Non-ribosomal nucleolar proteins in HeLa cells

The study of nucleolar macromolecular components has previously emphasized ribosomal precursor and mature RNA, or ribosomal structural proteins. In this work, we have stressed the purification of nucleoli, and have studied their protein composition. The results indicate that there exists in highly purified nucleoli of HeLa cells, a class of high molecular weight polypeptides which have been identified by means of size, kinetics of turnover, and metabolic behavior to be non-ribosomal nucleolar-specific proteins. The possibility that these proteins play a part in the assembly of mature ribosomes is discussed.