Paper BI-TuP21
First Direct Observation of Membrane Lipid Asymmetry Induced by Polypeptide Association

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Cellular membranes of eukaryotic cells are characterized by a heterogeneous distribution of phospholipids, which is key to many physiological functions. While some membranes (such as the endoplasmic reticulum) are symmetric, others (as the plasma membrane) are asymmetric with phosphatidylethanolamine and phosphatidylserines being primarily localized in the inner leaflet. The loss of asymmetry in plasma membranes has been thought to have direct relevance to numerous physiological and pathological events, such as phagocytosis and cell apoptosis. Although much is know about the biosynthesis of phospholipids, little is known about their mechanism of translocation and even less is known about the mechanism which produces or maintains lipid asymmetry in membranes. The present study shows for the first time, spontaneous development of asymmetry in a DSPC-d₇₀/DSPS (1,2-distearoyl-sn-glycero-3-phosphocholine /1,2-distearoyl-sn-glycero-3-[phospho-L-serine] planar supported lipid bilayer, in the presence of a positively charged polypeptide (polylsyine). The negatively charged DSPS molecules selectively localize in the top leaflet of the bilayer due to the electrostatic association with the polypeptide. The highly surface specific, second order nonlinear spectroscopy, sum frequency vibrational spectroscopy (SFVS), has been used since it offered the opportunity to study the kinetics of unlabeled lipids that were not modified by a fluorescent or spin-probe.