Parallel genetic screens identify nuclear envelope homeostasis as a key determinant of telomere entanglement resolution in fission yeast

Abstract

In fission yeast lacking the telomere binding protein, Taz1, replication stalls at telomeres, triggering deleterious downstream events. Strand invasion from one taz1{Delta} telomeric stalled fork to another on a separate (non-sister) chromosome leads to telomere entanglements, which are resolved in mitosis at 32{degrees}C; however, entanglement resolution fails at [≤]20{degrees}C, leading to cold-specific cell lethality. Previously, we found that loss of the mitotic function of Rif1, a conserved DNA replication and repair factor, suppresses cold sensitivity by promoting resolution of entanglements without affecting entanglement formation. To understand the underlying pathways of mitotic entanglement resolution, we performed a series of genomewide synthetic genetic array screens to generate a comprehensive list of genetic interactors of taz1{Delta} and rif1{Delta}. We modified a previously described screening method to ensure that the queried cells were kept in log phase growth. In addition to recapitulating previously identified genetic interactions, we find that loss of genes encoding components of nuclear pore complexes (NPCs) promotes telomere disentanglement and suppresses taz1{Delta} cold sensitivity; we attribute this to more rapid anaphase midregion nuclear envelope (NE) breakdown in the absence of these NPC components. Moreover, loss of genes involved in lipid metabolism reverses the ability of rif1+ deletion to suppress taz1{Delta} cold sensitivity, again pinpointing NE modulation. A rif1+ separation-of-function mutant that specifically loses Rif1s mitotic functions yields similar genetic interactions. Genes promoting membrane fluidity were enriched in a parallel taz1+ synthetic lethal screen at permissive temperature, cementing the idea that the cold specificity of taz1{Delta} lethality stems from altered NE homeostasis.