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 and recombination

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 (HORMADs), which function as master regulators of meiotic recombination through the recruitment and control of recombination proteins. We showed that the HORMADs 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, West et al. 2018). We are currently exploring how the HORMAD 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 meiotic nuclei and 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 HORMADs 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 HORMADs using a variety of structural and biochemical methods, along with analyses in both mitotic and meiotic cells. Our data has revealed 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 HORMADs (Ye et al. 2017).

 

HORMA domain proteins in bacterial signaling?

Much of our work focuses on proteins of the HORMA domain family, which until recently were thought to exist only in eukaryotes. We are studying a newly-discovered family of HORMA domain proteins, and associated TRIP13-like ATPases, that exist in a variety of bacteria including known pathogens. Using a combination of structure, biochemistry, and genetics, we are exploring the idea that these proteins play key roles in signaling pathways important for interactions with phages, other bacteria, or eukaryotic hosts.