Hypophosphorylated SR splicing factors transiently localize around active nucleolar organizing regions in telophase daughter nuclei

Upon completion of mitosis, daughter nuclei assemble all of the organelles necessary, for the implementation of nuclear functions. We found that upon entry into daughter nuclei, snRNPs and SIR proteins do not immediately colocalize in nuclear speckles. SIR proteins accumulated in patches around active nucleolar organizing regions (NORs) that we refer to as NOR.-associated patches (NAPs), whereas snRNPs were enriched at other nuclear regions. NAPs formed transiently, persisting for 15-20 min before dissipating as nuclear speckles began to form in G1. In the absence of RNA polymerase 11 transcription, NAPs increased in size and persisted for at least 2 h, with delayed localization of SR proteins to nuclear speckles. In addition, SR proteins in NAPs are hypophosphorylated, and the SR protein kinase Clk/STY colocalizes with SIR proteins in NAPs, suggesting that phosphorylation releases SIR proteins from NAPs and their initial target is transcription sites. This work demonstrates a previously unrecognized role of NAPs in splicing factor trafficking and nuclear speckle biogenesis.