A monoclonal antibody recognizes a human nuclear protein resembling Xenopus oocyte nucleoplasmin

The monoclonal antibody 10BG2 (IgG3) was derived from a mouse immunized with human pre B cells. In immunofluorescence studies the antibody revealed a human nuclear-associated determinant, which in interphase cells was entirely restricted to the nucleus. In metaphase cells 10BG2 antigen was detected throughout the cytoplasm with intensified staining at the periphery of chromosomes. 10BG2 antibody stained all human normal and transformed cells tested. In contrast, the antibody did not stain mouse 3T3 cells or Chinese hamster ovary (CHO) cells. Electrophoresis under denaturing and non-denaturing conditions revealed the 10BG2 antigen to be a 130 X 10(3) to 140 X 10(3) Mr protein with a subunit molecular weight of 29.5 X 10(3). This suggests the protein is at least a tetramer. On two-dimensional gels the 10BG2 antigen had a streaked appearance and separated into two isoelectric variants (pI5.2, 5.4). The protein was also shown to be phosphorylated and thermostable, and remained in solution at pH 3.6. 10BG2 antigen was highly soluble in aqueous buffers and co-migrated on non-denaturing gels with the nucleosome-assembly protein, nucleoplasmin, purified from Xenopus oocytes. The similarity of 10BG2 antigen to nucleoplasmin is discussed.