The role of the membrane in driving CLC-ec1 dimerization.

Abstract

Most membrane proteins oligomerize in the membrane. In the case of the bacterial chloride/proton antiporter CLC-ec1, the role of highly stable homodimers is not understood since individual CLC-ec1 protomers can maintain the chloride/proton transport stoichiometry. In a previous study, we used experimental and computational approaches to show that in C16:0/C18:1 palmitoyl-oleoyl lipid membranes, the monomeric state of CLC-ec1 creates a membrane defect that is ≈ 8 Å thinner than the bulk. However, in the dimer state, the membrane is restored to a near-native bilayer thickness and structure. We hence proposed that the dimerization is driven by the larger free-energy cost of lipid solvation of the dissociated monomers. Here, we carry out coarse-grained molecular dynamics simulations with umbrella sampling and free energy perturbation methods to quantify the membrane's contribution to free energy of dimerization. In the case of CLC-ec1, addition of 30% C12:0 di-lauryl chain lipids stabilizes the free energy of the monomeric state by ≈ 2 kcal/mole, in qualitative agreement with our experimental observations. Therefore, we have a physical model of how changes in lipid composition can regulate self-assembly reactions of membrane proteins in membranes. This work was partially performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-840782.