714B Poster - 11. Cell division and cell growth
Friday April 08, 2:00 PM - 4:00 PM

Authors:
Yuki Shinso; Amanda Amodeo

Affiliation: Dartmouth College

Keywords:
k. cell cycle control; l. checkpoint

The early embryos of many species including Drosophila spend the first few hours of development undergoing a series of rapid cell cycles to generate the large number of cells required for embryogenesis. These early reductive divisions are largely transcriptionally silenced and therefore depend upon pools of maternally provided components. Cessation of the rapid cell cycle program at the mid-blastula transition (MBT) is dependent on achieving a sufficient nuclear to cytoplasmic (N/C) ratio and the activity of the DNA-damage checkpoint kinase, Chk1 (grapes). How Chk1 activity is coupled to the N/C ratio has remained unclear. We have shown that large stores of extra-nucleosomal histone H3 accumulate in the early nuclei because the maternally provided histone pools vastly exceed the amount of DNA in the early embryo. As the N/C ratio increases these pools are exhausted. At the same time, histone H3 is a known Chk1 substrate. Therefore, we hypothesized that in the early nuclei H3 may compete with Chk1’s cell cycle regulatory targets (CDC25-string and twine) to prevent cell cycle slowing before the correct N/C ratio is reached at the MBT. To test this we expressed a version of the H3 N-terminal tail that contains the Chk1 binding site, but cannot be incorporated into chromatin in the pre-MBT embryo. We found that this 47-amino acid fragment is sufficient to delay cell cycle slowing and reduce Chk1 activity in vitro and in vivo. Mutation of the Chk1 phosphosite in the context of the full length H3 protein is sufficient to lengthen the cell cycle and accelerate the onset of the MBT. Together our data support a model in which hyper-abundant histone H3 in the early embryo competitively inhibits Chk1 to prevent cell cycle slowing until a sufficient N/C ratio is achieved. We are currently working to determine if H3 inhibits Chk1 in other developmental contexts including in response to DNA-damage.