Cells cycle with no poles
Cells cycle with no poles- no poles encodes a predicted E3 ubiquitin ligase required for early embryonic development of Drosophila
Julie A. Merkle1, Jamie L. Rickmyre1, Aprajita Garg2, Erin B. Loggins1, Jeanne N. Jodoin1, Ethan Lee1, Louisa P. Wu2 and Laura A. Lee1,*
1 Department of Cell and Developmental Biology, Vanderbilt University Medical Center, U-4200 MRBIII, 465 21st Avenue South, Nashville, TN 37232, USA.
2 Center for Biosystems Research, University of Maryland Biotechnology Institute, 5115 Plant Sciences Building, College Park, MD 20742, USA.
* Author for correspondence (e-mail: laura.a.lee@vanderbilt.edu) Accepted 25 November 2008
In a screen for cell-cycle regulators, we identified a Drosophila maternal effect-lethal mutant that we named `no poles' (nopo). Embryos from nopo females undergo mitotic arrest with barrel-shaped, acentrosomal spindles during the rapid S-M cycles of syncytial embryogenesis. We identified CG5140, which encodes a candidate RING domain-containing E3 ubiquitin ligase, as the nopo gene. A conserved residue in the RING domain is altered in our EMS-mutagenized allele of nopo, suggesting that E3 ligase activity is crucial for NOPO function. We show that mutation of a DNA checkpoint kinase, CHK2, suppresses the spindle and developmental defects of nopo-derived embryos, revealing that activation of a DNA checkpoint operational in early embryos contributes significantly to the nopo phenotype. CHK2-mediated mitotic arrest has been previously shown to occur in response to mitotic entry with DNA damage or incompletely replicated DNA. Syncytial embryos lacking NOPO exhibit a shorter interphase during cycle 11, suggesting that they may enter mitosis prior to the completion of DNA replication. We show that Bendless (BEN), an E2 ubiquitin-conjugating enzyme, interacts with NOPO in a yeast two-hybrid assay; furthermore, ben-derived embryos arrest with a nopo-like phenotype during syncytial divisions. These data support our model that an E2-E3 ubiquitination complex consisting of BEN-UEV1A (E2 heterodimer) and NOPO (E3 ligase) is required for the preservation of genomic integrity during early embryogenesis.