CLONING AND CHARACTERIZATION OF DLK, A NOVEL SERINE THREONINE KINASE THAT IS TIGHTLY ASSOCIATED WITH CHROMATIN AND PHOSPHORYLATES CORE HISTONES/

We cloned a cDNA coding for a novel serine/threonine kinase, Dlk, a pr otein of 448 amino acids with a predicted molecular weight of 51.3 kDa , The kinase domain shows 81% amino acid sequence identity to the rece ntly identified DAP kinase (death associated protein kinase) (Deiss et al,, Genes & Dev,, 9, 15-30, 1995), therefore, the new kinase was cal led Dlk, for DAP like kinase. Northern analyses revealed a single mRNA species of 1.7 kb which was ubiquitously expressed. However, expressi on levels varied considerably in different cell lines and tissues. Mor eover, expression was downregulated upon UV irradiation. Dlk exhibited autophosphorylation activity, predominantly towards threonine residue s and phosphorylated the regulatory subunit of myosin light chain, but in this case exclusively at serine residues. Dlk seems to be tightly associated with insoluble nuclear structures, presumably chromatin, si nce it was resistant to various rigorous extraction procedures but it was partially released upon DNase I digestion of nuclei. Consistent wi th this, purified Dlk phosphorylated core histones H3, H2A and H4 as e xogenous substrates and endogenous histone H3 in kinase assays with nu clear extracts. Expression as GFP-fusion protein revealed a diffuse as well as a speckled nuclear staining suggesting an association with re plication or transcription centers.