Ribosome assembly in HeLa cells: labeling pattern of ribosomal proteins by two-dimensional resolution.

A high resolution, two-dimensional gel electrophoresis of the proteins from HeLa cell large ribosomal subunits and their nucleolar precursor particles is described. There are 40 major spots in the mature particles and about 65 in the precursors. Proteins in the precursor particles include 30 spots which are similar to those in mature large subunits, and at least 33 major spots which are restricted to the precursor stage. Labeling patterns of ribosomes showed a limited number of proteins associated with mature large subunits that incorporate radioactive amino acids more rapidly, indicating those proteins that are recycled in the cytoplasm. Among the proteins associated with pre-ribosomal particles, those that are similar to the proteins of mature ribosomes labeled more rapidly than the precursor-specific nucleolar proteins. The latter are apparently reutilized for ribosome assembly in the nucleolus. Thus, in addition to resolution of the proteins only transiently associated with precursor particles, results indicate the differences in their labeling properties, consistent with their behaviour during ribosome assembly in HeLa cells.