Monoclonal antibodies against a nucleolar protein from differentiating chick muscle cells

Two monoclonal antibodies, NORM and NORA, bind specifically to a 37 000 molecular weight protein in total protein extracts of chick skeletal muscle cell cultures. They have been used to follow changes in the amount and distribution of this protein during terminal differentiation and the associated cessation of cell division. Immunofluorescence microscopy shows that the antigen is found almost exclusively in the nucleolus in interphase cells and is dispersed in the cytoplasm during mitosis. Low doses of actinomycin D cause segregation of nucleoli into DNA fibrils and ribonucleoprotein granules, and the antigen is clearly associated with the latter. High doses of actinomycin D cause shrinkage of the nucleolus and the antigen is dispersed within the nucleoplasm, but not released into the cytoplasm. The amount and distribution of antigen is similar in all cell types (myoblasts, myotubes and fibroblasts) and there are no major changes during muscle differentiation. From known changes in ribosomal RNA metabolism during myogenesis and after actinomycin D treatment, we suggest that the 37 000 Mr protein is involved in some post-transcriptional aspect of ribosome production and that its concentration and distribution may be related to the concentration of ribosomal RNA in the nucleolus.