLC dimerization has evolved to offer a mechanism for coupled inhibition of CLC function. It is postulated that dynamic oligomerization may be a common phenomenon in membrane proteins, as this offers an intrinsic mechanism for dynamic regulation and may be why so many membrane proteins are observed in oligomeric forms, even when the functional pathway is contained within each subunit.