Terminal mitoses require negative regulation of Fzr/Cdh1 by Cyclin A, preventing premature degradation of mitotic cyclins and String/Cdc25.
Cyclin A expression is only required for particular cell divisions during Drosophila embryogenesis. In the epidermis, Cyclin A is strictly required for progression through mitosis 16 in cells that become post-mitotic after this division. By contrast, Cyclin A is not absolutely required in epidermal cells that are developmentally programmed for continuation of cell cycle progression after mitosis 16. Our analyses suggest the following explanation for the special Cyclin A requirement during terminal division cycles. Cyclin E is known to be downregulated during terminal division cycles to allow a timely cell cycle exit after the final mitosis. Cyclin E is therefore no longer available before terminal mitoses to prevent premature Fizzy-related/Cdh1 activation. As a consequence, Cyclin A, which can also function as a negative regulator of Fizzy-related/Cdh1, becomes essential to provide this inhibition before terminal mitoses. In the absence of Cyclin A, premature Fizzy-related/Cdh1 activity results in the premature degradation of the Cdk1 activators Cyclin B and Cyclin B3, and apparently of String/Cdc25 phosphatase as well. Without these activators, entry into terminal mitoses is not possible. However, entry into terminal mitoses can be restored by the simultaneous expression of versions of Cyclin B and Cyclin B3 without destruction boxes, along with a Cdk1 mutant that escapes inhibitory phosphorylation on T14 and Y15. Moreover, terminal mitoses are also restored in Cyclin A mutants by either the elimination of Fizzy-related/Cdh1 function or Cyclin E overexpression.
Reber
Achim
A
BZMB, Department of Genetics, University of Bayreuth, 95440 Bayreuth, Germany.
Lehner
Christian F
CF
Jacobs
Henning W
HW
D000818:Animals; D064200:Cdh1 Proteins; D018797:Cell Cycle Proteins / Q000378:metabolism*; D019925:Cyclin A / Q000235:genetics / Q000378:metabolism*; D019927:Cyclin E / Q000235:genetics / Q000378:metabolism; D016213:Cyclins / Q000378:metabolism; D015536:Down-Regulation; D004330:Drosophila / Q000166:cytology / Q000196:embryology*; D029721:Drosophila Proteins / Q000378:metabolism*; D004625:Embryo, Nonmammalian / Q000378:metabolism; D047108:Embryonic Development / Q000235:genetics; D004817:Epidermis / Q000378:metabolism; D008938:Mitosis / Q000235:genetics; D009154:Mutation; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism*
D064200:Cdh1 Proteins; D018797:Cell Cycle Proteins; D019925:Cyclin A; D019927:Cyclin E; D016213:Cyclins; D029721:Drosophila Proteins; C577674:fzr protein, Drosophila; D017027:Protein Tyrosine Phosphatases; C059217:stg protein, Drosophila