Abstract:Plasma membrane (PM) of high purity was prepared through a two-phase aqueous system from roots of 3 ～ 4 d old com ( Zea mays L. ) seedling, and within which calcium-activated protein kinase was identified. The kinase was mainly located in the inner side of the PM with a semi-activation concentration to free calcium ion of 50 μmol/L. Exogenous CaM had no agonistic effect on the protein kinase but CaM antagonist TFP could strongly inhibit its activity with a semi-inhibition concentration of 75 μmol/L. The protein kinase showed high specificity for the exogenous substrate, Histone Ⅲ S. These results indicated that this protein kinase could be a calcium-dependent and calmodulin-independent protein kinase (CDPK). Moreover, two proteins (33 kD and 58 kD) in PM might be the endogenous substrates of the CDPK.