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Esf2p, a U3-Associated Factor Required for Small-Subunit Processome Assembly and Compaction

Tran Hoang,^1, Wen-Tao Peng,^2,, Emmanuel Vanrobays,^1, Nevan Krogan,^2,3 Shawna Hiley,² Ann L. Beyer,⁴ Yvonne N. Osheim,⁴ Jack Greenblatt,^2,3 Timothy R. Hughes,^2,3,* and Denis L. J. Lafontaine^1,*

Fonds National de la Recherche Scientifique, Université Libre de Bruxelles, Institut de Biologie et de Médecine Moléculaires, Charleroi-Gosselies, Belgium,¹ Banting and Best Department of Medical Research, University of Toronto, Toronto, Canada,² Department of Molecular and Medical Genetics, University of Toronto, Toronto, Canada,³ Department of Microbiology, University of Virginia Health System, Charlottesville, Virginia 22908⁴

Received 4 February 2005/ Returned for modification 4 March 2005/ Accepted 6 April 2005

Esf2p is the Saccharomyces cerevisiae homolog of mouse ABT1, a protein previously identified as a putative partner of the TATA-element binding protein. However, large-scale studies have indicated that Esf2p is primarily localized to the nucleolus and that it physically associates with pre-rRNA processing factors. Here, we show that Esf2p-depleted cells are defective for pre-rRNA processing at the early nucleolar cleavage sites A₀ through A₂ and consequently are inhibited for 18S rRNA synthesis. Esf2p was stably associated with the 5' external transcribed spacer (ETS) and the box C+D snoRNA U3, as well as additional box C+D snoRNAs and proteins enriched within the small-subunit (SSU) processome/90S preribosomes. Esf2p colocalized on glycerol gradients with 90S preribosomes and slower migrating particles containing 5' ETS fragments. Strikingly, upon Esf2p depletion, chromatin spreads revealed that SSU processome assembly and compaction are inhibited and glycerol gradient analysis showed that U3 remains associated within 90S preribosomes. This suggests that in the absence of proper SSU processome assembly, early pre-rRNA processing is inhibited and U3 is not properly released from the 35S pre-rRNAs. The identification of ABT1 in a large-scale analysis of the human nucleolar proteome indicates that its role may also be conserved in mammals.

These authors made equal contributions.

These two authors contributed equally to this work.

Present address: Building 10, Room 9D42, NIH, MSC 1830, 10 Center Dr., Bethesda, MD 20892-1830.

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