Abstract:Nucleoli were isolated from the interphase nuclei of Physarum polycephalum Schw. ‘lhe nucleolar skeleton was obtained after DNA and most of the nucleolar proteins were extracted with DNase I 0.25 mol/L ( NH4)2SO4 and 2 mol/L NaC1. The nucleolar skeleton appeared as a fibrous network structure composed of fibres about 10 to 30 nm in diameter when observed under the electron microscope. SDS-PAGE analyses revealed about 20 polypeptides in the nucleolar skeleton, including a 43 kD pelypeptide which is equivalent to actin in molecular weight, lmmunofiuorescence observations upon slide preparations of the nueleolar skeleton labeled with anti-actin antibody showed that the nucleolar skeleton emanated bright fluorescence, indicating the existence of thc antigen, lmmunodotting assays further localized actin in the protein preparations of the nucleolar skeleton. Results of immuno-electron microscopy, with anti-actin antibody and protein A-gold as probes, indicated that gold particles were distributed all over the nucleolus of the interphase nucleus.