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The synaptonemal complex protein, Zip1, promotes the segregation of nonexchange chromosomes at meiosis I.

ABSTRACT: Crossing over establishes connections between homologous chromosomes that promote their proper segregation at the first meiotic division. However, there exists a backup system to ensure the correct segregation of those chromosome pairs that fail to cross over. We have found that, in budding yeast, a mutation eliminating the synaptonemal complex protein, Zip1, increases the meiosis I nondisjunction rate of nonexchange chromosomes (NECs). The centromeres of NECs become tethered during meiotic prophase, and this tethering is disrupted by the zip1 mutation. Furthermore, the Zip1 protein often colocalizes to the centromeres of the tethered chromosomes, suggesting that Zip1 plays a direct role in holding NECs together. Zip3, a protein involved in the initiation of synaptonemal complex formation, is also important for NEC segregation. In the absence of Zip3, both the tethering of NECs and the localization of Zip1 to centromeres are impaired. A mutation in the MAD3 gene, which encodes a component of the spindle checkpoint, also increases the nondisjunction of NECs. Together, the zip1 and mad3 mutations have an additive effect, suggesting that these proteins act in parallel pathways to promote NEC segregation. We propose that Mad3 promotes the segregation of NECs that are not tethered by Zip1 at their centromeres.

SUBMITTER: Newnham L

PROVIDER: S-EPMC2818913 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Accurate chromosome segregation during meiosis relies on the prior establishment of at least one crossover recombination event between homologous chromosomes. Most meiotic recombination intermediates that give rise to interhomolog crossovers are embedded within a hallmark chromosomal structure called the synaptonemal complex (SC), but the mechanisms that coordinate the processes of SC assembly (synapsis) and crossover recombination remain poorly understood. Among known structural components of the budding yeast SC, the Zip1 protein is unique for its independent role in promoting crossover recombination; Zip1 is specifically required for the large subset of crossovers that also rely on the meiosis-specific MutSγ complex. Here we report that adjacent regions within Zip1's N terminus encompass its crossover and synapsis functions. We previously showed that deletion of Zip1 residues 21-163 abolishes tripartite SC assembly and prevents robust SUMOylation of the SC central element component, Ecm11, but allows excess MutSγ crossover recombination. We find the reciprocal phenotype when Zip1 residues 2-9 or 10-14 are deleted; in these mutants SC assembles and Ecm11 is hyperSUMOylated, but MutSγ crossovers are strongly diminished. Interestingly, Zip1 residues 2-9 or 2-14 are required for the normal localization of Zip3, a putative E3 SUMO ligase and pro-MutSγ crossover factor, to Zip1 polycomplex structures and to recombination initiation sites. By contrast, deletion of Zip1 residues 15-20 does not detectably prevent Zip3's localization at Zip1 polycomplex and supports some MutSγ crossing over but prevents normal SC assembly and Ecm11 SUMOylation. Our results highlight distinct N terminal regions that are differentially critical for Zip1's roles in crossing over and SC assembly; we speculate that the adjacency of these regions enables Zip1 to serve as a liaison, facilitating crosstalk between the two processes by bringing crossover recombination and synapsis factors within close proximity of one another.

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The synaptonemal complex protein, Zip1, promotes the segregation of nonexchange chromosomes at meiosis I.

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