F-box proteins Pof3 and Pof1 regulate Wee1 degradation and mitotic entry in fission yeast.

The key cyclin-dependent kinase Cdk1 (Cdc2) promotes irreversible mitotic entry, mainly by activating the phosphatase Cdc25 while suppressing the tyrosine kinase Wee1. Wee1 needs to be downregulated at the onset of mitosis to ensure rapid activation of Cdk1. In human somatic cells, one mechanism of suppressing Wee1 activity is mediated by ubiquitylation-dependent proteolysis through the Skp1/Cul1/F-box protein (SCF) ubiquitin E3 ligase complex. This mechanism is believed to be conserved from yeasts to humans. So far, the best-characterized human F-box proteins involved in recognition of Wee1 are β-TrCP (BTRCP) and Tome-1 (CDCA3). Although fission yeast Wee1 was the first identified member of its conserved kinase family, the F-box proteins involved in recognition and ubiquitylation of Wee1 have not been identified in this organism. In this study, our screen using Wee1- Renilla luciferase as the reporter revealed that two F-box proteins, Pof1 and Pof3, are required for downregulating Wee1 and are possibly responsible for recruiting Wee1 to SCF. Our genetic analyses supported a functional relevance between Pof1 and Pof3 and the rate of mitotic entry, and Pof3 might play a major role in this process.