Role of Protein Phosphorylation in the Maturation-induced Activation of a Myelin Basic Protein Kinase from Sea Star Oocytes

We have previously described the purification of a myelin basis protein (MBP) kinase from maturing sea star oocytes (Sanghera, J. S., Paddon, H. B., Bader, S. A., and Pelech, S. L. (1990) J. Biol. Chem. 265, 52-57). The ability of the purified 44-kDa protein to bind azido-ATP and undergo autophosphorylation on the serine residue implied that it is a protein kinase. Furthermore, partial amino acid sequence data has revealed that it is a novel protein kinase, which we have provisionally designated p44mpk. Autophosphorylation of p44mpk to 0.7 mol of phosphate/mol of enzyme was correlated with a modest (approximately 17%) increase in the MBP-phosphorylating activity of the kinase. Rabbit polyclonal antibody raised against purified p44mpk recognized on immunoblots the protein in highly purified preparations as well as crude oocyte extracts. The affinity-purified anti-p44mpk antibody could immunoprecipitate active kinase, but a subpopulation of the antibody also appeared to be inhibitory. Using this antibody, we have demonstrated that the up to 12-fold stimulation of the cytosolic MBP-phosphorylating activity of this kinase that occurs during sea star oocyte maturation is not due to an increase in the amount of enzyme protein, either from a redistribution within the oocyte or protein synthesis. A slight retardation of the migration of the activated p44mpk on sodium dodecyl sulfate-polyacrylamide gels and its tighter interaction with a MonoQ column is consistent with phosphorylation of the kinase during maturation. p44mpk underwent enhanced phosphorylation when oocytes prelabeled with [32P]orthophosphate were induced to mature with 1-methyladenine. The stimulated MBP-phosphorylating activity of p44mpk in cytosols from maturing oocytes was partly stabilized by the presence of the phosphatase inhibitor beta-glycerol phosphate. Furthermore, treatment of purified p44mpk with protein phosphatase 2A and alkaline phosphatase resulted in 56 and 86% decreases, respectively, in the activity of the kinase. Together, these findings strongly implicate a role for phosphorylation of p44mpk in its activation during sea star oocyte maturation.