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Amy J. MacQueen


Molecular Biology and Biochemistry




Meiosis is a specialized cell division wherein diploid cells produce haploid gametes by halving the chromosome number. During meiosis, the success of chromosome ploidy reduction relies on pre-established linkages between homologous chromosomes (homologs). Such associations between homologs occur through a series of highly ordered events during early meiotic prophase. The Synaptonemal Complex (SC) is a structurally conserved assembly of proteins that lies at the interface of lengthwise-aligned homologs. When homolog pairing fails, SC proteins usually aggregate into a polycomplex structure instead of assembling SCs between paired homologs. We use genetic and cytological approaches in Saccharomyces cerevisiae to identify protein regulators that ensure the SC assembles only in the right place at the right time.

We have identified several trans-acting factors that promote SC protein aggregation when recombination and/or homolog pairing is absent. Fpr3 and Rrd1 prolyl isomerases along with phosphatase subunit Pph3 are required for the aggregation of Zip1 (a SC central region protein) into a polycomplex at the nucleolus. In absence of such Zip1 regulators, polycomplex failure can correlate with promiscuous SC assembly on unpaired chromosomes, suggesting the importance of negatively regulating Zip1 to promote polycomplex formation. Like Fpr3, epitope-tagged Pph3 often co-aggregates within a polycomplex in a Zip1-dependent manner. However, epitope-tagged Rrd1 does not appear to localize to Zip1 polycomplex structures. Moreover, while Zip1 requires Fpr3 for its full capacity to form polycomplex, Fpr3 polycomplex forms independent of Zip1 at the nucleolus. Such Zip1-independent, nucleolus-anchored Fpr3 polycomplexes are devoid of SUMO (another SC central region protein) and Synapsis Initiation Complex 2 (SIC) proteins that are normally present within polycomplexes. Consistent with this idea, SUMO and SICs are dispensable for Zip1 and Fpr3 aggregation into a polycomplex. Taken together, our results are consistent with a model in which Fpr3 may directly act on Zip1 to promote polycomplex formation whereas Rrd1 may promote Zip1 polycomplex indirectly through activating and/or inhibiting the phosphatase components Pph3 and Sit4 respectively.

Finally, we asked whether Fpr3-overexpression interferes with SC assembly during an otherwise normal meiosis. Intriguingly, we found that overexpression of Fpr3 reduces steady-state SC protein levels, and results in a meiosis-specific AAA+ ATPase Pch2- dependent defect in early prophase (pre-pachytene) progression, SC assembly and meiotic spore formation. Together, these results suggest a novel function of Pch2 (that otherwise functions as a pachytene checkpoint protein) in monitoring early prophase (pre-pachytene) checkpoint, and we propose that the defect in SC assembly and sporulation caused by Fpr3-overexpression is an indirect consequence of this Pch2 mediated pre-pachytene checkpoint activation.



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