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Molecular and Cellular Biology, July 2004, p. 6324-6337, Vol. 24, No. 14
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.14.6324-6337.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Npa1p, a Component of Very Early Pre-60S Ribosomal Particles, Associates with a Subset of Small Nucleolar RNPs Required for Peptidyl Transferase Center Modification{dagger}

Christophe Dez,1,{ddagger} Carine Froment,2 Jacqueline Noaillac-Depeyre,1 Bernard Monsarrat,2 Michèle Caizergues-Ferrer,1 and Yves Henry1*

Laboratoire de Biologie Moléculaire Eucaryote, UMR5099 CNRS-Université Paul Sabatier, IFR 109, 31062 Toulouse cedex 04,1 Plate-forme protéomique, Institut de Pharmacologie et de Biologie Structurale (CNRS UMR 5089), 31077 Toulouse cedex, France2

Received 7 January 2004/ Returned for modification 8 February 2004/ Accepted 9 April 2004

We have identified a novel essential nucleolar factor required for the synthesis of 5.8S and 25S rRNAs termed Npa1p. In the absence of Npa1p, the pre-rRNA processing pathway leading to 5.8S and 25S rRNA production is perturbed such that the C2 cleavage within internal transcribed spacer 2 occurs prematurely. Npa1p accumulates in the immediate vicinity of the dense fibrillar component of the nucleolus and is predominantly associated with the 27SA2 pre-rRNA, the RNA component of the earliest pre-60S ribosomal particles. By mass spectrometry, we have identified the protein partners of Npa1p, which include eight putative helicases as well as the novel Npa2p factor. Strikingly, we also show that Npa1p can associate with a subset of H/ACA and C/D small nucleolar RNPs (snoRNPs) involved in the chemical modification of residues in the vicinity of the peptidyl transferase center. Our results suggest that 27SA2-containing pre-60S ribosomal particles are located at the interface between the dense fibrillar and the granular components of the nucleolus and that these particles can contain a subset of snoRNPs.

* Corresponding author. Mailing address: Laboratoire de Biologie Moléculaire Eucaryote, UMR5099 CNRS-Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 04, France. Phone: 33 5 61 33 59 53. Fax: 33 5 61 33 58 86. E-mail: henry{at}ibcg.biotoul.fr.

{dagger} We dedicate this work to the memory of Bernard Simon, Laboratoire de Biologie Moléculaire Eucaryote du CNRS.

{ddagger} Present address: Wellcome Trust Centre for Cell Biology, Institute for Cell and Molecular Biology, University of Edinburgh, Edinburgh EH9 3JR, United Kingdom.

Molecular and Cellular Biology, July 2004, p. 6324-6337, Vol. 24, No. 14
0022-538X/04/$08.00+0     DOI: 10.1128/MCB.24.14.6324-6337.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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