Negative regulation of the wee1 protein kinase by direct action of the nim1/cdr1 mitotic inducer.

The wee1 protein kinase suppresses the entry into mitosis by mediating the inhibitory tyrosine phosphorylation of p34cdc2. Genetic studies have suggested that the nim1 protein kinase (also known as cdr1) acts as a positive regulator of mitosis by down-regulating the wee1 pathway in yeast cells. We have overexpressed the nim1 protein in both bacteria and insect cells. The recombinant nim1 protein autophosphorylates on both tyrosine and serine residues and can phosphorylate the isolated wee1 protein directly in a cell-free system. The nim1-catalyzed phosphorylation of the wee1 protein occurs in its C-terminal region and leads to a substantial drop in its activity as a cdc2-specific tyrosine kinase. This nim1-dependent inhibition of the wee1 protein kinase can be reversed readily in vitro by treatment with a protein phosphatase. These experiments provide direct biochemical evidence that the wee1 protein is subject to negative regulation by phosphorylation and indicate that the nim1 protein acts as an inhibitory, wee1-specific kinase.