Critical Role of Nucleostemin in Pre-rRNA Processing*
- Liudmila Romanova,
- Anthony Grand,
- Liying Zhang,
- Samuel Rayner,
- Nobuko Katoku-Kikyo,
- Steven Kellner and
- Nobuaki Kikyo1
- Stem Cell Institute, Division of Hematology, Oncology and Transplantation, Department of Medicine, University of Minnesota, Minneapolis, Minnesota 55455
- 1 To whom correspondence should be addressed: Stem Cell Institute, University of Minnesota, Rm. 2-216, MTRF, Mail Code 2873, 2001 6th St. SE, Minneapolis, MN 55455. Tel.: 612-624-0498; Fax: 612-624-2436; E-mail: kikyo001{at}umn.edu.
Abstract
Nucleostemin is a nucleolar protein widely expressed in proliferating cells. Nucleostemin is involved in the regulation of cell proliferation, and both depletion and overexpression of nucleostemin induce cell cycle arrest through the p53 signaling pathway. Although the presence of p53-independent functions of nucleostemin has been previously suggested, the identities of these additional functions remained to be investigated. Here, we show that nucleostemin has a novel role as an integrated component of ribosome biogenesis, particularly pre-rRNA processing. Nucleostemin forms a large protein complex (>700 kDa) that co-fractionates with the pre-60 S ribosomal subunit in a sucrose gradient. This complex contains proteins related to pre-rRNA processing, such as Pes1, DDX21, and EBP2, in addition to several ribosomal proteins. We show that the nucleolar retention of DDX21 and EBP2 is dependent on the presence of nucleostemin in the nucleolus. Furthermore, the knockdown of nucleostemin delays the processing of 32 S pre-rRNA into 28 S rRNA. This is accompanied by a substantial decrease of protein synthesis as well as the levels of rRNAs and some mRNAs. In addition, overexpressed nucleostemin significantly promotes the processing of 32 S pre-rRNA. Collectively, these biochemical and functional studies demonstrate a novel role of nucleostemin in ribosome biogenesis. This is a key aspect of the role of nucleostemin in regulating cell proliferation.
Footnotes
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↵2 The abbreviations used are: NS, nucleostemin; EGFP, enhanced green fluorescent protein; FBS, fetal bovine serum; I, immunofluorescence staining; PBS, phosphate-buffered saline; W, Western blotting; RT, reverse transcription; siRNA, short interfering RNA.
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↵* This work was supported, in whole or in part, by National Institutes of Health Grant R01 GM068027 (to N. K.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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- Received June 16, 2008.
- Revision received December 4, 2008.
- The American Society for Biochemistry and Molecular Biology, Inc.
