Biology subjects

Boerner, G. V.

Publications and source records attributed to Boerner, G. V..

4 recordsLinked to original sources

bioRxiv2026

A Kinetochore-Associated Proteasome Pool Drives a Second Pathway of Cohesin Removal during Meiosis

Accurate chromosome segregation requires the spatiotemporally regulated removal of sister chromatid cohesion. Cohesin cleavage by the endopeptidase separase depends on destruction of its inhibitor securin by the ubiquitin-proteasome system (UPS) and on phosphorylation-mediated priming of the cohesin kleisin subunit. Whether the UPS also contributes to cohesin priming has remained unknown. Here, we show that the 26S proteasome mediates cohesin removal during meiosis II through branches of two parallel pathways. Using separation-of-function mutants targeting either the proteasome's core or regulatory particles, we identify a proteasome function that is required specifically for centromeric cohesin removal during meiosis II but dispensable for separase activation. Defects in this proteasome function causes centromeric accumulation of phosphatase anchor shugoshin (Sgo1), impaired cleavage of meiotic kleisin Rec8, and frequent failure of sister chromatid segregation. Bypassing the requirement for Rec8 priming, either through a phosphomimetic rec8 allele or by separase-independent Rec8 cleavage, restores chromosome segregation, demonstrating that the proteasome mediates cohesin removal independently of its established role in activating separase. Consistent with a direct role, proteasomes localize prominently to kinetochores during meiosis II. Together, these findings identify the proteasome as a dual-function regulator that mediates both separase activation and cohesin priming, revealing how a single proteolytic machine coordinates the two molecular pathways underlying stepwise chromosome segregation during meiosis.

genetics↗

bioRxiv2025

An H3K79 Methylation-Dependent Checkpoint Blocks Holliday Junction Resolution and Meiotic Divisions under Heat Stress

Meiosis, the specialized cell division that produces haploid gametes from diploid precursors, is markedly more sensitive to heat stress than mitosis in a wide range of organisms, despite the two processes utilizing largely overlapping cellular machinery. The mechanistic basis of meiotic heat sensitivity has remained unclear. Here, we show that in budding yeast, even moderate heat stress impairs the processing of programmed double-strand breaks into crossovers, resulting in permanent meiotic division arrest. This arrest depends on methylation of histone H3 at lysine 79 (H3K79) by Dot1, and a subset of other components of the meiotic recombination checkpoint. Thus, in contrast to the canonical heat shock response that adapts mitotically dividing cells to elevated temperatures, meiotic arrest is triggered by stalled recombination intermediates in the context of the H3K79 chromatin modification. These findings suggest a conserved epigenetic pathway responsible for heat-induced meiotic failure across eukaryotes. Genetically enhancing the heat tolerance of meiotic chromosome metabolism may increase reproductive resilience of organisms facing rising global temperatures, bridging the gap until mitigation strategies are in place. Significance StatementMeiosis generates haploid gametes through a specialized cell division that involves crossovers between homologous chromosomes. Crossovers promote genetic diversity and ensure proper chromosome segregation during meiosis I. In many organisms, meiosis stalls under moderate heat stress at temperatures permissive for mitosis, a phenomenon with implications for crop fertility and stability of ecosystems. Using yeast, we discovered that heat stress blocks meiosis at Holliday junctions - the four-armed DNA intermediates that resolve into crossovers - while alternative non-crossovers form normally. Arrest depends on the H3K79 methylation-dependent checkpoint involving Dot1. Thus, heat sensitivity of meiosis stems from checkpoint surveillance of impaired chromosome interactions. Modulating the underlying mechanism could enhance reproductive resilience to a warming climate.

genetics↗

bioRxiv2024

Temporal and Functional Relationship between Synaptonemal Complex Morphogenesis and Recombination during Meiosis

During prophase of meiosis I, programmed double strand breaks (DSBs) are processed into crossovers, a critical requirement for segregation of homologous chromosomes (homologs) and genome haploidization in sexually reproducing organisms. Crossovers form via homologous recombination in close temporospatial association with morphogenesis of the synaptonemal complex (SC), a proteinaceous structure that connects paired homologs along their length during the pachytene stage. Synapsis and recombination are a paradigm for the interplay between higher order chromosome structure and DNA metabolism, yet their temporal and functional relationship remains poorly understood. Probing linkage between these processes in budding yeast, we show that SC assembly is associated with a distinct threshold number of unstable D-loops. The transition from bona fide paranemic D-loops to plectonemic DSB single end invasions (SEIs) is completed during midpachynema, when the SC is fully assembled. Double Holliday junctions (dHJs) form at the time of desynapsis and are resolved into crossovers during diplonema. The SC central element component Zip1 shepherds recombination through three transitions, including DSB first end strand exchange and second end capture, as well as dHJ resolution. Zip1 mediates SEI formation independent of its polymerization whereas precocious Zip1 assembly interferes with double Holliday junction resolution. Together, our findings indicate that the synaptonemal complex controls recombination while assembled but also beyond its disassembly, possibly by establishing spatial constraints at recombination sites.

genetics↗

bioRxiv2023

Modeling identifies chromosome number, size, and non-homologous repulsion as novel determinants of meiotic pairing

During meiosis, pairing of homologous chromosomes (homologs) ensures the formation of haploid gametes from diploid precursor cells, a prerequisite for sexual reproduction. Pairing during meiotic prophase I facilitates crossover recombination and homolog segregation during the ensuing reductional cell division. Mechanisms that ensure stable homolog alignment in the presence of an excess of non-homologous chromosomes have remained elusive, but rapid chromosome movements during prophase I appear to play a role in the process. Apart from homolog attraction, provided by early intermediates of homologous recombination, dissociation of non-homologous associations also appears to contribute to homolog pairing, as suggested by the detection of stable non-homologous chromosome associations in pairing-defective mutants. Here, we have developed an agent-based model for homolog pairing derived from the dynamics of a naturally occurring chromosome ensemble. The model simulates unidirectional chromosome movements, as well as collision dynamics determined by attractive and repulsive forces arising from close-range physical interactions. In addition to homolog attraction, chromosome number and size as well as movement velocity and repulsive forces are identified as key factors in the kinetics and efficiency of homologous pairing. Dissociation of interactions between non-homologous chromosomes may contribute to pairing by crowding homologs into a limited nuclear area thus creating preconditions for close-range homolog attraction. Predictions from the model are readily compared to experimental data from budding yeast, parameters can be adjusted to other cellular systems and predictions from the model can be tested via experimental manipulation of the relevant chromosomal features. Author summaryPairing of homologous chromosomes (homologs) is a key feature of multiple cellular processes including gene expression control, chromosome break repair, and chromosome segregation. Homolog pairing during meiosis is shared among all sexually reproducing eukaryotes. Mechanistic determinants of homology-specific chromosome alignment are presently unknown. We have developed an agent-based model where contributions of the entire chromosome set to the pairing process is taken into account, comprising both homologous and non-homologous chromosomal encounters. Incorporating natural chromosome lengths, the model accurately recapitulates efficiency and kinetics of homolog pairing observed for wild-type and mutant meiosis in budding yeast, and can be adapted to nuclear dimensions and chromosome sets of other organisms.

biophysics↗