Research Article: Nucleolar asymmetry and the importance of septin integrity upon cell cycle arrest

Date Published: March 24, 2017

Publisher: Public Library of Science

Author(s): Urvashi Rai, Fadi Najm, Alan M. Tartakoff, Claude Prigent.


Cell cycle arrest can be imposed by inactivating the anaphase promoting complex (APC). In S. cerevisiae this arrest has been reported to stabilize a metaphase-like intermediate in which the nuclear envelope spans the bud neck, while chromatin repeatedly translocates between the mother and bud domains. The present investigation was undertaken to learn how other features of nuclear organization are affected upon depletion of the APC activator, Cdc20. We observe that the spindle pole bodies and the spindle repeatedly translocate across the narrow orifice at the level of the neck. Nevertheless, we find that the nucleolus (organized around rDNA repeats on the long right arm of chromosome XII) remains in the mother domain, marking the polarity of the nucleus. Accordingly, chromosome XII is polarized: TelXIIR remains in the mother domain and its centromere is predominantly located in the bud domain. In order to learn why the nucleolus remains in the mother domain, we studied the impact of inhibiting rRNA synthesis in arrested cells. We observed that this fragments the nucleolus and that these fragments entered the bud domain. Taken together with earlier observations, the restriction of the nucleolus to the mother domain therefore can be attributed to its massive structure. We also observed that inactivation of septins allowed arrested cells to complete the cell cycle, that the alternative APC activator, Cdh1, was required for completion of the cell cycle and that induction of Cdh1 itself caused arrested cells to progress to the end of the cell cycle.

Partial Text

Upon loss of the activator of the anaphase promoting complex, Cdc20, in S. cerevisiae, DNA replicates and the bud grows until it is essentially as large as the mother. The metaphase-like arrest at this point is known as “medial nuclear division arrest.” Cohesin keeps sister chromatids associated with each other and the spindle does not elongate. At this point, the cell is poised to enter into anaphase [1–7].

Orderly progression through the cell cycle depends on checkpoints that impose arrest if critical features of the cell are not prepared to advance. Without these surveillance mechanisms, catastrophic mistakes occur [66, 70, 73, 76–78]. From the point of view of nuclear dynamics, we find that the arrest response to DNA damage (elicited by exposing single-stranded DNA in cdc13-1 [79, 80]) is very similar to the Cdc20 arrest (S8 Fig).