Nuclear F-actin and myosins drive relocalization of heterochromatic breaks

Heterochromatin mostly comprises repeated DNA sequences prone to ectopic recombination. In Drosophila cells, ‘safe’ homologous recombination repair of heterochromatic double-strand breaks relies on the relocalization of repair sites to the nuclear periphery before strand invasion. Mechanisms responsible for this movement were unknown. Here, we show that relocalization occurs via directed motion along nuclear actin filaments assembled at repair sites by Arp2/3. Relocalization requires Smc5/6-associated nuclear myosins and the myosin activator Unc45, which is recruited to repair sites by Smc5/6. Arp2/3, actin nucleation and myosins also relocalize heterochromatic DSBs in mouse cells. Defects in this pathway result in heterochromatin repair defects and chromosome rearrangements. These findings uncover de novo nuclear actin filaments and myosins as effectors of chromatin dynamics for heterochromatin repair and stability in multi-cellular eukaryotes.