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Molecular and Cellular Biology, August 2007, p. 5414-5429, Vol. 27, No. 15
0270-7306/07/$08.00+0     doi:10.1128/MCB.00380-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The 90S Preribosome Is a Multimodular Structure That Is Assembled through a Hierarchical Mechanism ^,

Centro de Investigación del Cáncer and Instituto de Biología Molecular y Celular del Cáncer, CSIC-University of Salamanca, Campus Unamuno, E-37007 Salamanca, Spain

Received 2 March 2007/ Returned for modification 28 March 2007/ Accepted 10 May 2007

The 90S preribosomal particle is required for the production of the 18S rRNA from a pre-rRNA precursor. Despite the identification of the protein components of this particle, its mechanism of assembly and structural design remain unknown. In this work, we have combined biochemical studies, proteomic techniques, and bioinformatic analyses to shed light into the rules of assembly of the yeast 90S preribosome. Our results indicate that several protein subcomplexes work as discrete assembly subunits that bind in defined steps to the 35S pre-rRNA. The assembly of the t-UTP subunit is an essential step for the engagement of at least five additional subunits in two separate, and mutually independent, assembling routes. One of these routes leads to the formation of an assembly intermediate composed of the U3 snoRNP, the Pwp2p/UTP-B, subunit and the Mpp10p complex. The other assembly route involves the stepwise binding of Rrp5p and the UTP-C subunit. We also report the use of a bioinformatic approach that provides a model for the topological arrangement of protein components within the fully assembled particle. Together, our data identify the mechanism of assembly of the 90S preribosome and offer novel information about its internal architecture.

Published ahead of print on 21 May 2007.

Supplemental material for this article may be found at http://mcb.asm.org/.

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