Identification of MAPKAPK Homolog (MAPKAPK-4) as a Myosin II Regulatory Light-Chain Kinase in Sea Urchin Egg Extracts

We identified and cloned a homolog of mammalian mitogen-activated protein kinase-activated protein kinase (MAPKAPK)-2 and -3 from sea urchin,Hemicentrotus pulcherrimus.The obtained cDNA clone was composed of 350 amino acid residues which contain MAPK phosphorylation sites and the bipartite nuclear localization signal sites in its C-terminal domain. The clone showed 65.4 and 66.7% amino acid residue identity to human MAPKAPK-2 and -3, respectively. Phylogenetic analysis revealed that the homolog can be classified into a distinct group of MAPKAPK and, therefore, the identified homolog was designated as MAPKAPK-4. Biochemical characterization was performed using recombinant glutathioneS-transferase (GST)–MAPKAPK-4 fusion protein. The protein kinase activity of GST–MAPKAPK-4 was activated by MAPK and this enabled the kinase to phosphorylate both glycogen synthase N-terminal peptide and the regulatory light chain of myosin IIin vitro.Northern blot analysis showed that MAPKAPK-4 was expressed throughout the development of sea urchin embryos. These observations suggest that MAPKAPK-4 may play an important role in the regulation of myosin II activity during the development of sea urchin.