Regulation and function of the Synaptonemal Complex during meiosis in Saccharomyces cerevisiae

The Synaptonemal Complex (SC) is a proteinaceous structure that connects homologous chromosomes lengthwise during meiotic prophase. In budding yeast, the SC consists of two parallel axes that become connected by the central element protein, Zip1 that extends along the chromosome axes (Sym, Engebrecht et al. 1993). Extension of the SC is coordinated to crossover formation by a group of proteins known as the ‘ZMM’s (Zip1, Zip2, Zip3, Zip4, Msh4, Msh5 and Mer3) (Borner, Kleckner et al. 2004). Work outlined here demonstrates a role for the mismatch repair paralogue, Msh4 in preventing SC extension from being de-coupled from crossover formation. Furthermore, increased temperature serves as a positive effector for this decoupling. These findings suggest that SC extension is highly regulated to ensure that it is coupled with crossing over. As well as its role in crossover formation (Storlazzi, Xu et al. 1996), the work outlined here demonstrates an independent role for Zip1 in promoting the segregation of non-exchange chromosome pairs (NECs). Zip1 pairs the centromeres of NECs in pachytene through to metaphase I, where it aids their segregation at the first meiotic division. The localisation and function of Zip1 at the centromeres of non-exchange chromosomes depends on Zip3 and Zip2, respectively. Zip1 is observed at the centromeres of all chromosomes following SC disassembly through to the first meiotic division, where it promotes the segregation of exchange pairs also. A model is suggested whereby Zip1 promotes the segregation of both non-exchange and exchange chromosome pairs by tethering homologous centromeres throughout meiotic prophase. Finally, a parallel pathway for NEC segregation is also described that depends upon the spindle checkpoint component, Mad3. When both ZIP1 and MAD3 are deleted, NECs segregate at random.