Nuclear phosphoproteins of Physarum polycephalum: Characterization and phosphorus content of the phenol-soluble nuclear acidic proteins

Abstract

The phenol-soluble nuclear phosphoproteins of the slime mold Physarum polycephalum have been characterized using 32P isotopic labeling followed by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. Plasmodia were labeled continuously with 3H-amino acids and 32P i before extraction of the phenol-soluble nuclear phosphoproteins, which were then separated by acrylamide gel electrophoresis. After scintillation counting of gel slices, we were able to calculate the moles phosphate per mole polypeptide in individual gel slices. The results indicated minimum values of less than 2 phosphates per polypeptide for most resolvable 32P bands. The phenol-soluble nuclear acidic protein fraction contained less than 0.1% phosphorus by weight. Gel autoradiographs demonstrated that most of the phosphoprotein peaks did not correspond to major protein peaks, a situation that was also found in autoradiographic profiles of other nuclear protein fractions. Pronase digestion of samples before electrophoresis abolished the appearance on gels of bands with 32P activity. The results of this investigation suggest that in Physarum the phenol-soluble nuclear acidic proteins appear to be similar to other nuclear proteins in their phosphoprotein composition. The evidence presented suggests that phosphoproteins comprise a small part of the complement of nuclear acidic proteins.