Cell-cycle-dependent and ATM-independent expression of human Chk1 kinase.
Checkpoint genes cause cell cycle arrest when DNA is damaged or DNA replication is blocked. Although a human homolog of Chk1 (hChk1) has recently been reported to be involved in the DNA damage checkpoint through phosphorylation of Cdc25A, B, and C, it is not known at which phase(s) of the cell cycle hChk1 functions and how hChk1 causes cell cycle arrest in response to DNA damage. In the present study, we demonstrate that in normal human fibroblasts (MJ90), hChk1 is expressed specifically at the S to M phase of the cell cycle at both the RNA and protein levels and that it is localized to the nucleus at this time. hChk1 activity, as determined by phosphorylation of Cdc25C, is readily detected at the S to M phase of the cell cycle, and DNA damage induced by UV or ionizing radiation does not enhance the expression of hChk1 or its activity. Furthermore, hChk1 exists in an active form at the S to M phase in fibroblasts derived from patients with ataxia telangiectasia (AT) which lack the functional AT mutated (ATM) gene product, suggesting that hChk1 expression is independent of functional ATM. Taken together with the findings that phosphorylation of Cdc25C on serine 216 is increased at the S to M phase, it is suggested that at this particular phase of the cell cycle, even in the absence of DNA damage, hChk1 phosphorylates Cdc25C on serine 216, which is considered to be a prerequisite for the G2/M checkpoint. Thus, hChk1 may play an important role in keeping Cdc25C prepared for responding to DNA damage by phosphorylating its serine residue at 216 during the S to M phase.
Department of Geriatric Research, National Institute for Longevity Sciences, Obu, Aichi, Japan.
Y S
YS
Kaneko
N
N
Watanabe
H
H
Morisaki
H
H
Akita
A
A
Fujimoto
K
K
Tominaga
M
M
Terasawa
A
A
Tachibana
K
K
Ikeda
M
M
Nakanishi
Y
Y
Kaneko
D064007:Ataxia Telangiectasia Mutated Proteins; D002453:Cell Cycle; D018797:Cell Cycle Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000201:enzymology; D000071877:Checkpoint Kinase 1; D004247:DNA / Q000528:radiation effects; D004249:DNA Damage; D004268:DNA-Binding Proteins; D004790:Enzyme Induction; D005347:Fibroblasts / Q000201:enzymology; D006367:HeLa Cells / Q000201:enzymology; D006801:Humans; D008677:Metaphase; D010766:Phosphorylation; D020033:Protein Isoforms / Q000378:metabolism; D011494:Protein Kinases / Q000096:biosynthesis* / Q000235:genetics; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases; D011506:Proteins / Q000502:physiology*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D016196:S Phase; D025521:Tumor Suppressor Proteins; D014466:Ultraviolet Rays; D014965:X-Rays; D020666:ras-GRF1
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D020033:Protein Isoforms; D011506:Proteins; D011993:Recombinant Fusion Proteins; D025521:Tumor Suppressor Proteins; D020666:ras-GRF1; D004247:DNA; D011494:Protein Kinases; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; D017346:Protein Serine-Threonine Kinases