The cell cycle regulator, human p50weel, is a tyrosine kinase and not a serine/tyrosine kinase.

The human weel protein, a homologue of the yeast weel protein, was expressed in E. coli and purified to homogeneity. The purified weel protein phosphorylated the tyrosine residue of cdc2 kinase in HeLa cell extracts in the presence of human cyclin B1. It also phosphorylated the tyrosine but not the threonine residue in the peptide of the amino-terminal of cdc2 kinase, although both these residues have been shown to be phosphorylated in higher eukaryotes in vivo. Furthermore, serine and tyrosine residues of the yeast weel protein are reportedly autophosphorylated in vitro, however the tyrosine residue of the human weel protein was autophosphorylated whereas the serine and threonine residues were not. These data indicate that human p50weel is tyrosine kinase and that it phosphorylated the tyrosine residue of the amino-terminal of cdc2 kinase in the presence of cyclin B1 and that the threonine residue is phosphorylated by another, unknown kinase.