Histone modification regulated bysuppressor of Zeste 12 and Ipl1 Aurora-like kinase in Drosophila

Abstract: Polycomb group (PcG) proteins are a group of genome wide silencers that are crucial for many processes during the development. In Drosophila PcG proteins are organised into four different complexes: PRC1, PRC2, PhoRC and PR-DUB. PRC2 consists of four core proteins: Su(z)12, E(z), Esc and Nurf. E(z) is the only known proteins with a known function, it possess a methyltransferase activity that specifically methylates lysine 27 on histone 3 (H3K27). A novel PcG gene was identified in 2001 in screen for modifiers of zeste-white interaction. This gene suppressed zeste’s repression of white and gave it the name Suppressor of zeste 12 (Su(z)12). The Su(z)12 gene is alternatively spliced into two transcripts; a 4.1 kb mRNA called Su(z)12A and a 3.7 kb mRNA called Su(z)12B. These transcripts are translated into two isoforms; a 95 kDa Su(z)12A protein and 100 kDa Su(z)12B protein. These isoforms show a sequence similarity of 95% and the only difference is the C-terminal end. During development these two isoforms are present at different levels. Interaction of the two isoforms with the other core components in PRC2 showed that only Su(z)12B interacts with Nurf. Also the two isoforms showed interaction with each other with the exception of a single copy of Su(z)12A that couldn´t interact with Su(z)12B. Overexpression of Su(z)12B in vivo caused lethality and homeotic transformations. Aurora kinases belong to a conserved family of serine/threonine kinases that are important for many processes in mitotsis, such as spindle formation, chromosomal segregation and cytokinesis. Aurora kinases are overexpressed in many human cancers and inhibitors of Aurora A and Aurora B has shown to inhibit growth and induce apoptosis. There are three Aurora kinases in vertebrates; Aurora A, Aurora B and Aurora C and although they are highly similar, they have different roles and location during mitosis. Aurora B is a chromosomal passenger protein and forms the chromosomal passenger complex with INCENP, Survivin and Borealin. Depletion of Aurora B causes severe effects in mitosis and lead to large cells with several nuclei and polyploidy. The Drosophila homologue of Aurora B is called IpI1-like-Aurora kinase (ial). The c-Myc transcription factor, or its relatives N-Myc and L-Myc, are also overexpressed in many, if not all human cancers. Drosophila has only one Myc protein, dMyc, which is encoded by the diminutive (dm) locus. In Drosophila, dMyc is mostly associated with size and growth regulation and depletion of dm results in endoreplication and growth arrest in early development. Previous work has shown that mammalian c-Myc induces Aurora A and Aurora B kinases. When Myc-driven lymphomas are treated with Aurora B inhibitors, cells are accumulated in G2/M phase and apoptosis is induced. Here we show that these conserved proteins have a potential connection in Drosophila as well since knockdown of ial causes severe phenotypes and leads to larger cells. When ial is knocked down or when dMyc is overexpressed the flies become smaller. Interestingly however transgenic flies which overexpress dMyc and knock down ial exhibit a different pheontype - the flies become bigger. This showing evidence that a relationship between Myc and Aurora B is evolutionary conserved down to Drosophila.