Intranucleolar sites of ribosome biogenesis defined by the localization of early binding ribosomal proteins

Considerable efforts are being undertaken to elucidate the processes of ribosome biogenesis. Although various preribosomal RNP complexes have been isolated and molecularly characterized, the order of ribosomal protein (r-protein) addition to the emerging ribosome subunits is largely unknown. Furthermore, the correlation between the ribosome assembly pathway and the structural organization of the dedicated ribosome factory, the nucleolus, is not well established. We have analyzed the nucleolar localization of several early binding r-proteins in human cells, applying various methods, including live-cell imaging and electron microscopy. We have located all examined r-proteins (S4, S6, S7, S9, S14, and L4) in the granular component (GC), which is the nucleolar region where later pre-ribosomal RNA (rRNA) processing steps take place. These results imply that early binding r-proteins do not assemble with nascent pre-rRNA transcripts in the dense fibrillar component (DFC), as is generally believed, and provide a link between r-protein assembly and the emergence of distinct granules at the DFC–GC interface.