SR and SR-related proteins redistribute to segregated fibrillar components of nucleoli in a response to DNA damage

Pre-mRNA splicing factors are often redistributed to nucleoli in response to physiological conditions and cell stimuli. In telophase nuclei, serine-arginine rich (SR) proteins, which usually reside in nuclear speckles, localize transiently to active ribosomal DNA (rDNA) transcription sites called nucleolar organizing region-associated patches (NAPs). Here, we show that ultraviolet light and DNA damaging chemicals induce the redistribution of SR and SR-related proteins to areas around nucleolar fibrillar components in interphase nuclei that are similar to, but distinct from, NAPs, and these areas have been termed DNA damage-induced NAPs (d-NAPs). In vivo labeling of nascent RNA distinguished d-NAPs from NAPs in that d-NAPs were observed even after full rDNA transcriptional arrest as a result of DNA damage. Studies under a variety of conditions revealed that d-NAP formation requires both RNA polymerase II-dependent transcriptional arrest and nucleolar segregation, in particular, the disorganization of the granular nucleolar components. Despite the redistribution of SR proteins, splicing factor-enriched nuclear speckles were not disrupted because other nuclear speckle components, such as nuclear poly(A) RNA and the U5-116K protein, remained in DNA-damaged cells. These data suggest that the selective redistribution of splicing factors contributes to the regulation of specific genes via RNA metabolism. Finally, we demonstrate that a change in alternative splicing of apoptosis-related genes is coordinated with the occurrence of d-NAPs. Our results reveal a novel response to DNA damage that involves the dynamic redistribution of splicing factors to nucleoli.