A non-histone chromatin protein that is a specific phosphate acceptor of nuclear cAMP-independent protein kinase from mouse spleen cells

Abstract

A non-histone chromatin protein (NHCP), which is specifically phosphorylated by a nuclear cAMP-independent protein kinase from mouse spleen cells in vitro, has been purified from calf thymus chromatin. SDS-polyacrylamide gel electrophoresis shows that the molecular weight of the purified NHCP (single peptide) is 13,000. This peptide consists of 14 amino acid residues and has high serine content (17.9%). However, no methionine, proline, or half-cysteine have been detected. The NHCP is a basic protein (isoelectric point, approximately 9.0), although it has high acidic amino acid content (26.2%). This protein serves as the most effective phosphate acceptor for the kinase in vitro when compared with other chromatin proteins such as histones; the Km value of the kinase for the NHCP is 3.65 x 10(-6) M, whereas that for histone H2a (Mr = 14,000) is 1.08 x 10(-5) M. The present finding that the NHCP phosphorylation by the kinase in vitro is remarkably stimulated by double-stranded DNA but inhibited by whole histone or histone H2a suggests that the phosphorylation may be controlled by both double-stranded DNA and histone H2a.