Drosophila Separase binds to the securin PIM and to THR, a non-securin protein. S.K. Heidmann , H. Jaeger , A. Herzig , C. Lehner. Dept Genetics, Univ Bayreuth, Bayreuth, Germany.

   Sister chromatid separation in mitosis is regulated by complexes of securin and separase proteins. Proteolysis of securins at the metaphase to anaphase transition liberates separases which in turn trigger the dissolution of sister chromatid cohesion by proteolytic degradation of a conserved cohesin subunit. Drosophila PIM has been suggested to function as a securin. We show here that PIM is associated with Drosophila Separase (SSE). SSE is an unusual member of the separase family. SSE is only about one third of the size of the separase proteins found in other organisms. Furthermore, the homology in the conserved separase domain is rather weak which places the Drosophila protein as a very distant member in the separase family tree. Finally, SSE is also found in complexes with THR, which has no homology to other proteins but is known to bind PIM. Throughout embryogenesis, SSE is present in excess over both PIM and THR. Unlike PIM and THR, SSE is rather stable and does not seem to be subject to mitotic turnover. We have generated a deletion mutant of Sse by the male recombination technique. Individuals homozygous for the deletion die during late larval and early pupal stages. This lethality can be rescued, at least in part, by expression of Sse from a genomic transgene or by ubiquitous overexpression of an epitope tagged Sse cDNA. A detailed analysis of the mutant phenotype is underway and will be reported.