Compositional Heterogeneity In Biologically Relevant Membrane Models

Abstract

Lateral organization of the cellular plasma membrane promotes biological function by permitting regulation of cellular processes. Evidence now supports the hypothesis that the coexistence of a variety of lipid molecules, with different melting temperatures and acyl chain lengths, contribute to this lateral organization by forming lipid rafts, nanoscale domains with distinct biophysical properties. Phase separation into immiscible micron-sized domains is readily observed in model membranes, chemically simplified lipid mixtures that have been studied under equilibrium conditions to understand how composition and temperature affect domain properties. As lipid domains have yet to be imaged directly in live resting cells, the relevance of model membranes is uncertain. We have focused on characterizing the biologically relevant outer leaflet membrane model brain sphingomyelin (bSM)/1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC)/cholesterol (Chol) in which nanoscale domains challenge conventional imaging techniques. We have determined the temperature-dependent ternary phase diagrams for bSM/POPC/Chol and bSM/1,2-dioleoyl-sn-glycero-3phosphocholine (DOPC)/Chol using Förster resonance energy transfer (FRET) and differential scanning calorimetry, and we have confirmed the biologically relevant liquid-disordered (Ld) and liquid-ordered (Lo) coexistence region using electron spin resonance spectroscopy. We have determined that the size of coexisting Ld+Lo domains in bSM/POPC/Chol is 2-6 nm radius using FRET and small-angle neutron scattering. Ultimately, this careful characterization of model membrane will serve as a starting point for investigating the influence of peptides on domain size and other biophysical properties. Rafts can be stabilized to form larger platforms through protein-protein and protein-lipid interactions. Understanding how these domains form, grow, and stabilize in model systems is a first step toward elucidating their roles in important membrane-mediated processes