Biophysical Society Conference | Tahoe 2024

Molecular Biophysics of Membranes

Thursday Speaker Abstracts

ASYMMETRIC DISTRIBUTION OF PHOSPHOLIPIDS AND CHOLESTEROL RESULTS IN UNIQUE PLASMA MEMBRANE PROPERTIES Kandice R. Levental 1 ; Milka Doktorova 1 ; Fred Heberle 2 ; Ed Lyman 3 ; Ilya Levental 1 ; 1 University of Virginia, Molecular Physiology and Biophysics, Charlottesville, VA, USA 2 University of Tennessee, Chemistry, Knoxville, TN, USA 3 University of Delaware, Physics and Astronomy, Newark, DE, USA The plasma membrane is the interface between a cell and its environment and is therefore responsible for a myriad of parallel processing tasks that must be tightly regulated to avoid aberrant signaling. To achieve this functional complexity, mammalian cells produce hundreds of lipid species, nearly all of which are asymmetrically distributed between the two membrane leaflets. Using quantitative lipidomics we determined the asymmetric distribution of all phospholipids in human erythrocyte plasma membranes. In addition to defining the asymmetric lipidomes of the two PM leaflets, we discovered that the cytoplasmic leaflet contains 50% more phospholipids than the exoplasmic leaflet. This finding invalidates the long-standing assumption that the two leaflets of a lipid bilayer should contain equal numbers of phospholipids. We show that this imbalance of phospholipids in the plasma membrane is enabled by the large abundance of cholesterol (~40%) in the plasma membrane, which can rapidly flip between leaflets to buffer stresses. Through computational and experimental approaches, we find that the combination of the preference of cholesterol for the more saturated exoplasmic lipids and the overabundance of lipids in the cytoplasmic leaflet yields a dramatic enrichment of cholesterol in the exoplasmic leaflet. Furthermore, we show that this asymmetric distribution of phospholipids and cholesterol results in unique plasma membrane properties, namely low permeability, fast cholesterol diffusion, and more cytoplasmic hydrophobic defects which enables lipidated protein interactions with the cytoplasmic leaflet. Our observations of these previously overlooked aspects of membrane asymmetry represents an evolution of existing models of plasma membrane structure and physiology.

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