SummaryBackgroundAccurate chromosome segregation depends on the establishment of correct—amphitelic—kinetochore orientation. Merotelic kinetochore orientation is an error that occurs when a single kinetochore attaches to microtubules emanating from opposite spindle poles, a condition that hinders segregation of the kinetochore to a spindle pole in anaphase. To avoid chromosome missegregation resulting from merotelic kinetochore orientation, cells have developed mechanisms to prevent or correct merotelic attachment. A protein called Pcs1 has been implicated in preventing merotelic attachment in mitosis and meiosis II in the fission yeast S. pombe.ResultsWe report that Pcs1 forms a complex with a protein called Mde4. Both Pcs1 and Mde4 localize to the central core of centromeres. Deletion of mde4+, like that of pcs1+, causes the appearance of lagging chromosomes during the anaphases of mitotic and meiosis II cells. We provide evidence that the kinetochores of lagging chromosomes in both pcs1 and mde4 mutant cells are merotelically attached. In addition, we find that lagging chromosomes in cells with defective centromeric heterochromatin also display features consistent with merotelic attachment.ConclusionsWe suggest that the Pcs1/Mde4 complex is the fission yeast counterpart of the budding yeast monopolin subcomplex Csm1/Lrs4, which promotes the segregation of sister kinetochores to the same pole during meiosis I. We propose that the Pcs1/Mde4 complex acts in the central kinetochore domain to clamp microtubule binding sites together, the centromeric heterochromatin coating the flanking domains provides rigidity, and both systems contribute to the prevention of merotelic attachment.

We have designed the most efficient strategy to knock out genes in fission yeast Schizosaccharomyces pombe on a large scale. Our technique is based on knockout constructs that contain regions homologous to the target gene cloned into vectors carrying dominant drug-resistance markers. Most of the steps are carried out in a 96-well format, allowing simultaneous deletion of 96 genes in one batch. Based on our knockout technique, we designed a strategy for cloning knockout constructs for all predicted fission yeast genes, which is available in a form of a searchable database http://mendel.imp.ac.at/Pombe_deletion/. We validated this technique in a screen where we identified novel genes required for chromosome segregation during meiosis. Here, we present our protocol with detailed instructions. Using this protocol, one person can knock out 96 S. pombe genes in 8 days.

Segregation of chromosomes during meiosis depends on separase cleavage of Rec8, the meiosis-specific alpha-kleisin subunit of cohesin. We mapped Rec8 phosphorylation sites by mass spectrometry and show that, in fission yeast, Rec8 phosphorylation is required for proper chromosome disjunction during meiosis. We further show that the fission yeast casein kinase 1 (CK1) delta/epsilon isoforms Hhp1 and Hhp2 are required for full levels of Rec8 phosphorylation and for efficient removal of Rec8 at the onset of anaphase I. Our data are consistent with the model that Hhp1/Hhp2-dependent phosphorylation of Rec8 is required for separase-mediated cleavage of Rec8 during meiosis I.