Interaction Forces between Lipid Rafts.

Cellular membranes containing sphingolipids and cholesterol have been shown to self-organize into lipid rafts-specialized domains that host integral membrane proteins and modulate the bioactivity of cells. In this work, force-distance profiles between raft membranes in the liquid-ordered phase consisting of singly unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a complex mixture of brain sphingomyelin (BSM), and cholesterol were measured using the surface force apparatus (SFA). Two distinct force profiles were detected corresponding to uniform raft membranes and raft membranes with a higher level of topological membrane defects (heterogeneous) as corroborated by atomic force microscopy (AFM) scans. In all cases a weak, long-range electrostatic repulsion was observed with some variation in the surface charge density. The variation in electrostatic repulsion was attributed to charged lipid species primarily from the constituent lipids in the BSM mixture. The adhesion between the uniform raft membranes was comparable to our previous work with pure component, liquid-ordered POPC-DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine)-cholesterol membranes. Raft membranes with more topological defects adhered more strongly owing to hydrophobic attraction between exposed acyl chains. Even though the rafts were in the liquid-ordered phase and membrane defects were present in the contact region, the raft membranes were stable, and no structural rearrangement was observed throughout the measurements. Our findings demonstrate that liquid-ordered membranes are stable to mechanical loading and not particularly sensitive to compositional variation.