The Regulation of the Activity of Soluble Starch Synthase in Spinach Leaves by a Calcium-Calmodulin Dependent Protein Kinase

Soluble starch synthase from spinach leaves can use ADPG and UDPG for chain elongation in the presence of an appropriate primer (glycogen, amylopectin). Whereas calcium ions alone or in combination with calmodulin (CaM) insignificantly reduce the biosynthesis of starch, ATP causes a substantial decrease in activity. EDTA and ADP on the other hand promote the incorporation of glucose. Using CaM-Sepharose-4B, a protein kinase (molecular weight 68 kD) could be isolated. This protein kinase is able to eliminate the ATP-induced inhibition in a calcium-CaM dependent way in the presence of Mg⁺⁺. The enhancement of the starch synthase activity by the protein kinase is Ca⁺⁺ and CaM specific. The optimum is observed at 5 mM ca⁺⁺ and 1.45 · 10^-7 mM CaM. The Ca-CaM dependence of protein kinase was proven by CaM antagonists (fluphenazine, trifluoperazine and calmidazolium at 5 · w^-5 M). Fluoride on the other hand enhanced the starch synthesis via the inhibition of a protein phosphatase. The phosphorylation of starch synthase was demonstrated by the incorporation of ³²P after application of ³²P-γ-ATP in the presence of the protein kinase.