Calcium-dependent protein kinases were detected and characterized in the phloem sap of rice plants. Protein phosphorylation was activated in the presence of micromolar levels of free Ca2+ ions but was not activated by a polyamine in vitro. Mg2+ ions were essential for protein phosphorylation and K+ ions inhibited the protein phosphorylation. Analysis by two-dimensional polyacrylamide electrophoresis revealed that 17-kDa protein with a pI of 5.0 was the most highly phosphorylated protein in the phloem sap of rice plants. A protein of 65 kDa, which was autophosphorylated, had a Ca2+-dependent protein kinase activity and the mobility of this band in SDS gel was changed in the presence of calcium. These results suggest that a signal-transport system may exist in the sieve tubes of rice plants that operates via the phosphorylation of proteins by calcium dependent protein kinases.
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Nakamura et al. (1995) studied this question.