Coupling between sterol and sphingolipid structure in ordered membrane domains
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A hallmark of eukaryotic membranes is the pairing of lineage-specific sterols with characteristic sphingolipid species. Mammalian cell membranes are enriched in both cholesterol and long-chain sphingolipids like sphingomyelin, whereas fungi synthesize ergosterol and very long-chain sphingolipids with sugar-containing head groups. It has been proposed that these two lipid classes co-evolved to support membrane structure and organization. Here we investigated how sterol structure and sphingolipid chain length together control membrane order and phase behavior. In the yeast Saccharomyces cerevisiae , loss of very long-chain C26 sphingolipids disrupted formation of liquid-ordered ( L o ) domains in the vacuole membrane. Similarly, substitution of ergosterol synthesis for that of cholesterol also prevented vacuole L o domains. To determine a possible physical basis of these effects, we investigated synthetic membranes of defined composition containing either ergosterol or cholesterol and sphingomyelin with different chain lengths. In membranes containing egg sphingomyelin with C16 chains, ergosterol only sparsely supported L o domains, in contrast to cholesterol. Membranes containing sphingomyelin with C26 chains displayed a different pattern. Cholesterol mixtures were largely homogeneous across most compositions, with only a limited region that supported fluid domains. Ergosterol mixtures exhibited a distinct compositional window that supported fluid domains positioned between regimes of uniform membranes and gel phases. This window corresponded to stoichiometric changes in the vacuole as it phase-separates during nutritional restriction. Measurements of membrane order showed that cholesterol strongly increased membrane packing compared to ergosterol in membranes containing egg sphingomyelin, whereas this difference was lost in membranes containing C26 sphingomyelin. The results suggest that sphingolipid chain length can tune sterol interactions needed for membrane organization.
Significance
Membrane phase separation into coexisting ordered and disordered fluid domains has largely been investigated using characteristic mammalian lipid components, cholesterol and long-chain saturated lipids like sphingomyelin. Under nutrient limitation, vacuole membranes in yeast organize into micron-scale domains that are important for their physiology. Compared to mammals, yeast synthesize an alternative sterol, ergosterol, and sphingolipids with very long-chains. We show that vacuole membrane domains are sensitive to both these features, which also show preferential interactions in liposomes that support membrane ordering and phase properties. In lipid mixtures containing very long-chain sphingomyelins, stoichiometric regimes that support phase separation of fluid domains are similar to those of the vacuole lipidome under nutrient limitation. This finding supports a model in which sterols and sphingolipids co-evolved to support membrane structure.
