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Molecular and Cellular Biology, June 2009, p. 3007-3017, Vol. 29, No. 11
0270-7306/09/$08.00+0     doi:10.1128/MCB.00029-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

A Novel Small-Subunit Processome Assembly Intermediate That Contains the U3 snoRNP, Nucleolin, RRP5, and DBP4{triangledown}

Amy Jane Turner,1 Andrew Alexander Knox,1 José-Luis Prieto,2 Brian McStay,2 and Nicholas James Watkins1*

Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom,1 Biomedical Research Centre, University of Dundee, Dundee, United Kingdom2

Received 7 January 2009/ Returned for modification 12 February 2009/ Accepted 24 March 2009

Eukaryotic 18S rRNA processing is mediated by the small subunit (SSU) processome, a machine comprised of the U3 small nucleolar RNP (U3 snoRNP), tUTP, bUTP, MPP10, and BMS1/RCL1 subcomplexes. We report that the human SSU processome is a dynamic structure with the recruitment and release of subcomplexes occurring during the early stages of ribosome biogenesis. A novel 50S U3 snoRNP accumulated when either pre-rRNA transcription was blocked or the tUTP proteins were depleted. This complex did not contain the tUTP, bUTP, MPP10, and BMS1/RCL1 subcomplexes but was associated with the RNA-binding proteins nucleolin and RRP5 and the RNA helicase DBP4. Our data suggest that the 50S U3 snoRNP is an SSU assembly intermediate that is likely recruited to the pre-rRNA through the RNA-binding proteins nucleolin and RRP5. We predict that nucleolin is only transiently associated with the SSU processome and likely leaves the complex not long after 50S U3 snoRNP recruitment. The nucleolin-binding site potentially overlaps that of several other key factors, and we propose that this protein must leave the SSU processome for pre-rRNA processing to occur.

* Corresponding author. Mailing address: Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne NE2 4HH, United Kingdom. Phone: 0044 191 222 6991. Fax: 0044 191 222 7424. E-mail: n.j.watkins{at}ncl.ac.uk

{triangledown} Published ahead of print on 30 March 2009.

Molecular and Cellular Biology, June 2009, p. 3007-3017, Vol. 29, No. 11
0270-7306/09/$08.00+0     doi:10.1128/MCB.00029-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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