Monoclonal antibody 3C5 recognizes a family of protein antigens present predominantly within the nucleus of interphase cells. We have shown previously that the epitope recognized by 3C5 is phosphorylated and have concluded that the proteins defined by this antibody share a common phosphorylation site. Using a combination of immunofluorescence microscopy and immunogold labelling in conjunction with electron microscopy, we have studied the distribution of 3C5-reactive material within interphase and mitotic cells. Antibody 3C5 was found to label specific structures within the interphase nucleus that, on the basis of their characteristic granulofibrillar morphology and strong staining with bismuth, have been identified as clusters of interchromatin granules (IG clusters). Double-labelling experiments with 3C5 and monoclonal antibodies to DNA have shown that these structures contain no detectable DNA. However, by indirect immunofluorescence, we have shown that 3C5-reactive nuclear structures do label with human autoantibodies to the Sm antigen, a component of small nuclear RNP particles (snRNPs). Granulofibrillar structures that stained strongly with bismuth, and were morphologically identical to nuclear IG clusters, were observed in the cytoplasm of mitotic cells. These structures also labelled with 3C5 but not with anti-Sm antibodies. Our results suggest that IG clusters remain essentially intact through mitosis though some snRNP components are apparently lost. In situ extraction of cultured cells with Triton X-100, micrococcal nuclease and 1–2M-NaCl failed to deplete 3C5-reactive material in either interphase or mitotic cells, though some redistribution was evident. In addition, 3C5-reactive proteins were identified in nuclear matrices prepared from rat liver by high-salt extraction procedures. However, the recovery of such proteins was strongly influenced by the preparation technique employed. Our results suggest that 3C5-reactive proteins and IG clusters are anchored to, but not integral components of, salt-resistant structural elements of the interphase nucleus and the mitotic cytoplasm, presumably the nuclear matrix and the cytoskeleton, respectively.

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