Meiosis-Specific Loading of the Centromere-Specific Histone CENH3 in Arabidopsis thaliana

Abstract

Centromere behavior is specialized in meiosis I, so that sister chromatids of homologous chromosomes are pulled toward the same side of the spindle (through kinetochore mono-orientation) and chromosome number is reduced. Factors required for mono-orientation have been identified in yeast. However, comparatively little is known about how meiotic centromere behavior is specialized in animals and plants that typically have large tandem repeat centromeres. Kinetochores are nucleated by the centromere-specific histone CENH3. Unlike conventional histone H3s, CENH3 is rapidly evolving, particularly in its N-terminal tail domain. Here we describe chimeric variants of CENH3 with alterations in the N-terminal tail that are specifically defective in meiosis. Arabidopsis thaliana cenh3 mutants expressing a GFP-tagged chimeric protein containing the H3 N-terminal tail and the CENH3 C-terminus (termed GFP-tailswap) are sterile because of random meiotic chromosome segregation. These defects result from the specific depletion of GFP-tailswap protein from meiotic kinetochores, which contrasts with its normal localization in mitotic cells. Loss of the GFP-tailswap CENH3 variant in meiosis affects recruitment of the essential kinetochore protein MIS12. Our findings suggest that CENH3 loading dynamics might be regulated differently in mitosis and meiosis. As further support for our hypothesis, we show that GFP-tailswap protein is recruited back to centromeres in a subset of pollen grains in GFP-tailswap once they resume haploid mitosis. Meiotic recruitment of the GFP-tailswap CENH3 variant is not restored by removal of the meiosis-specific cohesin subunit REC8. Our results reveal the existence of a specialized loading pathway for CENH3 during meiosis that is likely to involve the hypervariable N-terminal tail. Meiosis-specific CENH3 dynamics may play a role in modulating meiotic centromere behavior. There are two types of cell division in eukaryotes. Mitosis produces cells with identical copies of the genome, while meiosis produces gametes with half the number of chromosomes found in the parent cell. Faithful genome inheritance is controlled by centromeres, chromosomal structures that allow duplicated chromosomes to be pulled apart correctly during cell division. Centromeres are differentially configured during meiosis (relative to mitosis) so chromosome number can be reduced by half. Centromeres are built upon a specialized DNA packing protein, CENH3. Here we describe altered forms of CENH3 that are loaded correctly during mitosis but are severely depleted from centromeres in meiotic cells. As CENH3 is essential for chromosome inheritance, plants expressing these versions of the protein are sterile because they produce very few viable gametes. Differential loading of CENH3 during meiosis may play a role in modulating chromosome inheritance to form haploid gametes.