Insights on plasma membrane asymmetry from studies of asymmetric model membranes.

Abstract

Cells spend a lot of energy to maintain the plasma membrane as asymmetric, with distinct leaflet compositions. In contrast, unhealthy cells, or cells with a signal to die, typically lose this asymmetry. Certainly, cells would evolve to such high complexity for a purpose. Studies in model membranes with rationally simplified lipid composition point to an exoplasmic model leaflet with coexistence of liquid disordered (Ld) and liquid ordered (Lo) phases. The apposed cytoplasmic leaflet lacks the large fraction of high melting lipids needed to sustain phase separation. To investigate properties of the plasma membrane asymmetry, we use a new method to engineer asymmetric bilayers by hemifusion between giant unilamellar vesicles (GUVs) and a supported lipid bilayer (SLB). The newly exchanged lipids from the SLB replace the lipids on the GUV outer leaflet, forming asymmetric vesicles (aGUVs). Little perturbation was observed in the vesicle content of these aGUVs. We assemble a phase-separated Ld+Lo leaflet apposed to a leaflet with a uniform Ld composition. For several different lipid mixtures, we observe an ordered phase inducing exactly apposed order on the fluid leaflet. Even with the challenge to track cholesterol movements, our results suggest a composition change, in which induced order domains become enriched with cholesterol. This result brings a new picture and a significant implication for the plasma membrane: The cytoplasmic leaflet has “raft-like” domains induced by the exoplasmic leaflet. Membrane-bound molecules in the cytoplasmic leaflet that can move laterally must partition between disordered regions and induced order regions, as we observe with lipid dyes. Based on line tension estimates, we explain why the induced order domains would be macroscopic domains, or modulated phases, or else nanodomains.