Proteolytic cleavage of the THR subunit during anaphase limits Drosophila separase function

Sister-chromatid separation in mitosis requires proteolytic cleavage of a cohesin subunit. Separase, the corresponding protease, is activated at the metaphase-to-anaphase transition. Activation involves proteolysis of an inhibitory subunit, securin, following ubiquitination mediated by the anaphase-promoting complex/cyclosome. In Drosophila, the securin PIM associates not only with separase (SSE), but also with an additional protein, THR. Here we show that THR is cleaved after the metaphase-to-anaphase transition. THR cleavage only occurs in functional SSE complexes and in a region that matches the separase cleavage-site consensus. Mutations in this region abolish mitotic THR cleavage. These results indicate that THR is cleaved by SSE. Expression of noncleavable THR variants results in cold-sensitive maternal-effect lethality. This lethality can be suppressed by a reduction of catalytically active SSE levels, indicating that THR cleavage inactivates SSE complexes. THR cleavage is particularly important during the process of cellularization, which follows completion of the last syncytial mitosis of early embryogenesis, suggesting that Drosophila separase has other targets in addition to cohesin subunits.