Lipid packing drives the segregation of transmembrane helices into disordered lipid domains in model membranes

Abstract

Cell membranes are comprised of multicomponent lipid and protein mixtures that exhibit a complex partitioning behavior. Regions of structural and compositional heterogeneity play a major role in the sorting and self-assembly of proteins, and their clustering into higher-order oligomers. Here, we use computer simulations and optical microscopy to study the sorting of transmembrane helices into the liquid-disordered domains of phase-separated model membranes, irrespective of peptide-lipid hydrophobic mismatch. Free energy calculations show that the enthalpic contribution due to the packing of the lipids drives the lateral sorting of the helices. Hydrophobic mismatch regulates the clustering into either small dynamic or large static aggregates. These results reveal important molecular driving forces for the lateral organization and self-assembly of transmembrane helices in heterogeneous model membranes, with implications for the formation of functional protein complexes in real cells.

Fig. 1.

Simulation of the sorting and clustering of WALP peptides in a model bilayer with coexisting fluid domains. (A) WALP peptides (colored spheres) embedded in a ternary mixture of di-C16∶0PC (green), di-C18∶2PC (red), and cholesterol (gray), solvated by water (blue). (B) Coarse-grained representation of WALP23 (cyan and yellow), di-C16∶0PC (green), di-C18∶2PC (red), and cholesterol (gray), shown as spheres and sticks. The atomistic structure of the peptide is also shown (sticks). (C and D) Sorting and clustering of WALP23 (C) and WALP31 peptides (D) in the disordered lipid domain.

Fig. 2.

Confocal images of GUVs containing Alexa Fluor 488-labeled WALP peptides. (A and B) The vesicles were composed of di-C18∶1PC/cholesterol/egg-sphingomyelin (1∶1∶1) and WALP23 (blue fluorescence channel) and DiD-C18∶0 (red). (C and D) The vesicles contained di-C14∶1PC/cholesterol/egg-sphingomyelin (1∶1∶1) and WALP27 (blue) and DiD-C18∶0 (red).

Fig. 3.

Clustering of WALP peptides during simulations. The number of WALP23 (A) or WALP31 (B) peptides that are present as a monomer, or part of a dimer, trimer, or a larger cluster are plotted over time. Two WALPs were considered bound if the distance between any two particles of the peptides was smaller than 0.7 nm.

Fig. 4.

Proteins moving lipids moving proteins. A WALP23 (orange spheres), initially located in the Lo phase (green), gets incorporated into the Ld phase (red) through concerted peptide and lipid motion. (A) A peninsula of di-C18∶2PC lipids bridges the peptide to the core of the Ld domain. (B) The peptide gathers an island of di-C18∶2PC lipids (red) in its environment. (C and D) The di-C18∶2PC island merges with the Ld domain, and the peptide subsequently diffuses away from the domain boundary.