MEIOTIC CENTROMERES · Function of centromeres during meiotic bouquet stage and protein composition of telomeres

Function of centromeres during meiotic bouquet stage and protein composition of telomeres

Meiosis is a fundamental process in the eukaryotic lifecycle, as all gametes are formed via this specialised cell division. Defects in the meiotic cell cycle, such as mis-segregation of chromosomes, may cause fetal abnormalities such as Down's syndrome, and could be responsible for most miscarriages. Meiosis is composed of a round of deoxyribonucleic acid (DNA) replication followed by two cell divisions. The first meiotic division is a reductional division, in which the homologues separate. In the second division, the sister chromatids separate, as in the equational divisions of mitosis. The telomere bouquet is a conserved feature of meiosis, characterised by the clustering of the telomeres at the nuclear periphery, usually near the centrosome. The function of the bouquet remains a mystery despite its remarkable conservation amongst eukaryotes. Suggestions for the role of the bouquet include the anchoring of chromosomes to a confined space, thus assisting in homologue pairing, and the prevention of ectopic recombination. Although these suggestions found some evidence in the study of the fission yeast bouquet stage (Ding et al. 2004, Davis and Smith 2006), it became clear that the fission yeast bouquet has further critical functions in meiosis in addition to the ones suggested so far. This came to light upon observation of the large difference in spore viability between bouquet deficient mutants and mutants defective in horsetail movement despite both these mutants displaying similar defects in pairing and recombination (Tomita and Cooper 2007). Therefore, the aim of our studies became the identification of these additional roles of the bouquet in fission yeast meiosis. Previous work from our lab has shown that disruption of bouquet formation impairs the meiotic spindle, such that spindle formation fails in approximately 45 % of meiotic cells. Surprisingly, we noticed that bouquet deficient cells with a functional meiotic spindle have further meiotic defects, such as lagging and un-attached chromosomes. These chromosome attachment defects suggest an impairment of kinetochore function. To directly assess this possibility, we checked the co-localisation of several kinetochore markers, such as Mis6, Cnp1 and Dad1, with centromeric DNA. Our results suggest that in the absence of the bouquet, kinetochore proteins fail to localise to several centromeres in 15 - 20 % of meiotic cells. Intriguingly, the HP1 homolog Swi6 also fails to localise to some centromeres in bouquet deficient meiotic cells, suggesting failure in heterochromatin formation and / or in Swi6 deposition, which may lead to kinetochore defects in a fraction of centromeres. Meiotic DSBs in the centromere are more frequent in heterochromatin mutants compared to wild-type. We therefore investigated the requirement for Spo11 homolog Rec12 in the regulation of kinetochore assembly during bouquet deficient meiosis. We show that, in the double mutant rec12Delta bqt1Delta the kinetochore defect is reduced compared to the bqt1Delta single mutant. Surprisingly, when a catalytically dead mutant of Rec12 (Y98F) is combined with the bqt1Delta mutation, no rescue was observed and the kinetochore defects were similar to the bqt1Delta single mutant. This shows the occurrence of a damaging activity of Rec12 on the kinetochore in the absence of the bouquet, which is unrelated to the DSB catalytic activity of Rec12. Taken together, the data we present suggests a previously unanticipated role for telomere bouquet formation in coordinating assembly of the meiotic kinetochore. We propose a model in which the transient co-localisation of telomeres and centromeres during meiotic prophase provides the telomere bouquet with the opportunity to influence kinetochore assembly through direct contact between telomeres and centromeres.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз