DNA-damage dependent isoform switching modulates RIF1 DNA repair complex assembly and phase separation.

How RIF1 (RAP1 interacting factor 1) fulfills its diverse roles in DNA double-strand break repair, DNA replication, and nuclear organization remains elusive. Here, we show that alternative splicing of a cassette exon (Ex32) encoding a Ser/Lys-rich cassette in the RIF1 C-terminal domain (CTD) gives rise to RIF1-Long (RIF1-L) and RIF1-Short (RIF1-S) isoforms with different functional characteristics. We demonstrate that RIF1-Ex32 splice-in is mediated by an exonic splicing enhancer that is recognized by the serine and arginine rich splicing factor 1 (SRSF1) and antagonized by SRSF3 and SRSF7. Exposure to DNA damage inhibited Ex32 splice-in, potentiated the association of SRSF3 and SRSF7 with RIF1 pre-mRNA, and caused an increase in RIF1-S protein expression, which was also observed across a diverse set of primary cancers. Isoform-specific proteomic analyses revealed RIF1-L preferentially associated with mediator of DNA damage checkpoint 1 (MDC1) and sustained MDC1 focus formation to a greater extent than RIF1-S. We further show that the Ser/Lys-rich cassette stabilized a novel phase separation activity of the RIF1 CTD and enhanced RIF1-L chromatin retention, which was reversed by cyclin-dependent kinase 1-dependent phosphorylation of the RIF1 CTD in response to G(2) DNA damage checkpoint inhibition. These combined findings suggest DNA damage-dependent RIF1 alternative splicing contributes to RIF1 functional diversification in genome protection.