AbstractMammalian cell sphingolipids, primarily with C24 and C16 acyl chains, reside in the outer leaflet of the plasma membrane. Curiously, little is known how C24 sphingolipids impact cholesterol and membrane microdomains. Here, we generated giant unilamellar vesicles and live mammalian cells with C24 or C16 sphingomyelin exclusively in the outer leaflet and compared microdomain formation. In giant unilamellar vesicles, we observed that asymmetrically placed C24 sphingomyelin suppresses microdomains. Conversely, C16 sphingomyelin facilitates microdomains. Replacing endogenous sphingolipids with C24 or C16 sphingomyelin in live HeLa cells has a similar impact on microdomains, characterized by FRET between GPI-anchored proteins: C24, but not C16, sphingomyelin suppresses submicron domains in the plasma membrane. Molecular dynamics simulations indicated that, when in the outer leaflet, the acyl chain of C24 sphingomyelin interdigitates into the opposing leaflet, thereby favouring cholesterol in the inner leaflet. We indeed found that cholesterol prefers the inner over the outer leaflet of asymmetric unilamellar vesicles (80/20) when C24 sphingomyelin is in the outer leaflet. However, when C16 sphingomyelin is in the outer leaflet, cholesterol is evenly partitioned between leaflets (50/50). Interestingly, when a mixture of C24/C16 sphingomyelin is in the outer leaflet of unilamellar vesicles, cholesterol still prefers the inner leaflet (80/20). Indeed, in human erythrocyte plasma membrane, where a mixture of C24 and C16 sphingolipids are naturally in the outer leaflet, cholesterol prefers the cytoplasmic leaflet (80/20). Therefore, C24 sphingomyelin uniquely interacts with cholesterol and governs the lateral organization in asymmetric membranes, including the plasma membrane, potentially by generating cholesterol asymmetry.Statement of SignificanceThe plasma membrane bilayer of mammalian cells has distinct phospholipids between the outer and inner leaflet, with sphingolipids exclusively in the outer leaflet. A large portion of mammalian sphingolipids have very long acyl chains (C24). Little is known how C24 sphingolipids function in the outer leaflet. Mutations in the ceramide synthase 2 gene is found to decrease C24. This severely perturbs homeostasis in mice and humans. Here, we investigated unilamellar vesicles and mammalian cells with C24 sphingomyelin exclusively in the outer leaflet. We provide evidence that outer leaflet C24 sphingomyelin suppresses microdomains in model membranes and live cells by partitioning cholesterol into the inner leaflet. We propose that C24 sphingolipids are critical to the function of the plasma membrane.
Biosynthesis and biology of mammalian GPI-anchored proteins
