Substrate Specificity of the Nuclear Protein Kinase NII from Porcine Liver

Abstract

The substrate specificity of the nuclear protein kinase NII from pig liver was studied using the purified casein variants α_s1B and βA² both in the phosphorylated and dephosphorylated form. Chymotryptic peptides of the phosphorylated casein were analyzed by ascending thin‐layer chromatography followed by thin‐layer electrophoresis. Phosphorylated peptides were detected by autoradiography. The determination of a phosphorylated site in a cluster of three serines was performed by solid phase Edman degradation of the whole βA² casein. Phosphorylated βA² casein was by far the best substrate. A threonine at position 41 in the amino acid sequence was found to be predominantly phosphorylated. Dephosphorylated βA² casein became rephosphorylated at serine‐17 whereas phosphorylation at threonine‐41 was greatly reduced. In both sites acidic amino acid side chains are present at the C‐terminal side though at different positions suggesting that acidic residues in the vicinity of the serine or threonine residue to be phosphorylated play an important role in the recognition by the nuclear protein kinase NIL