Securin regulates the spatiotemporal dynamics of separase.

Separase regulates multiple aspects of the metaphase-to-anaphase transition. Separase cleaves cohesin to allow chromosome segregation and localizes to vesicles to promote exocytosis. The anaphase-promoting complex/cyclosome (APC/C) activates separase by ubiquitinating its inhibitory chaperone, securin, triggering its degradation.

Securin Regulates the Spatiotemporal Dynamics of Separase.

Separase is a key regulator of the metaphase to anaphase transition with multiple functions. Separase cleaves cohesin to allow chromosome segregation and localizes to vesicles to promote exocytosis in mid-anaphase. The anaphase promoting complex/cyclosome (APC/C) activates separase by ubiquitinating its inhibitory chaperone, securin, triggering its degradation.

Strategies for Efficient Genome Editing Using CRISPR-Cas9.

The targetable DNA endonuclease CRISPR-Cas9 has transformed analysis of biological processes by enabling robust genome editing in model and nonmodel organisms. Although rules directing Cas9 to its target DNA via a guide RNA are straightforward, wide variation occurs in editing efficiency and repair outcomes for both imprecise error-prone repair and precise templated repair. We found that imprecise and precise DNA repair from double-strand breaks (DSBs) is asymmetric, favoring repair in one direction.

Condensin I protects meiotic cohesin from WAPL-1 mediated removal.

Condensin complexes are key determinants of higher-order chromatin structure and are required for mitotic and meiotic chromosome compaction and segregation. We identified a new role for condensin in the maintenance of sister chromatid cohesion during C. elegans meiosis.

Analysis of Meiotic Sister Chromatid Cohesion in Caenorhabditis elegans.

In sexually reproducing organisms, the formation of healthy gametes (sperm and eggs) requires the proper establishment and release of meiotic sister chromatid cohesion (SCC). SCC tethers replicated sisters from their formation in premeiotic S phase until the stepwise removal of cohesion in anaphase of meiosis I and II allows the separation of homologs and then sisters. Defects in the establishment or release of meiotic cohesion cause chromosome segregation errors that lead to the formation of aneuploid gametes and inviable embryos.

Oocyte Meiotic Spindle Assembly and Function.

Gametogenesis in animal oocytes reduces the diploid genome content of germline precursors to a haploid state in gametes by discarding ¾ of the duplicated chromosomes through a sequence of two meiotic cell divisions called meiosis I and II. The assembly of the microtubule-based spindle structure that mediates this reduction in genome content remains poorly understood compared to our knowledge of mitotic spindle assembly and function. In this review, we consider the diversity of oocyte meiotic spindle assembly and structure across animal phylogeny, review recent advances in our understanding of how animal oocytes assemble spindles in the absence of the centriole-based microtubule-organizing centers that dominate mitotic spindle assembly, and discuss different models for how chromosomes are captured and moved to achieve chromosome segregation during oocyte meiotic cell division.

Divergent kleisin subunits of cohesin specify mechanisms to tether and release meiotic chromosomes.

We show that multiple, functionally specialized cohesin complexes mediate the establishment and two-step release of sister chromatid cohesion that underlies the production of haploid gametes. In C. elegans, the kleisin subunits REC-8 and COH-3/4 differ between meiotic cohesins and endow them with distinctive properties that specify how cohesins load onto chromosomes and then trigger and release cohesion.

DNA helicase HIM-6/BLM both promotes MutSγ-dependent crossovers and antagonizes MutSγ-independent interhomolog associations during caenorhabditis elegans meiosis.

Meiotic recombination is initiated by the programmed induction of double-strand DNA breaks (DSBs), lesions that pose a potential threat to the genome. A subset of the DSBs induced during meiotic prophase become designated to be repaired by a pathway that specifically yields interhomolog crossovers (COs), which mature into chiasmata that temporarily connect the homologs to ensure their proper segregation at meiosis I. The remaining DSBs must be repaired by other mechanisms to restore genomic integrity prior to the meiotic divisions.

Condensin and cohesin complexity: the expanding repertoire of functions.

Condensin and cohesin complexes act in diverse nuclear processes in addition to their widely known roles in chromosome compaction and sister chromatid cohesion. Recent work has elucidated the contribution of condensin and cohesin to interphase genome organization, control of gene expression, metazoan development and meiosis. Despite these wide-ranging functions, several themes have come to light: both complexes establish higher-order chromosome structure by inhibiting or promoting interactions between distant genomic regions, both complexes influence the chromosomal association of other proteins, and both complexes achieve functional specialization by swapping homologous subunits.

The axial element protein HTP-3 promotes cohesin loading and meiotic axis assembly in C. elegans to implement the meiotic program of chromosome segregation.

Faithful transmission of the genome through sexual reproduction requires reduction of genome copy number during meiosis to produce haploid sperm and eggs. Meiosis entails steps absent from mitosis to achieve this goal. When meiosis begins, sisters are held together by sister chromatid cohesion (SCC), mediated by the cohesin complex.

Condensin restructures chromosomes in preparation for meiotic divisions.

The production of haploid gametes from diploid germ cells requires two rounds of meiotic chromosome segregation after one round of replication. Accurate meiotic chromosome segregation involves the remodeling of each pair of homologous chromosomes around the site of crossover into a highly condensed and ordered structure. We showed that condensin, the protein complex needed for mitotic chromosome compaction, restructures chromosomes during meiosis in Caenorhabditis elegans.

Myosin and the PAR proteins polarize microfilament-dependent forces that shape and position mitotic spindles in Caenorhabditis elegans.

In Caenorhabditis elegans, the partitioning proteins (PARs), microfilaments (MFs), dynein, dynactin, and a nonmuscle myosin II all localize to the cortex of early embryonic cells. Both the PARs and the actomyosin cytoskeleton are required to polarize the anterior-posterior (a-p) body axis in one-cell zygotes, but it remains unknown how MFs influence embryonic polarity. Here we show that MFs are required for the cortical localization of PAR-2 and PAR-3.

A Formin Homology protein and a profilin are required for cytokinesis and Arp2/3-independent assembly of cortical microfilaments in C. elegans.

Background: F-actin is enriched at the cortex of embryonic cells in the nematode Caenorhabditis elegans and is required for multiple processes that include the establishment of an anterior-posterior (A-P) axis and cytokinesis. However, the mechanisms that regulate cortical microfilament (MF) assembly remain poorly understood.

Results: We show here that a profilin called PFN-1 accumulates at the cortex independent of the actin cytoskeleton and is required for the assembly or maintenance of cortical MFs and myosin.

Centrosome maturation and mitotic spindle assembly in C. elegans require SPD-5, a protein with multiple coiled-coil domains.

The maternally expressed C. elegans gene spd-5 encodes a centrosomal protein with multiple coiled-coil domains. During mitosis in mutants with reduced levels of SPD-5, microtubules assemble but radiate from condensed chromosomes without forming a spindle, and mitosis fails.

Cytokinesis: closing in on the central spindle.

The central spindle is important for the completion of cytokinesis. Genetic and biochemical approaches have identified a tetrameric complex, made up of a mitotic kinesin-like protein and a Rho-GTPase activating protein, that mediates central spindle assembly.