The Assembly and Dynamics of Hop1 and Red1 at the Meiotic Chromosome Axis
The generation of viable gametes requires the proper pairing and physical association of homologous chromosomes prior to the first meiotic division. The extended prophase of meiosis I is dedicated to this process, and is regulated by cellular signals that delay cell division until the proper paring of homologs is achieved. In humans, misregulation of homolog pairing and segregation can lead to fertility issues, miscarriages, or developmental diseases such as Down Syndrome. Homologs identify and physically associate with one another through a modified homologous recombination pathway, governed by a conserved meiosis-specific protein assembly called the chromosome axis. The axis facilitates the introduction of programmed DNA double-strand breaks, and their repair via the homologous chromosome, thereby promoting crossover recombination that physically links homologs together. The axis also forms the foundation for the synaptonemal complex, which assembles between the paired homologs, joining them together during recombination. In the budding yeast S. cerevisiae, the chromosomes axis consists primarily of DNA-binding cohesin complexes and two meiosis-specific proteins, Hop1 and Red1. Here we present a biochemical and biophysical characterization of Hop1 and Red1 to understand the mechanisms governing chromosome axis assembly and function. We identify specific interactions between the Hop1 HORMA domain and conserved “closure motifs” in Red1 and the Hop1 C-terminus, which are responsible for the localization and self-assembly of Hop1 at the chromosome axis. We find that HORMA domain-closure motif interactions depend upon the conformational dynamics of the Hop1 HORMA domain in a manner reminiscent of a related HORMA domain protein, Mad2. We also find that Red1 contributes to the structure of the chromosome axis through the formation of high molecular weight oligomers assembled through its coiled-coil C-terminus. We find that previously-characterized mutations within this coiled-coil region that prevent formation of the chromosome axis and the synaptonemal complex, disrupt the oligomerization of the Red1 C-terminal domain, suggesting that Red1 oligomerization is essential for axis assembly and function. Together, the findings of this dissertation provide new mechanistic and functional insight into the assembly and dynamics of the meiotic chromosome axis and the means by which it coordinates the events of prophase I.