P107(WEE1) IS A DUAL-SPECIFICITY KINASE THAT PHOSPHORYLATES-P34(CDC2)ON TYROSINE-15

p107wee1 is a protein kinase that functions as a dose-dependent inhibi tor of mitosis through its interactions with p34cdc2 in Schizosaccharo myces pombe. To characterize the kinase activity of p107wee1, its carb oxyl-terminal catalytic domain was purified to homogeneity from overpr oducing insect cells. The apparent molecular mass of the purified prot ein (p37wee1 KD) was determined to be almost-equal-to 37 kDa by gel fi ltration, consistent with it being a monomer. Serine and tyrosine kina se activities cofiltered with p37wee1 KD, demonstrating that p107wee1 is a dual-specificity kinase. In vitro, p107wee1 phosphorylated p34cdc 2 on Tyr-15 only when p34cdc2 was complexed with cyclin. Neither monom eric p34cdc2 nor a peptide containing Tyr-15 was able to substitute fo r the p34cdc2/cyclin complex in this assay. Furthermore, the phosphory lation of p34cdc2 by p107wee1 in vitro inhibited the histone H-1 kinas e activity of p34cdc2. These results indicate that p107wee1 functions as a mitotic inhibitor by directly phosphorylating p34cdc2 on Tyr-15 a nd that the preferred substrate for phosphorylation is the p34cdc2/cyc lin complex.