AbstractWe have isolated and characterized glycopeptides, derived from mouse and bovine cerebral cortex cells, that inhibit protein synthesis and cell growth of normal but not transformed cells. The inhibitor binds to target cell surfaces, and gangliosides have previously been shown to influence cell sensitivity to the glycopeptides. Preincubation with 3.0 μg/ml ganglioside GM1at 0°C for 3 hr sensitized the mouse L‐cell line to the inhibitor, as determined by protein synthesis assays. Preincubation of LM cells with ganglioside GM1alone did not affect protein synthesis rates. In addition, the gangliosides GD1aand GM3also sensitized the LM cells to the protein synthesis inhibitory effect of the glycopeptide inhibitor. Binding experiments were performed with 3T3 (sensitive) and LM (insensitive) cells to determine if sensitivity to the glycopeptide inhibitor was reflected in binding of the inhibitor to these cells. Binding of125I‐labeled inhibitor to 3T3 cells was maximal after 60 min at 0°C and saturable at approximately 1 × 104molecules/cell. Furthermore, binding of the inhibitor was dose‐dependent, with half‐maximal binding at 1.5‐2.0 nM and saturation at 8.0‐10.0 nM. Scatchard plot analysis indicated that the Kdwas about 1 × 10−9M and that there are 1 × 104receptors/cell. Binding of the inhibitor to LM cells was maximal after 30 min at 0°C and saturation occurred at 5 × 103molecules/cell. We then examined the possibility that gangliosides are the cellular receptor or co‐receptor for the glycopeptide inhibitor. Binding of the inhibitor to ganglioside GM1was first examined after the ganglioside had been preadsorbed to polystyrene tubes. These experiments indicated that the ganglioside did not bind the inhibitor. Ganglioside‐containing liposomes from phosphatidylcholine or LM cell membrane components were also prepared; these artificial membranes did not bind appreciable amounts of the iodinated inhibitor. Competition experiments showed that the gangliosides GM1and GD1adid not neutralize the protein synthesis inhibitory activity of the glycopeptides, indicating that gangliosides do not directly interact with the glycopeptide inhibitor. In addition, binding of the inhibitor to LM cells preincubated with ganglioside GM1was studied. Although the binding of the inhibitor to LM cells was one‐half that observed for 3T3 cells, incorporation of exogenous gangliosides into LM cells did not result in increased binding of the inhibitor. Therefore, we conclude that the ganglioside‐induced sensitization of LM cells cannot be due to gangliosides serving as the cell‐surface receptor or co‐recept
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