No abstract

Two membrane subfractions, one enriched in GM3 ganglioside and the other containing caveolin, were separated from low density detergent-insoluble membrane fraction prepared by sucrose density gradient centrifugation of postnuclear fraction of mouse melanoma B16 cells. The GM3-enriched subfraction, separated by anti-GM3 monoclonal antibody DH2, contained sphingomyelin, cholesterol, c-Src, and Rho A but not caveolin. In contrast, the caveolin-containing subfraction, separated by anti-caveolin antibody, contained neither GM3, c-Src, nor Rho A but did contain glucosylceramide, Ras, a very small quantity of sphingomyelin, and a very large quantity of cholesterol. The GM3/c-Src-enriched membrane subfraction was characterized by (i) maintenance of GM3-dependent adhesion and (ii) susceptibility to being activated for signal transduction through GM3.32 P-Phosphorylation of c-Src (M r 60,000) together with two other components (M r 45,000 and 29,000) was enhanced in the fraction bound to dishes coated with asialo-GM2 (Gg3) or with anti-GM3 monoclonal antibody DH2, detected by incubation with [␥-32 P]ATP at 37°C for 5 min. GM3-dependent adhesion of B16 cells to Gg3-coated dishes and associated signaling were not reduced or abolished in the presence of either filipin or nystatin, which are cholesterol-binding reagents known to abolish caveolae structure and function. B16 melanoma cells incubated with filipin (0.16 -0.3 g/ml) or with nystatin (25 g/ml) for 30 min showed depletion of cholesterol in detergentinsoluble membrane fraction but were still capable of binding to Gg3-coated plate and capable of the associated signaling. Thus, the GM3-enriched subfraction, involved in cell adhesion and capable of sending signals through GM3, represents a membrane domain distinguishable from caveolin-containing subfraction or caveolae. This microdomain is hereby termed the "glycosphingolipid signaling domain" or "glycosignaling domain".