Npa1p, a component of very early pre-60S ribosomal particles, associates with a subset of small nucleolar RNPs required for peptidyl transferase center modification

Mol Cell Biol. 2004 Jul;24(14):6324-37. doi: 10.1128/MCB.24.14.6324-6337.2004.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Cell Nucleolus / metabolism
  • Humans
  • Macromolecular Substances
  • Molecular Sequence Data
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism*
  • Peptidyl Transferases / metabolism*
  • RNA Precursors / metabolism
  • RNA Processing, Post-Transcriptional
  • RNA, Ribosomal / metabolism*
  • RNA, Ribosomal, 5.8S / metabolism*
  • Ribonucleoproteins, Small Nuclear / metabolism*
  • Ribosomal Proteins / metabolism*
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / metabolism
  • Saccharomyces cerevisiae / ultrastructure
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism*
  • Sequence Alignment


  • Macromolecular Substances
  • Nuclear Proteins
  • RNA Precursors
  • RNA, Ribosomal
  • RNA, Ribosomal, 5.8S
  • Ribonucleoproteins, Small Nuclear
  • Ribosomal Proteins
  • Saccharomyces cerevisiae Proteins
  • URB1 protein, S cerevisiae
  • RNA, ribosomal, 25S
  • Peptidyl Transferases