| Abstract
| - A ganglioside-stimulated protein phosphorylation system was discovered in plasma membrane fractions of human neuroblastoma cells (GOTO). Gangliosides (GQ1b, GTla, GTlb, GDla, GDlb, GD3, and GM1) could stimulate this system. GQ1b showed the most effective stimulation among these gangliosides. The substrate specificity was rather broad. Not only some (de novo) proteins of the membranes but also purified histones and tubulin were phosphate-acceptable. This protein phosphorylation system specifically depended upon Ca2+ (optimum concentration: 50-100 μM). The optimum pH was 7.0-7.5. GQ1b/Ca2+ could not directly activate well known protein kinases (Ca2+/phospholipid-activated protein kinase, Ca2+/cal-modulin-activated protein kinase, and cyclic nucleotide-dependent protein kinases). Furthermore, GQlb could replace neither phospholipids nor calmodulin. Thus, an unknown, new type of protein kinase(s) may be involved in this system. Alternatively, GQlb may activate some known protein kinase(s) in cooperation with another unknown factor which may be removed during the preparation of the partially purified known protein kinase used in this experiment.
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