[
International Worm Meeting,
2021]
The centromere is a chromosomal region that recruits the kinetochore, a large multi-subunit complex that binds spindle microtubules enabling chromosome segregation. Centromeric chromatin is therefore placed on the surface of condensed chromosomes, and specifically adapted to withstand the spindle pulling forces. The cross-talk between centromeres and condensation machinery is an area of active study. In contrast to other commonly studied model organisms, C. elegans is a holocentric species: its centromeres are placed along the chromosome axis, which makes it an attractive model to study centromere organisation. Depletion of the centromeric histone H3 variant CENP-A (HCP-3 in C. elegans) and its main loading factor KNL-2 in embryos results in defective chromosome formation. However, little is known about the mechanism of their action and the temporal regulation. Using an immunoprecipitation followed by mass spectrometry, we identified regulatory phosphosites on KNL-2 that are targeted by CDK-1 kinase. Mutation of these residues to alanines to prevent their phosphorylation leads to defects in cell division and an increase in embryonic lethality. We show that compromised chromosome formation is underlying these phenotypes. Mutant embryos exhibit delayed prophase condensation, and the prometaphase chromosomes lack rigidity and twist around their axis. Interestingly, centromeric chromatin seems properly maintained, as evidenced by unchanged CENP-A genomic distribution and proper kinetochore recruitment in the mutant strain. We further found that condensin II levels are reduced on metaphase chromosomes in the mutant embryos, indicating that that KNL-2 may act through condensin II complex recruitment. We conclude that KNL-2 orchestrates centromeric chromatin maintenance as well as the establishment of the proper chromosome structure, and that these processes are independently regulated.