title: Chk1 protects against chromatin bridges by constitutively phosphorylating BLM serine 502 to inhibit BLM degradation.
abstract: Chromatin bridges represent incompletely segregated chromosomal DNA connecting the anaphase poles and can result in chromosome breakage. The Bloom's syndrome protein helicase (BLM, also known as BLMH) suppresses formation of chromatin bridges. Here, we show that cells deficient in checkpoint kinase 1 (Chk1, also known as CHEK1) exhibit higher frequency of chromatin bridges and reduced BLM protein levels compared to controls. Chk1 inhibition leads to BLM ubiquitylation and proteasomal degradation during interphase. Furthermore, Chk1 constitutively phosphorylates human BLM at serine 502 (S502) and phosphorylated BLM localises to chromatin bridges. Mutation of S502 to a non-phosphorylatable alanine residue (BLM-S502A) reduces the stability of BLM, whereas expression of a phospho-mimicking BLM-S502D, in which S502 is mutated to aspartic acid, stabilises BLM and prevents chromatin bridges in Chk1-deficient cells. In addition, wild-type but not BLM-S502D associates with cullin 3, and cullin 3 depletion rescues BLM accumulation and localisation to chromatin bridges after Chk1 inhibition. We propose that Chk1 phosphorylates BLM-S502 to inhibit cullin-3-mediated BLM degradation during interphase. These results suggest that Chk1 prevents deleterious anaphase bridges by stabilising BLM.
authors.affiliation: Department of Biology, University of Crete, Vassilika Vouton, Heraklion 70013, Greece.
authors.forename: Eleni
authors.initials: E
authors.lastname: Petsalaki
authors.affiliation: Department of Biology, University of Crete, Vassilika Vouton, Heraklion 70013, Greece.
authors.forename: Maria
authors.initials: M
authors.lastname: Dandoulaki
authors.affiliation: Beatson Institute for Cancer Research, Glasgow G61 1BD, UK.
authors.forename: Nick
authors.initials: N
authors.lastname: Morrice
authors.affiliation: Department of Biology, University of Crete, Vassilika Vouton, Heraklion 70013, Greece gzachos@biology.uoc.gr.
authors.forename: George
authors.initials: G
authors.lastname: Zachos
mesh_terms: D020816:Amino Acid Motifs; D000705:Anaphase; D045744:Cell Line, Tumor; D000071877:Checkpoint Kinase 1; D002843:Chromatin / Q000235:genetics / Q000378:metabolism*; D044842:Cullin Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D010766:Phosphorylation; D011485:Protein Binding; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D059748:Proteolysis; D053484:RecQ Helicases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D012694:Serine / Q000235:genetics / Q000378:metabolism*
chemical_list: C503645:CUL3 protein, human; D002843:Chromatin; D044842:Cullin Proteins; D012694:Serine; D011494:Protein Kinases; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; C097151:Bloom syndrome protein; D053484:RecQ Helicases
keywords: BLM; Chk1; Chromatin bridge; Cullin 3; Mitosis