Gauchos and ochos: a Wee1-Cdk tango regulating mitotic entry

Abstract

The kinase Wee1 has been recognized for a quarter century as a key inhibitor of Cyclin dependent kinase 1 (Cdk1) and mitotic entry in eukaryotes. Nonetheless, Wee1 regulation is not well understood and its large amino-terminal regulatory domain (NRD) has remained largely uncharted. Evidence has accumulated that cyclin B/Cdk1 complexes reciprocally inhibit Wee1 activity through NRD phosphorylation. Recent studies have identified the first functional NRD elements and suggested that vertebrate cyclin A/Cdk2 complexes also phosphorylate the NRD. A short NRD peptide, termed the Wee box, augments the activity of the Wee1 kinase domain. Cdk1/2-mediated phosphorylation of the Wee box (on T239) antagonizes kinase activity. A nearby region harbors a conserved RxL motif (RxL1) that promotes cyclin A/Cdk2 binding and T239 phosphorylation. Mutation of either T239 or RxL1 bolsters the ability of Wee1 to block mitotic entry, consistent with negative regulation of Wee1 through these sites. The region in human somatic Wee1 that encompasses RxL1 also binds Crm1, directing Wee1 export from the nucleus. These studies have illuminated important aspects of Wee1 regulation and defined a specific molecular pathway through which cyclin A/Cdk2 complexes foster mitotic entry. The complexity, speed, and importance of regulation of mitotic entry suggest that there is more to be learned.

Figure 1

Reciprocal regulation of Wee1 and Cdk1. The feedback loop is a double-negative one, resulting in positive regulation of cyclin B/Cdk1 as the activity of this Cdk complex rises. Wee1 inhibits Cdk1 by phosphorylating it on tyrosine 15. Myt1 also performs this modification, though Wee1 appears to be dominant. Cdc25 phosphatases (A, B, C) remove the phosphate group. Cdk1 can also phosphorylate Wee1, inhibiting it.

Figure 2

Primary structure of vertebrate somatic Wee1 proteins. The numbering is from human somatic Wee1. The NRD, kinase domain, and short carboxy-terminal domain are marked, with border amino acid residues numbered (below). RxL1 (residues 180-2) is embedded within the Crm1 binding site (175-184). The T239 Cdk phosphorylation site, an inhibitory modification, resides within the Wee box, a positive regulatory element.

Figure 3

Model for regulation of Wee1 by cyclin A/Cdk2 complexes. Wee1 is depicted as a cyclin B/Cdk1 Y15 kinase with a globular kinase domain (dark blue) and a relatively unstructured NRD (light blue). The Cyclin A/Cdk2 complex binds RxL1 and phosphorylates T239. It might also phosphorylate a Wee1 residue that contributes to Crm1 binding (dashed arrow to S/TX). Crm1 binds the NES and possibly the additional cyclin A/Cdk2 phosphorylation site (dashed line) and mediates Wee1 export from the nucleus. S phase substrates possessing RxL motifs compete with cyclin A/Cdk2 complexes for binding to Wee1.