Mnd2 is an Antagonist of the Meiosis Specific APC/C-Ama1, Essential to Sustain normal Chromosome Structure during Meiotic Prophase.
Alexandra M. Penkner, Martin Xaver, Franz Klein
Vienna Biocenter II, Max Perutz Laboratories, Department of Chromosome Biology, Dr. Bohr-Gasse 1, A-1030 Vienna, Austria
The anaphase promoting complex or cyclosome (APC/C) is a multi-subunit ubiquitin ligase, that triggers the metaphase-to-anaphase transition, by targeting its substrates for 26S proteasome degradation. Sister chromatid separation is mediated by the APC/C through destabilization of the separase inhibitor securin. Separase then cleaves the kleisin subunit of the cohesin complex, which is Rec8 during meiosis. We showed that Mnd2 prevents precocious activation of the meiosis specific APC/C-Ama1, and consequently of separase, thereby permitting the preservation of sister chromatid cohesion (SCC) during meiotic S- and prophase. Furthermore, Mnd2 is required for normal chromatin condensation and double strand break repair. Inhibition of Rec8 cleavage by interfering with separase activation, or by deleting the separase cleavage sites of Rec8, creating a non-cleavable Rec8N variant, restores SCC during prophase in mnd2Δ cells. However, the progression to paired and synapsed homologous chromosomes was supported by inactivation of separase, but not by expression of Rec8N. This suggested that unscheduled activation of APC/C-Ama1 interfered with normal synapsis in REC8N cells. Indeed, deletion of the APC/C activator AMA1 restored the ability of REC8N mnd2Δ cells to synapse. Inactivation of separase, however, failed to suppress this defect, suggesting the presence of a meiotic APC/C target, other than securin, the degradation of which disturbs synapsis in REC8N cells. Finally, shugoshin/Sgo1, the protector of centromeric Rec8 during the first meiotic division, is a substrate of APC/C-Ama1 and therefore requires Mnd2 for its own protection, similar to securin.