Special Session 22: Modeling and Analysis of Multi-Phase Lipid Bilayer Membranes

Lipid bilayer membranes of biological cells are likely to be non-random mixtures of membrane components. Lateral membrane domain formation is thought to be involved in essential functions of the membrane, including signaling, sorting, and trafficking. In order to elucidate the physical, mechanical, and physico-chemical basis and consequences of membrane heterogeneity, model systems have been developed. These typically consist of ternary lipid mixtures that under suitable conditions segregate into two fluid phases, a liquid ordered, and a liquid disordered phase. Of particular interest to us are the boundaries of domains in membranes with phase coexistence due to interfacial tension (line tension) at fluid phase boundaries. We find that this line tension couples to three dimensional membrane shape, modulating biologically relevant phenomena, including vesicle budding and fission. Line tension is also a control parameter regulating domain size and growth kinetics. We are therefore developing methods to precisely measure line tension as a function of membrane composition, using micropipette aspiration of giant vesicles, as well as capillary wave spectroscopy of thermal boundary fluctuations. We demonstrate that these two complementary techniques probe different line tension regimes. Furthermore, we are developing experimental methods to investigate the partitioning of both lipids and proteins among curvature gradients. We find that lipids are not detectably sorted among membrane with steep curvature difference, whereas peripherally membrane binding proteins are efficiently sorted. We discuss the biological relevance of our findings.