Affiliations: 1) Department of Molecular Biology and Computational Biology, University of Southern California, Los Angeles; 2) Department of Genetics, Harvard Medical School, Boston, MA; 3) Dept of Developmental and Cell Biology, University of California, Irvine, CA
Keywords: e. heterochromatin; e. heterochromatin
Pericentromeric Heterochromatin occupies ~30% of fly and human genomes and is mostly composed of highly repeated DNA sequences prone to aberrant recombination. And yet, repair mechanisms in heterochromatin remain poorly understood. Our previous studies in Drosophila cells showed that ‘safe’ homologous recombination (HR) repair of heterochromatic double-strand breaks (DSBs) requires the movement of repair sites to the nuclear periphery, where the strand invasion protein Rad51 is recruited to continue repair. This likely prevents aberrant recombination by isolating broken sites and their homologous templates away from ectopic (non-allelic) repair templates before strand invasion. The post-translational modification SUMOylation is essential for this pathway, as it blocks Rad51 recruitment and HR progression inside the heterochromatin domain, thus preventing aberrant recombination. Here we show that overexpression of the SUMO protease Ulp1 leads to abnormal formation of Rad51 foci inside the heterochromatin domain. Ulp1 is normally tethered to the nuclear periphery by the nuclear pore ‘basket’ protein Mtor/Tpr, suggesting that Ulp1 overexpression might affect repair progression by releasing some Ulp1 molecules to the nucleoplasm. We tested the importance of Ulp1 compartmentalization to the nuclear periphery in heterochromatin repair, using Mtor/Tpr RNAi or Ulp1 mutants that lose the nuclear periphery anchoring domain. These conditions result in abnormal Rad51 foci inside the heterochromatin domain. Together, these results support the hypothesis that Ulp1 compartmentalization to the nuclear periphery is necessary for preventing abnormal HR progression inside the heterochromatin domain, while enabling ‘safe’ HR progression at the nuclear periphery.