You Know You That Zip When You Get That Modification: Is Steady-State S. cerevisiae Zip1 Modified?

The proper segregation of homologous chromosomes (homologs) during meiosis is a paramount task for sexually reproducing organisms. Homologs successfully disjoin from one another on the meiosis I spindle so long as they have undergone prior alignment and have formed at least one crossover attachment. The Zip1 protein promotes an intimate alignment between homologs during budding yeast meiosis through its role as a building block of the synaptonemal complex (SC), a proteinaceous structure that assembles along the full length of aligned partner chromosomes. Independent of its role in building SC, Zip1 also plays a critical role in promoting interhomolog crossover recombination, although the molecular underpinnings of this activity are unclear. Through a structure-function study to identify residues that are essential for Zip1?s crossover recombination or SC assembly functions, we identified two zip1 mutants that produce an unexpectedly smaller form of Zip1. In meiotic cells that express either a zip1 allele in which five contiguous residues are changed to alanine, or an allele in which only two of the aforementioned five residues are changed to alanine, a large fraction of Zip1 migrates on a protein gel as if it has lost 10-20 kilodaltons of its wild-type molecular weight. The change in apparent molecular weight of Zip1 observed in our mutants raises the unanticipated possibility that most Zip1 proteins within the wild-type meiotic cell are constitutively modified, perhaps by a 10-20 kilodalton protein, and that this post-translational modification is dependent on the residues that are altered in our two zip1 mutant alleles. Here I describe various experiments designed to characterize and identify this putative second form of Zip1.