NS mediates DNA damage–increased interaction between PML-IV and SUMOylated TRF1. (A) The APB/PML percentage of TRF1-So foci is increased by etoposide (Etop) treatment (from 51.9% to 83.6%). NS-KD abolishes most of the etoposide-induced APB increase (61.2%). (B) CoIP demonstrated that the interaction between PML-IV and TRF1-So is increased by TRF2-DN–induced telomere damage (left) or etoposide-induced DNA damage (right). (C) Etoposide treatment also increases TRF1-So binding to NS. (D) NS-KD (shNS) significantly reduces the etoposide-increased interaction between PML-IV and TRF1-So. (E) Endogenous coIP results confirmed that the native NS and TRF1 proteins bind each other under the undamaged condition. The interaction between these two proteins is increased by etoposide treatment. (F) Immunoprecipitation of endogenous PML proteins by mouse anti-PML antibody (PG-M3) coprecipitates only a minimal amount of TRF1 under the normal condition (lane 2). Under the DNA damage condition, a significant amount of endogenous TRF1 proteins is copurified with PML proteins (lane 3). A high molecular weight product of TRF1 is copurified with PML proteins only under the DNA damage condition. The same TRF1 product can be recognized by the anti–SUMO-1 antibody on the same blot (open arrows). Open arrowheads and asterisks mark IgG (H+L) and SUMO-modified proteins copurified with PML proteins, respectively.