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Molecular and Cellular Biology, May 2007, p. 3682-3694, Vol. 27, No. 10
0270-7306/07/$08.00+0 doi:10.1128/MCB.00172-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Institute of Clinical Molecular Biology and Tumor Genetics,1 Institute of Molecular Immunology, GSF Research Center, Marchioninistrasse 25, D81377 Munich, Germany2
Received 30 January 2007/ Returned for modification 26 February 2007/ Accepted 1 March 2007
The PeBoW complex is essential for cell proliferation and maturation of the large ribosomal subunit in mammalian cells. Here we examined the role of PeBoW-specific proteins Pes1, Bop1, and WDR12 in complex assembly and stability, nucleolar transport, and preribosome association. Recombinant expression of the three subunits is sufficient for complex formation. The stability of all three subunits strongly increases upon incorporation into the complex. Only overexpression of Bop1 inhibits cell proliferation and rRNA processing, and its negative effects could be rescued by coexpression of WDR12, but not Pes1. Elevated levels of Bop1 induce Bop1/WDR12 and Bop1/Pes1 subcomplexes. Knockdown of Bop1 abolishes the copurification of Pes1 with WDR12, demonstrating Bop1 as the integral component of the complex. Overexpressed Bop1 substitutes for endogenous Bop1 in PeBoW complex assembly, leading to the instability of endogenous Bop1. Finally, indirect immunofluorescence, cell fractionation, and sucrose gradient centrifugation experiments indicate that transport of Bop1 from the cytoplasm to the nucleolus is Pes1 dependent, while Pes1 can migrate to the nucleolus and bind to preribosomal particles independently of Bop1. We conclude that the assembly and integrity of the PeBoW complex are highly sensitive to changes in Bop1 protein levels.
Published ahead of print on 12 March 2007.
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