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Vol. 15, Issue 1, 281-293, January 2004

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Components of U3 snoRNA-containing Complexes Shuttle between Nuclei and the Cytoplasm and Differentially Localize in Nucleoli: Implications for Assembly and Function

Daniel J. Leary ^*, Michael P. Terns , and Sui Huang ^* 

^* Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611; Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602

Submitted June 5, 2003; Revised July 31, 2003; Accepted August 26, 2003
Monitoring Editor: Joseph Gall

U3 small nucleolar RNA (snoRNA) and associated proteins are required for the processing of preribosomal RNA (pre-rRNA) and assembly of preribosomes. There are two major U3 snoRNA-containing complexes. The monoparticle contains U3 snoRNA and the core Box C/D snoRNA-associated proteins and an early preribosome-associated complex contains the monoparticle and additional factors that we refer to as preribosome-associated proteins. To address how and where the U3 snoRNA-containing preribosome assembles and how these processes are temporally and spatially regulated, we have examined the dynamics and distribution of human U3 complex-associated components in cells with active or inactive transcription of rDNA. We found that U3 complex-associated proteins shuttle between the nucleus and the cytoplasm independent of the synthesis and export of preribosomal particles, suggesting that the shuttling of these proteins may either provide opportunities for their regulation, or contribute to or modulate ribosome export. In addition, monoparticle and preribosome associated components predominantly localize to different nucleolar substructures, fibrillar components, and granular components, respectively, in active nucleoli, and partition separately into the two components during nucleolar segregation induced by inhibition of pol I transcription. Although the predominant localizations of these two sets of factors differ, there are significant areas of overlap that may represent the sites where they reside as a single complex. These results are consistent with a model in which U3 monoparticles associate with the fibrillar components of nucleoli and bind pre-rRNA during transcription, triggering recruitment of preribosome-associated proteins to assemble the complex necessary for pre-rRNA processing.

Abbreviations used: ActD, actinomycin D; DFC, dense fibrillar component; FC, fibrillarin center; GC, granular component; hnRNP, heterogenous ribonucleoprotein; pol I, RNA polymerase I; pre-mRNA, pre-messenger RNA; pre-rRNA, pre-ribosomal RNA; snoRNP, small nucleolar ribonucleoprotein; snoRNA, small nucleolar RNA; rRNA, ribosomal RNA; rDNA, ribosomal DNA; UBF, upstream binding factor.

Corresponding author. E-mail address: s-huang2{at}northwestern.edu.

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