Drosophila separase is required for sister chromatid separation and binds to PIM and THR

Drosophila PIM and THR are required for sister chromatid separation in mito sis and associate in vivo. Neither of these two proteins shares significant sequence similarity with known proteins. However, PIM has functional simil arities with securin proteins. Like securin, PIM is degraded at the metapha se-to-anaphase transition and this degradation is required for sister chrom atid separation. Securin binds and inhibits separase, a conserved cysteine endoprotease. Proteolysis of securin at the metaphase-to-anaphase transitio n activates separase, which degrades a conserved cohesin subunit, thereby a llowing sister chromatid separation. To address whether PIM regulates separ ase activity or functions with THR in a distinct pathway, we have character ized a Drosophila separase homolog (SSE). SSE is an unusual member of the s eparase family. SSE is only about one-third the size of other separases and has a diverged endoprotease domain. However, our genetic analyses show tha t SSE is essential and required for sister chromatid separation during mito sis. Moreover, we show that SSE associates with both PIM. and THR. Although our work shows that separase is required for sister chromatid separation i n higher eukaryotes, in addition, it also indicates that the regulatory pro teins have diverged to a surprising degree, particularly in Drosophila.