Published online
doi:10.1083/jcb.200612048
The Journal of Cell Biology, Vol. 177, No. 4, 573-578
The Rockefeller University Press, 0021-9525 $30.00
© Krüger et al.
Intranucleolar sites of ribosome biogenesis defined by the localization of early binding ribosomal proteins
Tim Krüger1,
Hanswalter Zentgraf2, and
Ulrich Scheer1
1 Department of Cell and Developmental Biology, Biocenter, University of Würzburg, D-97074 Würzburg, Germany
2 Department of Tumor Virology, German Cancer Research Center, D-69120 Heidelberg, Germany
Correspondence to Ulrich Scheer: scheer{at}biozentrum.uni-wuerzburg.de
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 DFCGC interface.
Abbreviations used in this paper: AMD, actinomycin D; DFC, dense fibrillar component; DRB, 5,6-dichloro-1-ß-D-ribofuranosylbenzimidazole; FC, fibrillar center; GC, granular component; mRFP, monomeric red fluorescent protein; NOR, nucleolus organizing region; r-protein, ribosomal protein; rRNA, ribosomal RNA; snoRNP, small nucleolar RNP; SSU processome, small subunit processome.

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