Activation of MEK family kinases requires phosphorylation of two conserved Ser/Thr residues

EMBO J. 1994 Mar 1;13(5):1123-31.


MEK is a family of dual specific protein kinases which activate the extracellular signal-regulated kinases by phosphorylation of threonine and tyrosine residues. MEK itself is activated via serine phosphorylation by upstream activator kinases, including c-raf, mos and MEK kinase. Here, we report the activation phosphorylation sites of human MEK1 and yeast STE7 kinase as determined by a combination of biochemical and genetic approaches. In human MEK1, substitution of either serine residue 218 or 222 with alanine completely abolished its activation by epidermal growth factor-stimulated Swiss 3T3 cell lysates or immunoprecipitated c-raf, suggesting that both serine residues are required for MEK1 activation. Phosphopeptide analysis demonstrated that serine residues 218 and 222 of human MEK1 are the primary sites for phosphorylation by c-raf. These two serine residues are highly conserved in all members of the MEK family, including the yeast STE7 gene product, a MEK homolog in the yeast mating pheromone response pathway. Mutation of the corresponding residues in STE7 completely abolished the biological functions of this gene. These data demonstrate that MEK is activated by phosphorylation of two adjacent serine/threonine residues and this activation mechanism is conserved in the MEK family kinases.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Conserved Sequence*
  • Enzyme Activation
  • Fungal Proteins / metabolism
  • Glutathione Transferase / metabolism
  • Humans
  • MAP Kinase Kinase 1
  • Mitogen-Activated Protein Kinase Kinases
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Peptide Mapping
  • Phosphopeptides / analysis
  • Phosphorylation
  • Point Mutation
  • Protein Kinases / metabolism*
  • Protein-Serine-Threonine Kinases / biosynthesis
  • Protein-Serine-Threonine Kinases / metabolism*
  • Protein-Tyrosine Kinases / biosynthesis
  • Protein-Tyrosine Kinases / metabolism*
  • Proto-Oncogene Proteins / metabolism
  • Proto-Oncogene Proteins c-raf
  • Recombinant Fusion Proteins
  • Saccharomyces cerevisiae
  • Serine*
  • Threonine*


  • Fungal Proteins
  • Phosphopeptides
  • Proto-Oncogene Proteins
  • Recombinant Fusion Proteins
  • Threonine
  • Serine
  • Glutathione Transferase
  • Protein Kinases
  • Protein-Tyrosine Kinases
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-raf
  • MAP Kinase Kinase 1
  • MAP2K1 protein, human
  • Mitogen-Activated Protein Kinase Kinases