Nucleolar localization of nucleophosmin/B23 requires GTP

Incubation of HeLa cells with the IMP dehydrogenase inhibitors: ribavirin (100 μM, 4 h), tiazofurin (100 μM, 4 h), selenazofurin (100 μM, 4 h), or mycophenolic acid (10 μM, 4 h) resulted in ∼70% reduction in cellular GTP pools and shifting of nucleophosmin/B23 from nucleoli to nucleoplasm as detected by immunofluorescence (B23-translocation). Enzyme-linked immunosorbent assay and Western blot assay showed there is no loss or degradation of nucleophosmin/B23 protein during drug treatment. This translocation effect could be prevented by co-incubation with guanosine (100 μM) or reversed by addition of guanosine (100 μM) to the culture medium after B23-translocation had been induced by these inhibitors. Under these conditions of guanosine supplementation, cellular GTP pool concentrations were maintained at the control level. These results indicate that localization of nucleophosmin/B23 into the nucleolus is dependent on the cellular GTP level.