A Calcium-dependent Protein Kinase (CDPK) in the Unicellular Green Alga Closterium ehrenbergii

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An in-gel kinase assay revealed that homogenates of Closterium ehrenbergii contain Ca²⁺-dependent protein kinases (CDPKs) ranging in relative molecular mass from 47 to 60 kDa, that phosphorylated histone H1, myelin basic protein, and casein as substrates in a Ca²⁺-dependent manner. Immunoblotting and immunoprecipitation assays of Closterium extracts using anti-Dunaliella CDPK serum showed that the antibody cross-reacted with a Closterium CDPK with a relative molecular mass of 55 kDa in the soluble fraction. These results indicate that Closterium possesses CDPK-related polypeptides: one is a 55-kDa CDPK that is immunologically related to Dunaliella CDPK and the others have various molecular masses.