The Interaction of Proteins with Asymmetric Lipid Bilayers

Abstract

Cell membranes can be characterized by their thermodynamic, structural and mechanical properties, such as phase behavior, thickness and rigidity. Experimental and computational studies of membrane properties and their effects on the function of proteins embedded in the lipid environment have focused largely on symmetric bilayers, i.e., with the same lipid composition in the two leaflets. However, the mammalian plasma membrane (PM) has an asymmetric lipid distribution, with an outer leaflet having a significant fraction of high-melting lipids, and an inner leaflet containing low-melting and negatively charged lipids. This asymmetry is often ignored in both experimental and modeling studies especially given the difficulty of preparing asymmetric liposomes in vitro. However, the different elastic and phase properties of symmetric bilayer models of the two PM leaflets, as well as the energy that cells spend to maintain their PM transverse structure, make it likely that this compositional asymmetry provides unique bilayer properties of functional significance. Our goal is to evaluate this hypothesis and its consequences both in vitro and in silico, with a systematic study of symmetric and asymmetric bilayers composed of phosphatidylcholine, sphingomyelin, phosphatidylserine and/or phosphatidylethanolamine lipid molecules both in the presence and absence of the transmembrane peptides WALP and Gramicidin. We examine the structural manifestations and mechanisms of interleaflet coupling and their effect on protein-membrane interactions, focusing on changes in bending rigidity, acyl chain order, and surface tension. We then evaluate the implications of these properties for the unique deformation of the asymmetric bilayers around the protein inclusions. Results from computation regarding inter-leaflet coupling are compared to in vitro measurements of the thermodynamics properties of asymmetric giant unilamellar vesicles prepared with a newly developed cyclodextrin-mediated exchange protocol.