Detection and Characterization of Protein Kinases in Rice Phloem Sap

Calcium-dependent protein kinases were detected and characterizedin the phloem sap of rice plants. Protein phosphorylation wasactivated in the presence of micromolar levels of free Ca²⁺ions but was not activated by a polyamine in vitro. Mg²⁺ ionswere essential for protein phosphorylation and K⁺ ions inhibitedthe protein phosphorylation. Analysis by two-dimensional polyacrylamideelectrophoresis revealed that 17-kDa protein with a pI of 5.0was the most highly phosphorylated protein in the phloem sapof rice plants. A protein of 65 kDa, which was autophosphorylated,had a Ca²⁺-dependent protein kinase activity and the mobilityof this band in SDS gel was changed in the presence of calcium.These results suggest that a signal-transport system may existin the sieve tubes of rice plants that operates via the phosphorylationof proteins by calcium dependent protein kinases. (Received December 20, 1993; Accepted October 8, 1994)