We investigated at the electron microscope level the fate of the three intranuclear structures known to accumulate snRNPs, and which correspond to the punctuate immunofluorescent staining pattern (the coiled bodies, the clusters of interchromatin granules and the interchromatin granule-associated zones) after exposure to either a low salt medium which induces a loosening and partial spreading of nucleoprotein fibers or a high ionic strength salt medium and subsequent DNase I digestion, in order to obtain DNA-depleted nuclear matrices. The loosened clusters of interchromatin granules and the coiled bodies could no longer be distinguished from surrounding nucleoprotein fibers solely by their structure, but constituents of the clusters of interchromatin granules could be detected by in situ hybridization with both U1 and U2 DNA probes, and constituents of the coiled bodies were detectable mainly with the U2 DNA probe. The interchromatin granule-associated zones, the electron-opacity and compactness of which were preserved despite the loosening treatment, remained labeled with the U1 DNA probe only. In DNA-depleted nuclear matrices, the snRNA content of the coiled bodies, the clusters of interchromatin granules and their associated zones, which were all easily recognizable within the residual nuclear ribonucleoprotein network, was unmodified. The data indicate, therefore, that the loosening procedure as well as the high salt extraction procedure preserve the snRNA content of all three spliceosome component-accumulation sites and reveal that interchromatin granule-associated zones are elements of the nuclear matrix. The p80-coilin content coiled bodies was also preserved whatever the salt treatment used. An intriguing new finding is the detection of abundant p80-coilin within the interchromatin granule-associated zones, both before and after either low or high salt treatment of cells. Therefore, p80-coilin is an integral constituent of the interchromatin granule-associated zones.

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