Importin β-dependent nuclear import of TopBP1 in ATR-Chk1 checkpoint in Xenopus egg extracts.
TopBP1, a multiple-BRCT-containing protein, plays diverse functions in DNA metabolism including DNA replication, DNA damage response and transcriptional regulation. The cytoplasmic localization of TopBP1 has been found to be associated with breast cancer susceptibility in clinical studies, suggesting the biological significance of TopBP1's sub-cellular localization. However, it remains elusive how TopBP1 is shuttled into nucleus and recruited to chromatin under normal or stressful conditions. Taking advantage of Xenopus egg extract, we identified Importin β as a new interacting protein of the TopBP1 C-terminus. We verified the TopBP1-Importin β association via GST pulldown and coimmunoprecipitation assays. We then demonstrated that TopBP1's C-terminal motif (designated as CTM, 23 amino acids) containing a putative NLS (nuclear localization signal) was required for Importin β interaction and that CT100 of Importin β (100 amino acids of extreme C-terminus of Importin β) was required for TopBP1 interaction. Further structure-function analysis reveals that the CTM of TopBP1 is essential for TopBP1's nuclear import and subsequent chromatin recruitment, thereby playing important roles in DNA replication and mitomycin C (MMC)-induced Chk1 phosphorylation. In addition, Importin β-specific inhibitor importazole inhibits TopBP1's nuclear import and the MMC-induced Chk1 phosphorylation. With ongoing DNA replication, the Importin β-dependent nuclear import of TopBP1 was indeed required for the MMC-induced Chk1 phosphorylation. Our data also suggest that checkpoint activation requires more TopBP1 than DNA replication does. The requirement of TopBP1's CTM motif for ATR-Chk1 checkpoint can be bypassed in a nucleus-free AT70 system. Taken together, our findings suggest that the CTM motif-mediated TopBP1 shuttling into nucleus via Importin β plays an important role in the ATR-Chk1 checkpoint signaling in Xenopus egg extracts.
Department of Biology, University of North Carolina at Charlotte, Charlotte, NC 28223, USA.
Liping
L
Bai
Molecular and Computational Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089, USA.
W Matthew
WM
Michael
Department of Biology, University of North Carolina at Charlotte, Charlotte, NC 28223, USA. Electronic address: Shan.Yan@uncc.edu.
Shan
S
Yan
D021581:Active Transport, Cell Nucleus / Q000187:drug effects; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D064007:Ataxia Telangiectasia Mutated Proteins / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism*; D000071877:Checkpoint Kinase 1; D004260:DNA Repair; D004268:DNA-Binding Proteins; D016685:Mitomycin / Q000494:pharmacology; D008969:Molecular Sequence Data; D010063:Ovum / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011799:Quinazolines / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000254:growth & development / Q000378:metabolism*; D028961:beta Karyopherins / Q000037:antagonists & inhibitors / Q000378:metabolism*
D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D011799:Quinazolines; C509993:TopBP1 protein, Xenopus; D029867:Xenopus Proteins; D028961:beta Karyopherins; C568452:importazole; D016685:Mitomycin; D011494:Protein Kinases; C576898:Atr protein, Xenopus; D064007:Ataxia Telangiectasia Mutated Proteins; D000071877:Checkpoint Kinase 1; C000605875:Chek1 protein, Xenopus
ATR–Chk1 checkpoint; DNA damage response; DNA replication; Importin β; Nuclear import; TopBP1