Transbilayer Asymmetry in the Lipid Distribution and Stability of Nanoscale Lipid Domains

Mohamed Laradji

Physics Department, The University of Memphis

Biomembraes are self-assembled quasi-two-dimensional complex fluids composed essentially of phospholipids and cholesterol. The primary roles of biomembranes are the separation between the inner and outer environments of the cell or inner organelles and the support of a specialized structural and physiological protein-based machinery. Studies have shown that plasma membranes of eukaryotic cells are characterized by both compositional and conformational heterogeneities. Among these heterogeneities are nanoscale lipid domains on the outer leaflet, called rafts, that are mainly composed of saturated sphingomyelin and cholesterol, and are believed to be essential for signaling, recruitment of specific proteins and endocytosis. Elucidation of the mechanisms leading to the stability of finite size lipid rafts remains elusive. In particular, recent experiments on multicomponent giant vesicles observed micron-scale lipid domains that are rich in cholesterol. These domains are, however, many orders of magnitude larger than lipid rafts in plasma membranes. In this talk, I will present results of large scale dissipative particle dynamics simulations of self-assembled multicomponent lipid membranes in explicit solvent. I will show that the account for the asymmetry in lipid distribution across the membrane--which is not accounted for in the experimental multicomponent lipid vesicles--induces a spontaneous curvature and lead to finite size domains for intermediate values of the membrane tension. The stability of finite size domains results from the interplay between line tension, surface tension and spontaneous curvature. I will also discuss other possible mechanisms that can also contribute to the stability of nanoscale lipid domains in biomembranes.