Postmetaphase nuclear formation: loss of a chromosomal epitope coincident with apparent chromatid coalescence

Previously, we have conceptualized mitotic nuclear formation following metaphase as a morphogenic process and have suggested that sets of chromatids, after separation from a metaphase plate, can be thought of as prenuclei. Such structures can be grouped temporally as either early or late prenuclei based on morphologic, morphometric and density characteristics. Sequential ordering of early prenuclei is of particular interest because it reveals that condensed chromatids coalesce with the resulting formation of a unique chambered structure. In this paper we describe data obtained with a newly raised monoclonal antibody (mAb-2) that initially recognizes an epitope(s) on metaphase chromosomes. Light and confocal fluorescent microscopy of early prenuclei reveal that the chromosomal epitope can no longer be detected about chromatids after their apparent coalescence. Immunoblot analysis of dispersed polypeptides of metaphase plates and early prenuclei indicates that the major protein antigens recognized by mAb-2 have apparent molecular masses of approximately 106000 and 80500 and that each is likely composed of multiple charge isomers. A dual fluorescent analysis using mAb-2 and high-titer anti-lamin B serum provides additional evidence that chromatid coalescence is a separate, early event that precedes nuclear lamina formation.