Background: Members of the MAP kinase superfamily play important roles in a wide variety of signal transduction pathways, and several members have been identified. However, the diversity and complexity of cellular responses in mammalian systems may imply existence of hitherto unidentified members of the MAP kinase superfamily.
Results: We report the molecular cloning and characterization of a novel member of the MAP kinase superfamily. We isolated full-length mouse and human cDNAs that encode complete open reading frames of a novel protein kinase, termed MOK. MOK consists of 419 (human) and 420 (mouse) amino acids, with a calculated molecular weight of 48kDa. MOK contains all of the protein serine/threonine kinase consensus motifs and shows a modest similarity to members of the MAP kinase superfamily and MAK and MAK-related kinase (MRK). In addition, MOK possesses a Thr-Glu-Tyr (TEY) motif in the activation loop domain, as do classical MAP kinases. MOK is widely expressed in normal tissues and organs and localizes to the cytoplasm. MOK is able to phosphorylate several known MAP kinase substrates and to undergo autophosphorylation. A mutation in the TEY motif to AEF abolished the kinase activity of MOK, and the treatment of cells with a phosphatase inhibitor, okadaic acid, enhanced the kinase activity of MOK, suggesting the existence of an upstream kinase. Phorbol ester TPA was found to stimulate the kinase activity of MOK, whereas serum stimulation, osmotic shock, or anisomycin treatment did not significantly activate MOK.
Conclusion: These results indicate that MOK is distantly related to members of known subfamilies of the MAP kinase superfamily and can therefore be classified as a novel member.