THE B1C8 PROTEIN IS IN THE DENSE ASSEMBLIES OF THE NUCLEAR MATRIX ANDRELOCATES TO THE SPINDLE AND PERICENTRIOLAR FILAMENTS AT MITOSIS

The B1C8 monoclonal antibody detects a 180-kDa nuclear matrix-specific protein. The protein is a component of the dense, metabolically activ e bodies or assemblies revealed by resinless section electron microsco py of the nuclear matrix. These assemblies are scattered through the n uclear interior, emmeshed in a complex network of 11-nm filaments. Res inless section electron microscopy of immunogold-stained nuclear matri x preparations shows B1C8 located in many but apparently not all the a ssemblies. In this regard, the B1C8 antigen resembles previously studi ed nuclear matrix proteins such as the H1B2 protein. The speckled patt ern of nuclear immunofluorescence by B1C8 reflects this labeling of th e dense assemblies in the nuclear matrix. Somewhat unusual is the fain t staining of cytoplasmic microtubules by B1C8, which appears to be du e to a weakly cross-reacting protein. During fell division, the B1C8 a ntigen redistributed drastically, showing the dispersion of nuclear ma trix assemblies at mitosis. Speckles of B1C8 fluorescence first coales ced at prophase within the nuclear interior and then scattered into nu merous cytoplasmic speckles by prometaphase. At metaphase, the B1C8 sp eckled cytoplasmic staining had become even more widely distributed an d finely grained. Also, intense labeling appeared at the mitotic pole and on the spindle fibers themselves. The reassembly of B1C8 antigens into larger cytoplasmic speckles began at anaphase and finally, at tel o-phase, most B1C8 labeling redistributed into speckles in the re-form ing nuclei.