IBD2 encodes a novel component of the Bub2p-dependent spindle checkpoint in the budding yeast Saccharomyces cerevisiae

Hypoxia induces p53-dependent transactivation and Fas/CD95-dependent apoptosis

p53 triggers apoptosis in response to cellular stress. We analyzed p53-dependent gene and protein expression in response to hypoxia using wild-type p53-carrying or p53 null HCT116 colon carcinoma cells. Hypoxia induced p53 protein levels and p53-dependent apoptosis in these cells. cDNA microarray analysis revealed that only a limited number of genes were regulated by p53 upon hypoxia. Most classical p53 target genes were not upregulated. However, we found that Fas/CD95 was significantly induced in response to hypoxia in a p53-dependent manner, along with several novel p53 target genes including ANXA1, DDIT3/GADD153 (CHOP), SEL1L and SMURF1. Disruption of Fas/CD95 signalling using anti-Fas-blocking antibody or a caspase 8 inhibitor abrogated p53-induced apoptosis in response to hypoxia. We conclude that hypoxia triggers a p53-dependent gene expression pattern distinct from that induced by other stress agents and that Fas/CD95 is a critical regulator of p53-dependent apoptosis upon hypoxia.

The spindle position checkpoint in budding yeast: the motherly care of MEN

Mitotic exit and cytokinesis must be tightly coupled to nuclear division both in time and space in order to preserve genome stability and to ensure that daughter cells inherit the right set of chromosomes after cell division. This is achieved in budding yeast through control over a signal transduction cascade, the mitotic exit network (MEN), which is required for mitotic CDK inactivation in telophase and for cytokinesis. Current models of MEN activation emphasize on the bud as the place where most control is exerted. This review focuses on recent data that instead point to the mother cell as being the residence of key regulators of late mitotic events.

The C-terminus of Bfa1p in budding yeast is essential to induce mitotic arrest in response to diverse checkpoint-activating signals

During mitosis, genomic integrity is maintained by the proper coordination of anaphase entry and mitotic exit through mitotic checkpoints. In budding yeast, exit from mitosis is triggered by the activation of the small GTPase Tem1p. Bfa1p in association with Bub2p negatively regulates Tem1p in response to spindle damage, spindle misorientation, and DNA damage, resulting in cell cycle arrest. To delineate the Bfa1p domains that respond to distinct checkpoint signals, we constructed 13 Bfa1 deletion mutants. The C-terminal 184 amino acids of Bfa1p (Bfa1-D8(391-574)) contained the entire capacity of Bfa1p to generate mitotic arrest in response to spindle damage, spindle misorientation, and DNA damage. This domain was also enough to interact with the mitotic exit network proteins Tem1p, Bub2p, and Cdc5p, and to localize to the spindle pole body (SPB). Over-expression of Bfa1-D8(391-574) induced late anaphase arrest as efficient as the full-length Bfa1p in a Bub2p-dependent manner. In contrast, the N-terminal portion of Bfa1p (Bfa1-D16(1-376)) could not localize to SPB and did not block mitotic exit in response to diverse checkpoint signals. Bfa1-D16(1-376) interacted with Tem1p but not with Bub2p and its over-expression partially arrested cells in mitosis in the absence of Bub2p. By random mutagenesis of Bfa1-D8(391-574) with hydroxylamine, we isolated a point mutant of D8, D8(E438K), which interacts with both Tem1p and Bub2p but cannot respond to checkpoint signals. This mutant also showed reduced efficiency in the localization to SPB. Taken together, our study demonstrated that various checkpoint signals are transmitted to the C-terminal domain of Bfa1 (Bfa1-D8(391-574)) and that Bfa1p localization to SPB is necessary but not sufficient to regulate mitotic exit in response to various checkpoint signals.

Essential tension and constructive destruction: the spindle checkpoint and its regulatory links with mitotic exit

Replicated genetic material must be partitioned equally between daughter cells during cell division. The precision with which this is accomplished depends critically on the proper functioning of the mitotic spindle. The assembly, orientation and attachment of the spindle to the kinetochores are therefore constantly monitored by a surveillance mechanism termed the SCP (spindle checkpoint). In the event of malfunction, the SCP not only prevents chromosome segregation, but also inhibits subsequent mitotic events, such as cyclin destruction (mitotic exit) and cytokinesis. This concerted action helps to maintain temporal co-ordination among mitotic events. It appears that the SCP is primarily activated by either a lack of occupancy or the absence of tension at kinetochores. Once triggered, the inhibitory circuit bifurcates, where one branch restrains the sister chromatid separation by inhibiting the E3 ligase APC(Cdc20) (anaphase-promoting complex activated by Cdc20) and the other impinges on the MEN (mitotic exit network). A large body of investigations has now led to the identification of the control elements, their targets and the functional coupling among them. Here we review the emerging regulatory network and discuss the remaining gaps in our understanding of this effective mechanochemical control system.