title: Xenopus MAP kinase activator is a serine/threonine/tyrosine kinase activated by threonine phosphorylation.
abstract: Xenopus MAP kinase activator, a 45 kDa protein, has been shown to function as a direct upstream factor sufficient for full activation and both tyrosine and serine/threonine phosphorylation of inactive MAP kinase. We have now shown by using an anti-MAP kinase activator antiserum that MAP kinase activator is ubiquitous in tissues and is regulated post-translationally. Activation of MAP kinase activator is correlated precisely with its threonine phosphorylation during the oocyte maturation process. It is a key question whether MAP kinase activator is a kinase or not. We have shown that Xenopus MAP kinase activator purified from mature oocytes is capable of undergoing autophosphorylation on serine, threonine and tyrosine residues. Dephosphorylation of purified activator by protein phosphatase 2A treatment inactivates its autophosphorylation activity as well as its activator activity. Thus, Xenopus MAP kinase activator is a protein kinase with specificity for both serine/threonine and tyrosine. Partial protein sequencing of purified activator indicates that it contains a sequence homologous to kinase subdomains VI and VII of two yeast protein kinases, STE7 and byrl.
authors.affiliation: Department of Biophysics and Biochemistry, Faculty of Science, University of Tokyo, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Kosako
authors.forename: Y
authors.initials: Y
authors.lastname: Gotoh
authors.forename: S
authors.initials: S
authors.lastname: Matsuda
authors.forename: M
authors.initials: M
authors.lastname: Ishikawa
authors.forename: E
authors.initials: E
authors.lastname: Nishida
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D007106:Immune Sera; D007118:Immunoassay; D007700:Kinetics; D020929:Mitogen-Activated Protein Kinase Kinases; D008969:Molecular Sequence Data; D009865:Oocytes / Q000201:enzymology / Q000502:physiology; D010455:Peptides / Q000138:chemical synthesis / Q000276:immunology; D010766:Phosphorylation; D011494:Protein Kinases / Q000302:isolation & purification / Q000378:metabolism*; D012689:Sequence Homology, Nucleic Acid; D013912:Threonine; D014981:Xenopus
chemical_list: D007106:Immune Sera; D010455:Peptides; D013912:Threonine; D011494:Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D020929:Mitogen-Activated Protein Kinase Kinases