Fxc3X96Rster Resonance Energy Transfer In Lipid Bilayers: Theory And Applications For Examining Phase Behavior

Abstract

The spatial organization of lipids and proteins in biological membranes is of considerable interest. Multiple lines of evidence support the existence of specialized lipid microdomains or "membrane rafts", that are involved in processes as diverse as protein sorting, vesicular transport, viral entry and exit from cells, and signaling. The chemical complexity of cell membranes, and the small size and fleeting nature of rafts, pose serious challenges to experimentalists who seek to understand their thermodynamic origins. Using high-resolution FRET and ESR measurements, we have examined phase behavior, order parameters, and the partitioning behavior of fluorescent probes in three ternary lipid mixtures that are models for the outer leaflet of mammalian plasma membranes. In two of these mixtures, we observe a region of coexisting fluid phases characterized by small (nanometer scale) phase domains. We developed a FRET model suitable for characterizing the size of these small domains, and were able to recover domain sizes and probe partition coefficients with good accuracy in a global analysis of simulated FRET data. These studies confirm that many of the critical properties of membrane rafts can be reproduced in lipid-only mixtures. Systematic study of these nanodomain mixtures will provide insight into the thermodynamic origins of membrane rafts.