Fused sister kinetochores initiate the reductional division of mieosis I & phosphoserines on histone H3 and CENH3 demarcate the centromere and pericentromere during chromosome segregation
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The kinetochore, a protein complex that assembles on the centromere, tethers the chromosome to the microtubules and plays fundamental roles in chromosome orientation and faithful segregation during cell division. Here we show that kinetochore structural component MIS12 forms a visible bridge between sister kinetochores that is required for reductional chromosome segregation in meiosis I. MIS12 and microtubule binder NDC80 appear as a bridge between sister kinetochores. In Mis12 knockdown mutants, the visible MIS12/NDC80 bridge between sister kinetochores is lost, and chromosomes orient randomly. The outcome is severe meiosis II defects and overall meiotic failure. Meiosis-specific Rec8 cohesion and its protector Shugoshin (SGO) have also been implicated in controlling sister chromatid co-orientation in meiosis I. Our analysis shows that the MIS12 function is distinct from the Shugoshin/cohesion system between chromosome arms. The fusion of sister kinetochores by the MIS12/NDC80 bridge provides a unified microtubule binding interface and promotes sister chromatid co-segregation in meiosis I. Second, we show that phosphorylation of H3 on serine 28 (phH3-Ser28) in maize is a cell cycle dependent and pericentromere-specific posttranslational modification. It is undetectable in interphase, becomes increasingly apparent with cell cycle progression, and disappears in telophase. A unique feature of H3-Ser28 phosphorylation is that it is strictly limited to the pericentromeric domains during cell division. Considering the densely distributed cohesion in this heterochromatic domain, H3-Ser28 phosphorylation may serve as an epigenetic marker to label the cohesive region. Interestingly, CENH3, a histone H3 variant exclusively recruited in the centromere, is phosphorylated on serine 50 (phCENH3-Ser50) following the same temporal pattern as H3Ser28 phosphorylation. Together, we propose that the primary role of the CENH3Ser50 and H3Ser28 phosphorylation is to demarcate the centromere and its flanking pericentromere domains during the cell division. Lastly, we performed functional analysis of plant aurora kinases in the model plant Arabidopsis thaliana. Aurora kinases play pivotal roles in regulating the cell cycle in animals and yeast by phoshorylating versatile substrates including histone H3 and CENH3. Our data shows that plant aurora kinases have distinct functions implicated in cell division and many developmental pathways. Knockdown or overexpression of Arabidopsis aurora kinases leads to pleiotropic developmental defects in plant growth.