Inter- and Intra-Plane Interactions Control the Existence of Macrodomains in Asymmetric Giant Unilamellar Vesicles

Abstract

The complexity of the plasma membrane (PM) is in part related to the complexity of its composition. Beyond the hundreds of lipid components in the PM, compositional differences between the exoplasmic and the cytoplasmic leaflets leads to different physical-chemical properties in each leaflet affecting the interaction between them. Model membranes prepared in vitro that reproduce salient properties of the PM can reveal molecular determinants of these interactions. We investigate asymmetric giant unilamellar vesicles (aGUVs) prepared with different lipid compositions by hemifusion. Symmetric GUVs composed of the lipid mixture high-melting lipid (HTm)/DOPC/chol (0.39/0.39/0.22), where HTm saturated lipids ranging from 15-18 carbons, were hemifused with a supported lipid bilayer (SLB) composed of DOPC/chol (0.8/0.2). Fusion of the GUV and SLB outer leaflets promoted a range of lipid exchange, creating asymmetric vesicles. The symmetric GUVs composed of the ternary mixtures exhibited coexistence of liquid-disordered (Ld) and liquid-ordered (Lo) phases. The change in the chain lengths implies different intra-plane interactions in which the domain line tension or the difference in order between the Ld and the Lo phases decreases with the saturated chain length. We found that replacement of the GUV outer leaflet by a composition that forms Ld phase can prevent the formation of macrodomains in a manner that depends on the chain length of the saturated lipid and the percentage of lipid exchange. Interestingly, longer saturated chain length promoted the persistence of macroscopic domains to greater degrees of asymmetry. The domain size transition accompanying changes in the degree of asymmetry can be explained by intra- and inter-leaflet interactions. Our results suggest that cells can control the presence or absence of lipid rafts by controlling the types of lipids present in the two leaflets.