A minor species of a type I casein kinase from yeast phosphorylating threonine residues of protein substrate.

Protein kinase of Mr 23 000 was isolated from yeast and purified to apparent homogeneity. The enzyme preferentially phosphorylated casein and phosvitin in the presence of ATP as a phosphoryl donor. Its activity was neither affected by cyclic nucleotides nor by heparin. The kinase displayed practically the same substrate specificity as a typical casein kinase I from yeast (Kudlicki, W., Szyszka, R., Paleń, E. and Gasior, E. (1980) Biochim. Biophys. Acta 633, 376-385) except that it phosphorylated threonine instead of serine residues in protein substrates.