title: Novel positive feedback loop between Cdk1 and Chk1 in the nucleus during G2/M transition.
abstract: Chk1, one of the critical transducers in DNA damage/replication checkpoints, prevents entry into mitosis through inhibition of Cdk1 activity. However, it has remained unclear how this inhibition is cancelled at the G(2)/M transition. We reported recently that Chk1 is phosphorylated at Ser(286) and Ser(301) by Cdk1 during mitosis. Here, we show that mitotic Chk1 phosphorylation is accompanied by Chk1 translocation from the nucleus to the cytoplasm in prophase. This translocation advanced in accordance with prophase progression and was regulated by Crm-1-dependent nuclear export. Exogenous Chk1 mutated at Ser(286) and Ser(301) to Ala (S286A/S301A) was observed mainly in the nuclei of prophase cells, although such nuclear accumulation was hardly observed in wild-type Chk1. Induction of S286A/S301A resulted in the delay of mitotic entry. Biochemical analyses using immunoprecipitated cyclin B(1)-Cdk1 complexes revealed S286A/S301A expression to block the adequate activation of Cdk1. In support of this, S286A/S301A expression retained Wee1 at higher levels and Cdk1-induced phosphorylation of cyclin B(1) and vimentin at lower levels. A kinase-dead version of S286A/S301A also localized predominantly in the nucleus but lost the ability to delay mitotic entry. These results indicate that Chk1 phosphorylation by Cdk1 participates in cytoplasmic sequestration of Chk1 activity, which releases Cdk1 inhibition in the nucleus and promotes mitotic entry.
authors.affiliation: Division of Biochemistry, Aichi Cancer Center Research Institute, Nagoya, Aichi 464-8681, Japan.
authors.forename: Masato
authors.initials: M
authors.lastname: Enomoto
authors.forename: Hidemasa
authors.initials: H
authors.lastname: Goto
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Tomono
authors.forename: Kousuke
authors.initials: K
authors.lastname: Kasahara
authors.forename: Kunio
authors.initials: K
authors.lastname: Tsujimura
authors.forename: Tohru
authors.initials: T
authors.lastname: Kiyono
authors.forename: Masaki
authors.initials: M
authors.lastname: Inagaki
mesh_terms: D021581:Active Transport, Cell Nucleus; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D002453:Cell Cycle; D002455:Cell Division; D002467:Cell Nucleus / Q000378:metabolism*; D000071877:Checkpoint Kinase 1; D056744:Cyclin B1 / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D016195:G2 Phase; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D008938:Mitosis; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D014746:Vimentin / Q000378:metabolism
chemical_list: D056744:Cyclin B1; D014746:Vimentin; D011494:Protein Kinases; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; D016203:CDC2 Protein Kinase