The Drosophila ATM homologue Mei-41 has an essential checkpoint function at the midblastula transition.
BACKGROUND
Drosophila embryogenesis is initiated by 13 rapid syncytial mitotic divisions that do not require zygotic gene activity. This maternally directed cleavage phase of development terminates at the midblastula transition (MBT), at which point the cell cycle slows dramatically, membranes surround the cortical nuclei to form a cellular blastoderm, and zygotic gene expression is first required.


RESULTS
We show that embryos lacking Mei-41, a Drosophila homologue of the ATM tumor suppressor, proceed through unusually short syncytial mitoses, fail to terminate syncytial division following mitosis 13, and degenerate without forming cells. A similar cleavage-stage arrest is produced by mutations in grapes, which encodes a homologue of the Checkpoint-1 kinase. We present biochemical, cytological and genetic data indicating that Mei-41 and Grapes are components of a conserved DNA-replication/damage checkpoint pathway that triggers inhibitory phosphorylation of the Cdc2 kinase and mediates resistance to replication inhibitors and DNA-damaging agents. This pathway is nonessential during postembryonic development, but it is required to terminate the cleavage stage at the MBT. Cyclins are required for Cdc2 kinase activity, and mutations in cyclin A and cyclin B bypass the requirement for mei-41 at the MBT. These mutations do not restore wild-type syncytial cell-cycle timing or the embryonic replication checkpoint, however, suggesting that Mei-41-mediated inhibition of Cdc2 has an additional essential function at the MBT.


CONCLUSIONS
The Drosophila DNA-replication/damage checkpoint pathway can be activated by externally triggered DNA damage or replication defects throughout the life cycle, and under laboratory conditions this inducible function is nonessential. During early embryogenesis, however, this pathway is activated by developmental cues and is required for the transition from maternal to zygotic control of development at the MBT.
Sibon
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OC
Department of Biochemistry and Cell Biology, Institute for Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York, USA.
Laurençon
A
A
Hawley
R
R
Theurkauf
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WE
D000818:Animals; D016590:Aphidicolin / Q000494:pharmacology; D064007:Ataxia Telangiectasia Mutated Proteins; D001755:Blastocyst / Q000166:cytology*; D016203:CDC2 Protein Kinase / Q000037:antagonists & inhibitors / Q000378:metabolism; D002453:Cell Cycle; D018797:Cell Cycle Proteins; D002455:Cell Division; D000071877:Checkpoint Kinase 1; D019925:Cyclin A / Q000235:genetics / Q000502:physiology; D019926:Cyclin B / Q000235:genetics / Q000502:physiology; D004249:DNA Damage; D004261:DNA Replication / Q000502:physiology; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D029721:Drosophila Proteins; D004331:Drosophila melanogaster / Q000196:embryology* / Q000235:genetics; D004625:Embryo, Nonmammalian / Q000166:cytology; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D017344:Genes, Insect; D005804:Genes, Lethal; D016147:Genes, Tumor Suppressor; D006801:Humans; D007247:Infertility, Female / Q000235:genetics; D019476:Insect Proteins / Q000235:genetics / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D009687:Nuclear Proteins; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000502:physiology; D011499:Protein Processing, Post-Translational / Q000502:physiology; D017346:Protein Serine-Threonine Kinases; D011506:Proteins / Q000502:physiology*; D013045:Species Specificity; D035381:TATA-Binding Protein Associated Factors; D013997:Time Factors; D035362:Transcription Factor TFIID; D051758:Transcription Factor TFIIH; D014157:Transcription Factors / Q000502:physiology; D019473:Transcription Factors, TFII; D025521:Tumor Suppressor Proteins
D018797:Cell Cycle Proteins; C503698:CycB protein, Drosophila; D019925:Cyclin A; D019926:Cyclin B; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D019476:Insect Proteins; D009687:Nuclear Proteins; D011506:Proteins; D035381:TATA-Binding Protein Associated Factors; C084976:Taf6 protein, Drosophila; D035362:Transcription Factor TFIID; D014157:Transcription Factors; D019473:Transcription Factors, TFII; D025521:Tumor Suppressor Proteins; C080833:run protein, Drosophila; D051758:Transcription Factor TFIIH; D016590:Aphidicolin; D011494:Protein Kinases; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; D000071877:Checkpoint Kinase 1; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase