REGULATION OF THE HUMAN WEE1HU CDK TYROSINE 15-KINASE DURING THE CELL-CYCLE

In higher eukaryotes, the cyclin-dependent kinases (CDKs) are negative ly regulated by phosphorylation on threonine 14 (T14) and tyrosine 15 (Y15), In fission yeast, the Wee1 and mitosis inhibitory kinase 1 (Mik 1) protein kinases phosphorylate Y15 in Cdc2. WEE1Hu is the only known protein kinase that can carry out this inhibitory phosphorylation on Y15 in higher eukaryotes. In the present study, we examined the endoge nous products of WEE1Hu in human cells and found that the original WEE 1Hu cDNA lacked 214 amino acids at the N-terminus. The predicted full- length protein has weak, but significant, similarity over its entire l ength with Mik1. Thus, we suggest that 'WEE1Hu' is a Mik1-related prot ein rather than a Wee1 homologue. When isolated in immunoprecipitates, the endogenous WEE1Hu phosphorylated several cyclin-associated CDKs o n Y15, WEE1Hu activity increased during S and G(2) phases in parallel with the level of protein. Its activity decreased at M phase when WEE1 Hu became transiently hyperphosphorylated. In addition, a decrease in WEE1Hu protein level was observed at M/G(1) phase. Apparently, the hyp erphosphorylation and degradation in combination caused inactivation o f WEE1Hu at M phase and the following G(1) phase. These results sugges t that the activity of WEE1Hu is regulated by phosphorylation and prot eolytic degradation, and that WEE1Hu plays a role in inhibiting mitosi s before M phase by phosphorylating cyclin B1-Cdc2.