Calcium/Ganglioside‐Dependent Protein Kinase Activity in Rat Brain Membrane

Abstract: The effects of gangliosides on phosphorylation were studied in rat brain membrane. Gangliosides stimulated phosphorylation only in the presence of Ca²⁺ with major phosphoproteins of 45,000, 50,000, 60,000, and 80,000 daltons and high‐molecular‐weight species. In addition, gangliosides inhibited the phosphorylation of three proteins with molecular weights of 15,000, 20,000, and 78,000 daltons. The two low‐molecular‐weight proteins comigrated with rat myelin basic proteins. Ganglioside stimulation was dependent on the formation of a Ca²⁺ ganglioside complex since the calcium salt of gangliosides stimulated phosphorylation maximally. Disialo and trisialo gangliosides were more potent stimulators of kinase activity than the monosialo G_M1. G_D1a was the most potent activator tested. Asialo‐G_M1, cerebroside, sialic acid, neuraminyllactose, sulfatide, and the acidic phospholipids phosphatidylserine and phosphatidylinositol did not stimulate kinase activity. The Ca²⁺ ‐dependent, ganglioside‐stimulated phosphorylation was qualitatively similar to the pattern for calmodulin‐dependent phosphorylation. However, while calmodulin‐dependent kinase activity was inhibited with an IC⁵⁰ of 10 μM trifluoperazine, ganglioside‐stimulated kinase was inhibited with an IC₅₀ of 200 μM trifluoperazine. These results indicate that gangliosides have complex effects on membrane‐associated kinase activities and suggest that Ca²⁺ ‐ganglioside complexes are potent stimulators of membrane kinase activity.