page 51 by Bubulya et al.
During interphase, premRNA splicing factors such as the serine arginine–rich (SR) proteins reside in nuclear speckles, along with other pre-mRNA processing proteins. The speckles disassemble during mitosis, and then reform in G1. By viewing speckle reformation, the group finds that SR proteins first make a side trip and gather in nucleolar organizing region–associated patches (NAPs), around areas where rDNA is transcribed.
Inhibiting mRNA transcription prolonged the life of NAPs and also caused SR proteins to accumulate near nucleoli in interphase cells. Without their pre-mRNA transcript targets available, SR protein traffic was backed up at a location it would otherwise move through rapidly.
NAPs contained hypophosphorylated SR proteins, which may self-associate and thus accumulate in patches. NAPs also contained an SR protein kinase, whose activity may release the splicing factors to transcription sites, although it is not clear how active RNA polymerase II might signal to effect such a change. At NAPs, the SR proteins are sequestered from other splicing factors. Their isolation during telophase may be needed for modification or SR complex assembly in reforming daughter nuclei. Why SR proteins choose NAPs for their meeting point remains to be determined.