A nucleolar auto-antigen is part of a major chromosomal surface component

Several nucleolar antigens are defined by human autoantibodies. These antigens can therefore be used to follow the fate of nucleolar components through mitosis when this major nuclear structure disintegrates and becomes reassembled in G1-phase. We found that fibrillarin leaves the nucleolus before complete breakdown of this structure and attaches to chromosomes before nuclear envelope breakdown. In mouse, fibrillarin attaches over the chromosomal surface except for the excluded centromeric region. The antigen is transported to the new nucleus via the chromosomes and is last seen on chromosomal surfaces facing the cytoplasm during nuclear envelope reformation. Lamin B reappears on the same chromosomal surfaces before the nucleolar antigen is removed and aggregates for new nucleolar reformation in G1-phase cells. From our observations, we postulate that the antigen acts in concert with other proteins as a nuclear envelope equivalent by forming a protective sheath around the chromosome, that it excludes larger molecules, and helps to separate the chromosomes, in addition to segregation of the ribonucleoprotein (RNP) back to the nucleus for nucleolar reconstruction. We also suggest that the selective retention of these antigens from certain areas on individual chromosomes together with specific lamin B attachment over these chromosomal surfaces allows for a nonrandom positioning of chromosomes in the nucleus.