PROTEIN-KINASE ACTIVITY AND PROTEIN-PHOSPHORYLATION IN RICE (ORYZA-SATIVA L) LEAF

Calcium-, phospholipid- and phorbol ester-dependent protein kinase (Ca2+-, PS- and PA-dependent protein kinase) has been found present in the rice (Oryza sativa L.) leaf. Ca2+, PS and PA dependent protein kinase activity was detected in the cytosol fraction of the leaf and root during the early stages of seedling growth in rice. In vitro phosphorylation experiments, using extract or cytosol fraction from rice leaf showed that phosphorylation of two specific proteins with molecular masses of 45 000 and 43 000 was significantly stimulated by phorbol ester. In addition, despite cyclic AMP (cAMP) concentration being less than 1 pmol/g fresh weight, cAMP-dependent protein kinase activity was detected in both the leaf and root during seedling development in rice. In vitro phosphorylation of three proteins with molecular masses of 55 000, 50 000 and 40 000 was significantly stimulated by cAMP. It is suggested that Ca2+, PS- and PA-dependent or cAMP-dependent protein kinase in the rice leaf might be involved in cell regulatory systems through phosphorylation.