The transbilayer distribution of lipids in mammalian plasma membranes (PMs) is functionally important and incompletely understood. While the two leaflets’ lipid compositions have recently been determined in full detail, much less is known about their respective total abundances of phospholipids (PLs) and cholesterol. Contrary to the widely held assumption of approximately equivalent amounts of PLs in the two PM leaflets, our lipidomic analysis of human erythrocytes indicates instead a large phospholipid imbalance between them, with the cytoplasmic leaflet possessing almost 2-fold more PLs than the exoplasmic one. This surprising finding is consistent with several previous analyses and challenges our understanding of living membrane organization and structure. To examine the biophysical manifestations of this differential PL abundance, we performed a long atomistic molecular dynamics simulation of a PM model with data-driven leaflet compositions and overpopulated cytoplasmic leaflet. The integrity of the bilayer was preserved in part by (re)distribution of the PM’s other major component, cholesterol, which showed a strong preference for the sphingomyelin-rich exoplasmic leaflet. We therefore hypothesized that PM cholesterol in mammalian cells is enriched in the outer leaflet with its distribution strongly dependent on PL abundance asymmetry. To test this hypothesis, we performed additional simulations of compositionally similar PM models, but with exoplasmic leaflets that were either equally- or over-populated with PLs. Consistent with our predictions, cholesterol’s distribution changed systematically depending on the differential PL abundance. To confirm the biological relevance of the proposed PM architecture, we validated our simulation against in vivo measurements of lipid packing and cholesterol diffusion. Our results not only reaffirm the relative depletion of PLs and enrichment of cholesterol in the exoplasmic PM leaflet, but also reveal the unique biophysical signatures of PM number and composition asymmetries in differential pressure distribution, water permeability, and propensity for hydrophobic defects of the two leaflets.

Challenging the dogma: Cell plasma membranes are asymmetric not only in lipid composition but also phospholipid and cholesterol abundance

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.