Nucleolar components involved in ribosome biogenesis cycle between the nucleolus and nucleoplasm in interphase cells.
We examined the mobilities of nucleolar components that act at various steps of the ribosome biogenesis pathway. Fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) analyses demonstrate that factors involved in rRNA transcription (upstream-binding factor [UBF]), processing (nucleolin, fibrillarin, and RNase MRP subunits, Rpp29), and ribosome assembly (B23) exchange rapidly between the nucleoplasm and nucleolus. In contrast, the mobilities of ribosomal subunit proteins (S5, L9) are much slower. Selective inhibition of RNA polymerase I transcription does not prevent the exchanges but influences the rates of exchange differentially for different nucleolar components. These findings suggest that the rapid exchange of nucleolar components between the nucleolus and nucleoplasm may represent a new level of regulation for rRNA synthesis. The different dynamic properties of proteins involved in different steps of ribosome biogenesis imply that the nucleolar association of these proteins is due to their specific functional roles rather than simply their specific nucleolar-targeting events.
Chen
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Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611, USA.
Huang
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D002466:Cell Nucleolus / Q000378:metabolism*; D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D007399:Interphase; D008164:Luminescent Proteins / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D035463:Pol1 Transcription Initiation Complex Proteins; D012318:RNA Polymerase I / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012260:Ribonucleases / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic; D000097796:Nucleolin
D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins; D010750:Phosphoproteins; D035463:Pol1 Transcription Initiation Complex Proteins; D012318:RNA Polymerase I; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D012260:Ribonucleases; D014157:Transcription Factors; D000097796:Nucleolin; C047458:fibrillarin; C063844:transcription factor UBF