Biochemical evidence for a calcium-dependent protein kinase from Pharbitis nil and its involvement in photoperiodic flower induction

A soluble Ca(2+)-dependent protein kinase (CDPK) was isolated from seedlings of the short-day plant Pharbitis nil and purified to homogeneity. Activity of Pharbitis nil CDPK (PnCDPK) was strictly dependent on the presence of Ca(2+) (K(0,5)=4,9 microM). The enzyme was autophosphorylated on serine and threonine residues and phosphorylated a wide diversity of substrates only on serine residues. Histone III-S and syntide-2 were the best phosphate acceptors (K(m) for histone III-S=0,178 mg ml(-1)). Polyclonal antibodies directed to a regulatory region of the soybean CDPK recognized 54 and 62 kDa polypeptides from Pharbitis nil. However, only 54 kDa protein was able to catalyse autophosphorylation and phosphorylation of substrates in a Ca(2+)-dependent manner. CDPK autophosphorylation was high in 5-day-old Pharbitis nil seedlings grown under non-inductive continuous white light and was reduced to one-half of its original when plants were grown in the long inductive night. Also, the pattern of proteins phosphorylation has changed. After 16-h-long inductive night phosphorylation of endogenous target (specific band of 82 kDa) increased in the presence of calcium ions. It may suggest that Ca(2+)-dependent protein kinase is involved in this process and it is dependent on light/dark conditions.