Existence and Characteristics of Tonoplast-bound Protein Kinase in the Tip Cell of Maize Root

摘要

For understanding the function of tonoplast protein in plant cell signal pathway, we have identified an integral protein kinase activity from the highly purified tonoplast isolated from maize (Zea mays L.) root by a new nonradioactive method in which a color labeled peptide was used as substrate. The protein kinase was Ca~(2+)-dependent and CaM and phosphatidylserine-independent, like the calmodulin-like domain protein kinase (CDPK) in many plants. The optimal pH value and Ca~(2+) concentration were 6.5 and 10 μmol/L, respectively. According to the optimal pH value and the effect of detergent, it could be inferred that the active site of this protein kinase is oriented toward the cytoplasm. Zn~(2+) had no obvious effect on its activity, indicating that this protein kinase has no zinc-finger domain that exists in some mammalian protein kinases. At the same time, when tonoplast proteins were prephosphorylated in the presence of Ca~(2+) and ATP, both the ATP-hydrolysis and the proton-transport activity of vacuolar H~+-ATPase were stimulated. This stimulation could be reversed by an alkaline-phosphatase. These results indicate that a Ca~(2+)-dependent protein kinase was located in the tonoplast , and a Ca~(2+)-dependent phosphorylation, probably caused by this kinase, activated the vacuolar H~+ -ATPase activity. These results are helpful for further research on the function of CDPK in the course of signal transduction in plants.