Characterization of multiple forms of maize seedling protein kinases reminiscent of animal casein kinases S (type 1) and TS (type 2)

Abstract

Three protein kinase fractions active on casein and phosvitin but not on histones, have been partially purified from maize seedlings, and operationally termed casein kinases I, IIA and IIB after their elution order from DEAE-cellulose and Ultrogel AcA 34. Casein kinase I is almost indistinguishable from animal type-1 (S) casein kinase according to several criteria, including molecular mass (35 kDa); insensitivity to heparin, polyglutamates and polyamines; K m values for ATP (20 μM) and casein (0.7 mg/ml); use of just ATP as phosphate donor; relatively high I 50 for quercetin inhibition (50 μM); and specificity toward the single residue Ser-22 of β-casein. On the other hand both casein kinase IIA and casein kinase IIB meet most criteria of animal type-2 (TS) casein kinases: they are inhibited by low concentrations of heparin, polyglutamate and quercetin, they are stimulated by polylysine; they can use also GTP besides ATP as phosphate donor; they specifically phosphorylate Thr-41 of β-casein and are also very active toward the model peptide substrate Ser-Glu-Glu-Glu-Glu-Glu. While, however, casein kinase IIA displays a 135 kDa mass (by gel filtration) and shares with typical casein kinases 2 also stimulation by polyamines, casein kinase IIB has a lower mass (65 kDa) and is not stimulated, but rather inhibited, by polyamines. Two phosphorylatable polypeptides of 19 and 16.5 kDa copurify with casein kinase IIB. They are also readily phosphorylated by casein kinase IIA and by rat liver casein kinase 2 (TS).