Challenging the Dogma - Cell Plasma Membranes are Asymmetric not only in Phospholipid Composition but also Abundance

Abstract

The transbilayer distribution of lipids in mammalian plasma membranes (PMs) is functionally important and incompletely understood. It has been generally assumed that the two PM leaflets contain similar number of phospholipids (PLs) due to the constraint that their areas must be matched in a bilayer. Contrary to this widely held assumption, our recent detailed lipidomics analysis of human erythrocytes reveals a large phospholipid imbalance between PM leaflets, with the cytoplasmic leaflet possessing almost 2-fold more PLs as the exoplasmic one. This surprising finding is consistent with several previous observations and challenges our understanding of living membrane organization and structure. To investigate the effects of differential PL abundance on membrane properties, we performed long atomistic molecular dynamics simulations of various PM models with data-driven leaflet compositions and different PL imbalances between the leaflets. The integrity of the bilayer was preserved in part by (re)distribution of the PM's other major component, cholesterol, which shows a strong preference for the exoplasmic leaflet. Detailed leaflet-specific analysis of the physical properties of the membranes revealed systematic changes as the relative number of PLs in the exoplasmic leaflet decreased. These changes were accompanied by differences in the water permeability of the leaflets, their pressure distribution and propensity for hydrophobic defects. To determine which PM model most closely represents the cell PM we calibrated an in vitro reporter of membrane packing to simulations using experimental measurements of model membranes, and compared the simulated models to live cells PMs. Our results were consistent with a model of living PMs with far more cholesterol and fewer PLs in the exoplasmic leaflet. They also reveal a substantial amount of stored tension in the bilayer leaflets, uncovering a host of previously unconsidered physiological implications.