Nucleophosmin/B23 (NPM) oligomer is a major and stable entity in HeLa cells

HeLa cell extract was separated by 7% polyacrylamide gel electrophoresis without SDS and subsequently stained with anti-nucleophosmin/B23 (NPM) antibody in a Western blot analysis. Two immunobands were obtained. The major band with a slower electromobility (RF = 0.23) is the NPM oligomer, while the fast-moving minor band is the monomer (RF = 0.56). The oligomer constitutes about 95% of total NPM. The oligomer sedimented faster (10 s) than the monomer in sucrose density gradient centrifugation. Three oligomer bands were identified. NPM oligomer is not affected by treatments with DNase. RNase, 10 mM EDTA, 1 M NaCl, and lyophilization. However, 3 M urea causes reversible dissociation of NPM oligomer into monomer. The level of NPM oligomer remains unchanged in HeLa cells after treatment with the cytotoxic agents, actinomycin D, toyocamycin and camptothecin. These results indicate that NPM oligomer is the major and stable NPM entity in HeLa cells.