MAP kinase kinase from rabbit skeletal muscle A novel dual specificity enzyme showing homology to yeast protein kinases involved in pheromone‐dependent signal transduction

Abstract
MAP kinase kinase (MAPKK) was purified 30,000‐fold to homogeneity from extracts of rabbit skeletal muscle and shown to be a monomeric protein of apparent molecular mass 44 kDa, MAPKK activated the 42 kDa isoform of MAP kinase by phosphorylation of Thr‐183 and Tyr‐185, and phosphorylated itself slowly on tyrosine, threonine and serine residues, establishing that it is a ‘dual specificity’ protein kinase. Peptide sequences from MAPKK were homologous to other protein serine/threonine kinases, especially to the subfamily that includes yeast protein kinase that lie upstream of yeast MAP kinase homologues in the pheromone‐dependent mating pathways.