ER-tethering directs TREX1 penetration of a BAF-dependent barrier at micronuclei

Micronuclei are membrane-encapsulated nuclear aberrations that form following chromosome segregation errors. Micronuclear membrane collapse permits access of the pattern recognition receptor cGAS and its antagonist, the TREX1 exonuclease. TREX1 carboxy-terminal domain mediated endoplasmic reticulum tethering association is essential for invasion into ruptured micronuclei, however the mechanisms underlying this dependency are unknown. Here, we identify barrier-to-autointegration nuclear assembly factor 1 (BAF) as a key regulator of TREX1 activity at micronuclei. BAF accumulates on micronuclei following membrane collapse and augments TREX1 recruitment in a manner that depends on BAF interactions with membrane-associated LEM-domain proteins. Despite delayed entry, TREX1 exhibits enhanced micronuclear DNA degradation and independence from ER-tethering in BAF-deficient cells. In accordance, recombinant BAF protein inhibits TREX1-mediated DNA degradation in vitro in a manner that depends on BAF DNA-binding. BAF similarly outcompetes cGAS for micronuclear DNA interaction and reduces cGAS activation at micronuclei. These findings reveal, a BAF-dependent protective barrier to diffusive entry of DNA binding proteins at ruptured micronuclei, explaining the requirement of TREX1 ER-tethering for micronuclear localization and suppression of productive cGAS DNA substrate interactions that activate innate immune responses in chromosomally unstable cells.