Drosophila grapes/CHK1 mutants are defective in cyclin proteolysis and coordination of mitotic events.
The Drosophila grapes (grp) gene, which encodes a homolog of the Schizosaccharomyces pombe Chk1 kinase, provides a cell-cycle checkpoint that delays mitosis in response to inhibition of DNA replication [1]. Grp is also required in the undisturbed early embryonic cycles: in its absence, mitotic abnormalities appear in cycle 12 and chromosomes fail to fully separate in subsequent cycles [2] [3]. In other systems, Chk1 kinase phosphorylates and suppresses the activity of Cdc25 phosphatase: the resulting failure to remove inhibitory phosphate from cyclin-dependent kinase 1 (Cdk1) prevents entry into mitosis [4] [5]. Because in Drosophila embryos Cdk1 lacks inhibitory phosphate during cycles 11-13 [6], it is not clear that known actions of Grp/Chk1 suffice in these cycles. We found that the loss of grp compromised cyclin A proteolysis and delayed mitotic disjunction of sister chromosomes. These defects occurred before previously reported grp phenotypes. We conclude that Grp activates cyclin A degradation, and functions to time the disjunction of chromosomes in the early embryo. As cyclin A destruction is required for sister chromosome separation [7], a failure in Grp-promoted cyclin destruction can also explain the mitotic phenotype. The mitotic failure described previously for cycle 12 grp embryos might be a more severe form of the phenotypes that we describe in earlier embryos and we suggest that the underlying defect is reduced degradation of cyclin A.
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Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, California 94143-0448, USA.
Campbell
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O'Farrell
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D000705:Anaphase / Q000235:genetics; D000818:Animals; D015153:Blotting, Western; D002453:Cell Cycle / Q000187:drug effects / Q000235:genetics; D000071877:Checkpoint Kinase 1; D019925:Cyclin A / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology; D004247:DNA / Q000032:analysis; D004330:Drosophila / Q000166:cytology / Q000201:enzymology / Q000235:genetics* / Q000254:growth & development*; D029721:Drosophila Proteins; D008856:Microscopy, Fluorescence; D011494:Protein Kinases / Q000235:genetics / Q000502:physiology*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D029702:Schizosaccharomyces pombe Proteins
D019925:Cyclin A; D029721:Drosophila Proteins; D011500:Protein Synthesis Inhibitors; D029702:Schizosaccharomyces pombe Proteins; D004247:DNA; D003513:Cycloheximide; D011494:Protein Kinases; D000071877:Checkpoint Kinase 1; C000605873:Chk1 protein, S pombe; C000605877:grp protein, Drosophila