The Phase Behavior Of A Model Plasma Membrane

Abstract

The governing paradigm in biology states that understanding the structure of biological macromolecules and macromolecular assemblies will lead to an understanding of their cellular functions. While much progress has been made in understanding proteins/nucleic acids and their assemblies, the progress in understanding lipids/proteins and their assemblies, or biological membranes, has lagged behind. The main reason lies with the different interactions between the molecules. Proteins/nucleic acids tend to interact with high specificity at certain binding sites, but the interactions between lipids and proteins, especially between lipids themselves, tend not to be specific but show a high level of cooperativity among many molecules. Regarding just the lipids, this cooperativity manifests itself as phase behavior, both chemically, as coexisting phases, and mechanically, as phase domain morphology and dynamics. The chemical and mechanical phase behavior of lipid/protein mixtures directly relates to the physical structure of biological membranes; however, these membranes are much too complex to study directly. Therefore, membranes of simple but controllable compositions are required to model biological membranes. In this study, we explored the chemical and mechanical phase behavior of a ternary lipid mixture meant to model the plasma (outer) membrane of a mammalian cell. We used confocal fluorescence microscopy (CFM) and electron spin resonance (ESR) to construct a compositional phase diagram at 23°C that represents the chemical equilibrium of the model membrane. This phase diagram contains three two-phase coexistence regions (Ld + Lo, Ld + gel, and Lo + gel) and a three-phase coexistence region (Ld + Lo + gel). In addition, using ESR exclusively, we developed and applied a method to determine the infinite number of tie-lines, called the tie-line field, that partition the Ld + Lo coexistence region into the compositions of the coexisting phases. Finally, CFM was used exclusively to explore the phase morphology and dynamics of this model membrane. We observed fluid phase percolation, long-range order/patterns among fluid phase domains, unusual shapes of solid phase domains, phase domain transitions within fluid-fluid phase coexistence, and light-induced phase separation. Elucidating the phase behavior of this model membrane is a step towards understanding the structure of biological membranes.