The Corbett lab uses biochemistry, structural biology, and genetics to study chromosome organization, recombination, and segregation in mitosis and meiosis. We explore the fundamental mechanisms of cellular processes impacting human health in several areas: fertility, aneuploidy-related developmental disorders, and cancer. Our work falls into three main areas:

 

Meiotic Chromosome Organization

In meiosis, homologous chromosomes must find one another and physically associate through DNA recombination to form “crossovers”, in order to segregate properly to produce haploid gametes (in humans, sperm and egg cells). We study the architecture of a conserved meiosis-specific protein complex called the chromosome axis, which organizes each chromosome into a linear array of chromatin loops, and regulates meiotic recombination.

Our work has focused mainly on the HORMA domain proteins, which function as master regulators of meiotic recombination through the recruitment and control of recombination proteins. We showed that the HORMA domain proteins self-associate through interactions between their N-terminal HORMA domains and short C-terminal peptide motifs we call “closure motifs” (Kim, Rosenberg et al. 2014). We are currently exploring how the HORMA domain assembly regulates meiotic recombination, and how its assembly and disassembly are controlled.

Our current efforts are focused on elucidating the overall architecture of the meiotic chromosome axis, which includes the DNA-binding cohesin complexes and “linker” proteins that, while poorly conserved on a sequence level, are nonetheless shared across eukaryotes and constitute a critical component of the chromosome axis structure. We are also exploring how recombination regulators, called the “ZMM” proteins in S. cerevisiae, recognize specific DNA recombination intermediates and promote inter-homolog crossover formation.

Finally, we are exploring how cryo-electron microscopy on intact chromosomes, as well as chromosome architecture analysis using high-throughput sequencing, can be combined with our structural work to yield a comprehensive multi-scale picture of meiotic chromosome organization and recombination.

Structure of C. elegans HIM-3. The HORMA domain (rainbow) binds a short "closure motif" peptide (blue, surface) in HTP-3 to mediate self-assembly on the chromosome axis.

Structure of C. elegans HIM-3. The HORMA domain (rainbow) binds a short "closure motif" peptide (blue, surface) in HTP-3 to mediate self-assembly on the chromosome axis.

Organization of the meiotic HORMA domain protein assembly in C. elegans. Other eukaryotes including S. cerevisiae and mammals share a similar, but less complex, self-assembly mechanism.

Organization of the meiotic HORMA domain protein assembly in C. elegans. Other eukaryotes including S. cerevisiae and mammals share a similar, but less complex, self-assembly mechanism.

 

TRIP13 and The Spindle Assembly Checkpoint

Structure of the C. elegans TRIP13 ortholog PCH-2

Structure of the C. elegans TRIP13 ortholog PCH-2

In all eukaryotic cells, chromosome segregation depends on the proper attachment of chromosomes to the microtubule-based spindle, mediated by large protein complexes called kinetochores. When not properly attached, kinetochores assemble a multi-protein complex called the “Mitotic Checkpoint Complex” (MCC) that prevents chromosome segregation by inhibition of an ubiquitin E3 ligase, the Anaphase Promoting Complex/Cyclosome (APC/C). One component of the MCC is Mad2, which shares a fold with the meiotic HORMA domain proteins. Both Mad2 and the meiotic HORMA proteins are regulated by a hexameric AAA+ family ATPase, TRIP13. We showed that TRIP13, along with an adapter protein p31(comet), disassembles the MCC by causing a specific conformational change in Mad2 (Ye et al. 2015). Thus, TRIP13 and p31(comet) are necessary for turning off the Spindle Assembly Checkpoint through MCC disassembly.

Currently, we are exploring the interactions of TRIP13 with both Mad2 and the meiotic HORMA domain proteins using a variety of structural and biochemical methods, along with analyses in both mitotic and meiotic cells. Our data is revealing that TRIP13 uses a common mechanism to promote HORMA domain dynamics in both systems, and that these dynamics are critical for both assembly and disassembly of the Mad2-containing MCC, as well as for proper regulation of the meiotic HORMA domain proteins.

 

The Yeast Monopolin Complex

In meiosis I, kinetochores of sister chromosomes must co-orient, or attach to microtubules from the same spindle pole, to enable the bi-orientation and segregation of homologs. In S. cerevisiae and related fungi, the four-protein monopolin complex cross-links sister kinetochores to mediate their co-orientation. In a series of works, we have outlined the overall architecture of the monopolin complex and defined how it interacts with kinetochores (Corbett et al. 2010, Corbett & Harrison 2012, Sarangapani et al. 2014, Ye et al. 2016). Our current efforts are focused on regulation of the complex by its resident kinase, Hrr25, whose activity is crucial for specific sister kinetochore cross-linking. We are also exploring other functions of the monopolin complex, including its role in regulating silencing and illegitimate recombination in the ribosomal DNA repeats.

Overall architecture of the S. cerevisiae monopolin complex, assembled from multiple crystal structures. Shown in gray is the zone over which Hrr25's kinase activity can act when incorporated into the complex.

Overall architecture of the S. cerevisiae monopolin complex, assembled from multiple crystal structures. Shown in gray is the zone over which Hrr25's kinase activity can act when incorporated into the complex.

Model for sister kinetochore cross-linking by the monopolin complex in meiosis I. 

Model for sister kinetochore cross-linking by the monopolin complex in meiosis I.