MIK1(-15() ENCODES A TYROSINE KINASE THAT PHOSPHORYLATES P34(CDC2) ONTYROSINE)

mik1(+) and wee1(+) function to regulate the tyrosine phosphorylation of p34(cdc2) in, Schizosaccharomyces pombe (Lundgren, K., Walworth, N. , Booher, R., Dembski, M., Kirschner, M., and Beach, D. (1991) Cell 64 , 1111-1122). wee1(+) encodes a tyrosine kinase that directly phosphor ylates p34(cdc2) on tyrosine 15, resulting in the inactivation of the cyclin B/p34(cdc2) complex. We have overproduced the miK1(+) gene prod uct in insect cells and in S. pombe in order to characterize it bioche mically. Immunoprecipitates of Mik1 from both sources catalyzed the ph osphorylation of p34(cdc2) on tyrosine 15 whereas immunoprecipitates o f a kinase-deficient mutant of Mik1 were negative in this assay. Mik1 overproduced in insect cells was partially purified by column chromato graphy, and column fractions were assayed for their ability to phospho rylate p34(cdc2) on tyrosine 15. Two major peaks of Mik1 protein were detected by gel filtration chromatography. One peak eluted in the void volume, and a second peak eluted with an apparent molecular mass expe cted for monomeric Mik1 (similar to 68 kDa). The tyrosine 15 kinase ac tivity co-eluted with the 68 kDa form of Mik1. These results indicate that mik1(+) encodes a tyrosine kinase that directly phosphorylates p3 4(cdc2) on tyrosine 15.