Rec8 phosphorylation and recombination promote the step-wise loss of cohesins in meiosis.

During meiosis, cohesins--protein complexes that hold sister chromatids together--are lost from chromosomes in a step-wise manner. Loss of cohesins from chromosome arms is necessary for homologous chromosomes to segregate during meiosis I. Retention of cohesins around centromeres until meiosis II is required for the accurate segregation of sister chromatids. Here we show that phosphorylation of the cohesin subunit Rec8 contributes to step-wise cohesin removal. Our data further implicate two other key regulators of meiotic chromosome segregation, the cohesin protector Sgo1 and meiotic recombination in bringing about the step-wise loss of cohesins and thus the establishment of the meiotic chromosome segregation pattern. Understanding the interplay between these processes should provide insight into the events underlying meiotic chromosome mis-segregation, the leading cause of miscarriages and mental retardation in humans.

Spo13 maintains centromeric cohesion and kinetochore coorientation during meiosis I.

BACKGROUND: The meiotic cell cycle, the cell division cycle that leads to the generation of gametes, is unique in that a single DNA replication phase is followed by two chromosome segregation phases. During meiosis I, homologous chromosomes are segregated, and during meiosis II, as in mitosis, sister chromatids are partitioned. For homolog segregation to occur during meiosis I, physical linkages called chiasmata need to form between homologs, sister chromatid cohesion has to be lost in a stepwise manner, and sister kinetochores must attach to microtubules emanating from the same spindle pole (coorientation). RESULTS: Here we show that the meiosis-specific factor Spo13 functions in two key aspects of meiotic chromosome segregation. In cells lacking SPO13, cohesin, which is the protein complex that holds sister chromatids together, is not protected from removal around kinetochores during meiosis I but is instead lost along the entire length of the chromosomes. We furthermore find that Spo13 promotes sister kinetochore coorientation by maintaining the monopolin complex at kinetochores. In the absence of SPO13, Mam1 and Lrs4 disassociate from kinetochores prematurely during pro-metaphase I and metaphase I, resulting in a partial defect in sister kinetochore coorientation in spo13 Delta cells. CONCLUSIONS: Our results indicate that Spo13 has the ability to regulate both the stepwise loss of sister chromatid cohesion and kinetochore coorientation, two essential features of meiotic chromosome segregation.

Shugoshin promotes sister kinetochore biorientation in Saccharomyces cerevisiae.

Chromosome segregation must be executed accurately during both mitotic and meiotic cell divisions. Sgo1 plays a key role in ensuring faithful chromosome segregation in at least two ways. During meiosis this protein regulates the removal of cohesins, the proteins that hold sister chromatids together, from chromosomes. During mitosis, Sgo1 is required for sensing the absence of tension caused by sister kinetochores not being attached to microtubules emanating from opposite poles. Here we describe a differential requirement for Sgo1 in the segregation of homologous chromosomes and sister chromatids. Sgo1 plays only a minor role in segregating homologous chromosomes at meiosis I. In contrast, Sgo1 is important to bias sister kinetochores toward biorientation. We suggest that Sgo1 acts at sister kinetochores to promote their biorientation.

The core centromere and Sgo1 establish a 50-kb cohesin-protected domain around centromeres during meiosis I.

The stepwise loss of cohesins, the complexes that hold sister chromatids together, is required for faithful meiotic chromosome segregation. Cohesins are removed from chromosome arms during meiosis I but are maintained around centromeres until meiosis II. Here we show that Sgo1, a protein required for protecting centromeric cohesins from removal during meiosis I, localizes to cohesin-associated regions (CARs) at the centromere and the 50-kb region surrounding it. Establishment of this Sgo1-binding domain requires the 120-base-pair (bp) core centromere, the kinetochore component Bub1, and the meiosis-specific factor Spo13. Interestingly, cohesins and the kinetochore proteins Iml3 and Chl4 are necessary for Sgo1 to associate with pericentric regions but less so for Sgo1 to associate with the core centromeric regions. Finally, we show that the 50-kb Sgo1-binding domain is the chromosomal region where cohesins are protected from removal during meiosis I. Our results identify the portions of chromosomes where cohesins are protected from removal during meiosis I and show that kinetochore components and cohesins themselves are required to establish this cohesin protective domain.