Sorting of streptavidin protein coats on phase-separating model membranes.

Heterogeneities in cell membranes due to the ordering of lipids and proteins are thought to play an important role in enabling protein and lipid trafficking throughout the secretory pathway and in maintaining cell polarization. Protein-coated vesicles provide a major mechanism for intracellular transport of select cargo, which may be sorted into lipid microdomains; however, the mechanisms and physical constraints for lipid sorting by protein coats are relatively unexplored. We studied the influence of membrane-tethered protein coats on the sorting, morphology, and phase behavior of liquid-ordered lipid domains in a model system of giant unilamellar vesicles composed of dioleoylphosphatidylcholine, sphingomyelin, and cholesterol. We created protein-coated membranes by forming giant unilamellar vesicles containing a small amount of biotinylated lipid, thereby creating binding sites for streptavidin and avidin proteins in solution. We found that individual tethered proteins colocalize with the liquid-disordered phase, whereas ordered protein domains on the membrane surface colocalize with the liquid-ordered phase. These observations may be explained by considering the thermodynamics of this coupled system, which maximizes its entropy by cosegregating ordered protein and lipid domains. In addition, protein ordering inhibits lipid domain rearrangement and modifies the morphology and miscibility transition temperature of the membrane, most dramatically near the critical point in the membrane phase diagram. This observation suggests that liquid-ordered domains are stabilized by contact with ordered protein domains; it also hints at an approach to the stabilization of lipid microdomains by cross-linked protein clusters or ordered protein coats.