Alterations in chromatin conformation are accompanied by reorganization of nonchromatin domains that contain U-snRNP protein p28 and nuclear protein p107.

The intranuclear distribution of nuclear matrix-associated protein p107 and the 28-kD Sm antigen of U-snRNPs have been studied using double-label immunofluorescence and immunoperoxidase electron microscopy. In interphase nuclei of HeLa cells, Novikoff hepatoma cells, and rat kangaroo kidney cells, p107 was confined to discrete interchromatin domains. The domains had an irregular contour, with an average diameter of 1-1.5 micron. Each domain appeared to be composed of interconnected granules. The Sm antigen colocalized and appeared concentrated in these domains but also showed some general nucleoplasmic distribution. During mitosis, the interchromatin domains disassembled such that the Sm portion redistributed to the perichromosomal and spindle regions and the p107 component redistributed throughout the mitotic cytoplasm. During anaphase, p107 assembled into discrete clusters throughout the mitotic cytoplasm. The Sm antigen was not a component of these clusters. Double-label immunofluorescence with anti-p107 and the anti-DNA tight-binding protein, AhNa1, showed that the extranuclear p107 domains assumed an interchromatin localization only after the chromosomes had decondensed. The correlation between chromosome decondensation and the occurrence of p107 within interchromatin domains was also observed during chicken erythrocyte nuclear reactivation. We propose that the discrete interchromatin domains that contain p107 and p28 may be important for processing and splicing of RNA and that their structural assembly within nuclei is sensitive to the presence of the transcriptionally active conformation of chromatin.