Small changes in lipid backbone linkages can lead to big changes in liquid-liquid phase separation in model membranes.

Abstract

Cells can dynamically alter the spatial organization of lipids and proteins in the plasma membrane (PM) to regulate processes including cell signaling. The size and morphology of membrane domains, and how these depend on lipid composition, have drawn attention as potentially important variables in these processes. Model membrane studies have proven invaluable for elucidating the influence of phospholipid chains and headgroups, as well as cholesterol concentration, on membrane phase behavior. Following this approach, we have investigated simplified three- and four-component models for the PM outer leaflet to understand the role of the lipid backbone. Specifically, we replaced ester-linked DPPC and DOPC with their ether-linked counterparts to selectively change the membrane dipole potential of coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases, while titrating DOPC with POPC to control the line tension. Fluorescence imaging of GUVs gave insight into the macroscopic phase behavior, while FRET and cryo-EM of LUVs allowed us to probe nanoscale heterogeneity. Our results can be understood in terms of a model in which the tendency to minimize line tension is counteracted by dipole repulsion, which can in some cases result in stable arrays of small domains or modulated phases. These findings suggest that relatively small changes in the structure of the lipid backbone can influence domain formation and morphology in biomimetic membranes.