The association of ganglioside GD3 with TAG-1, a glycosylphosphatidylinositol-anchored neuronal cell adhesion molecule, was examined by coimmunoprecipitation experiments. Previously, we have shown that the anti-ganglioside GD3 antibody (R24) immunoprecipitated the Src family kinase Lyn from the rat cerebellum, and R24 treatment of primary cerebellar cultures induced Lyn activation and rapid tyrosine phosphorylation of an 80-kDa protein (p80). We now report that R24 coimmunoprecipitates a 135-kDa protein (p135) from primary cerebellar cultures. Treatment with phosphatidylinositol-specific phospholipase C revealed that p135 was glycosylphosphatidylinositol-anchored to the membrane. It was identified as TAG-1 by sequential immunoprecipitation with an anti-TAG-1 antibody. Antibody-mediated cross-linking of TAG-1 induced Lyn activation and rapid tyrosine phosphorylation of p80. Selective inhibitor for Src family kinases reduced the tyrosine phosphorylation of p80. Sucrose density gradient analysis revealed that the TAG-1 and tyrosine-phosphorylated p80 in cerebellar cultures were present in the lipid raft fraction. These data show that TAG-1 transduces signals via Lyn to p80 in the lipid rafts of the cerebellum. Furthermore, degradation of cell-surface glycosphingolipids by endoglycoceramidase induced an alteration of TAG-1 distribution on an OptiPrep gradient and reduced the TAG-1-mediated Lyn activation and tyrosine phosphorylation of p80. These observations suggest that glycosphingolipids are involved in TAG-1-mediated signaling in lipid rafts.Gangliosides, sialic acid-containing glycosphingolipids (GSLs), 1 are found in the outer leaflet of the plasma membrane of all vertebrate cells and are thought to play functional roles in cellular interactions and the control of cell proliferation (1-4).In the nervous system, where gangliosides are particularly abundant, the species and amounts of gangliosides undergo profound changes during development, suggesting that they may play fundamental roles in this process (5). The accumulation of gangliosides within the neurons in ganglioside storage diseases results in extensive neurite outgrowth (6). Exogenously administered gangliosides accelerate the regeneration of neurons in the central nervous system in vivo after lesioning (7). The addition of exogenous gangliosides to primary cultures of neurons and neuroblastoma cells in vitro stimulates cellular differentiation with concomitant neurite sprouting and extension (8 -10). Glucosylceramide synthesis, the first glycosylation step of GSL synthesis, is required for axonal growth in hippocampal neurons (11) and for embryonic development (12). Transfection of the ganglioside GD3 2 synthase cDNA into neuroblastoma cells induces their cholinergic differentiation and neurite sprouting (13). Finally, mice lacking complex gangliosides exhibit axonal degeneration (14). These data show that gangliosides are involved in neural cell differentiation and brain development. However, the molecular mechanisms underlying the ganglioside-depend...

[1] The vertical turbulent nitrate flux was calculated across the Kuroshio based on direct turbulence measurements and nitrate concentrations. Profile data were analyzed relative to the main axis of the Kuroshio front, the location of maximum surface velocity. Large upward nitrate flux on the northern side of the front, O(10, was observed from 300 m up to the base of the euphotic layer. Enhanced turbulence and a large vertical nitrate gradient contributed to this flux. On this side, high concentrations of chlorophyll-a were also observed near the surface, and the large nitrate flux is consistent with a previous estimate of the rate of new production. In contrast, south of the axis, the nitrate flux was O(10 -8 -10 À7

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