DIFFERENTIAL INTERACTION OF SPLICING SNRNPS WITH COILED BODIES AND INTERCHROMATIN GRANULES DURING MITOSIS AND ASSEMBLY OF DAUGHTER CELL-NUCLEI

In the interphase nucleus of mammalian cells the U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins (snRNPs), which are subunits of spli ceosomes, associate with specific subnuclear domains including interch romatin granules and coiled bodies. Here, we analyze the association o f splicing snRNPs with these structures during mitosis and reassembly of daughter nuclei. At the onset of mitosis snRNPs are predominantly d iffuse in the cytoplasm, although a subset remain associated with remn ants of coiled bodies and clusters of mitotic interchromatin granules, respectively The number and size of mitotic coiled bodies remain appr oximately unchanged from metaphase to early telophase while snRNP-cont aining clusters of mitotic interchromatin granules increase in size an d number as cells progress from anaphase to telophase. During telophas e snRNPs are transported into daughter nuclei while the clusters of mi totic interchromatin granules remain in the cytoplasm. The timing of n uclear import of splicing snRNPs closely correlates with the onset of transcriptional activity in daughter nuclei. When transcription restar ts in telophase cells snRNPs have a diffuse nucleoplasmic distribution . As cells progress to G1 snRNP-containing clusters of interchromatin granules reappear in the nucleus. Coiled bodies appear later in G1, al though the coiled body antigen, p80 coilin, enters early into telophas e nuclei. After inhibition of transcription we still observe nuclear i mport of snRNPs and the subsequent appearance of snRNP-containing clus ters of interchromatin granules, but not coiled body formation. These data demonstrate that snRNP associations with coiled bodies and interc hromatin granules are differentially regulated during the cell divisio n cycle and suggest that these structures play distinct roles connecte d with snRNP structure, transport, and/or function.