Fate of telomere entanglements is dictated by the timing of anaphase midregion nuclear envelope breakdown

Abstract

Persisting replication intermediates can confer mitotic catastrophe if left unresolved. Loss of the fission yeast telomere protein Taz1 (ortholog of mammalian TRF1/TRF2) causes telomeric replication fork stalling and in turn, telomere entanglements that stretch between the segregating chromosomes at anaphase. At [≤]20{degrees}C, these entanglements fail to resolve, resulting in lethality. Rif1, a conserved DNA replication/repair protein, localizes between the segregating chromosomes and specifically hinders the resolution of telomere entanglements without affecting their formation. During anaphase, the spindle and the last segments of segregating chromatin are encased by the nuclear envelope (NE), creating a microdomain termed the anaphase midregion. In the final stages of fission yeast mitosis, this midregion undergoes local NE breakdown. Here we demonstrate that in response to taz1{Delta} telomeric entanglements, Rif1 delays midregion NE breakdown, and this delay disfavors entanglement resolution. Accordingly, gene deletions that hasten midregion NE breakdown phenocopy, and are epistatic with, rif1+ deletion. Conversely, gene deletions that delay midregion NE breakdown block the taz1{Delta} telomere detanglement afforded by loss of Rif1. Overexpression of Rif1 in a wild type background causes cold-specific NE defects and lethality, which are rescued by treatment with a membrane fluidizing agent. We propose that delayed NE breakdown normally favors the resolution of simple entanglements, arising from incomplete replication, by delaying exposure to the cytoplasm. In contrast, resolution of more complex entanglements involving strand invasion, like entanglements between nonsister taz1{Delta} telomeres, requires more rapid exposure to the cytoplasm. These observations uncover an unexpected coordination between NE remodeling and DNA processing events that can prevent or promote aneuploidy.