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Ribosome-associated complex and Ssb are required for translational repression induced by polylysine segments within nascent chains.

When a polyadenylated nonstop transcript is fully translated, a complex consisting of the ribosome, the nonstop mRNA, and the C-terminally polylysine-tagged protein is generated. In Saccharomyces cerevisiae, a 3-step quality control system prevents formation of such dead-end complexes. Nonstop mRNA is rapidly degraded, translation of nonstop mRNA is repressed, and finally, nonstop proteins are cotranslationally degraded. Nonstop mRNA degradation depends on Ski7 and the exosome; nonstop protein degradation depends on the ribosome-bound E3 ligase Ltn1 and the proteasome. However, components which mediate translational repression of nonstop mRNA have previously not been identified. Here we show that the ribosome-bound chaperone system consisting of the ribosome-associated complex (RAC) and the Hsp70 homolog Ssb is required to stabilize translationally repressed ribosome-polylysine protein complexes, without affecting the folding or the degradation of polylysine proteins. As a consequence, in the absence of RAC/Ssb, polylysine proteins escaped translational repression and subsequently folded into their native conformation. This active role of RAC/Ssb in the quality control of polylysine proteins significantly contributed to the low level of expression of nonstop transcripts in vivo.

Pubmed ID: 23007158


  • Chiabudini M
  • Conz C
  • Reckmann F
  • Rospert S


Molecular and cellular biology

Publication Data

December 12, 2012

Associated Grants


Mesh Terms

  • Adenosine Triphosphatases
  • Gene Expression Regulation, Fungal
  • HSP70 Heat-Shock Proteins
  • Molecular Chaperones
  • Nonsense Mediated mRNA Decay
  • Polylysine
  • Proteasome Endopeptidase Complex
  • Protein Folding
  • Protein Stability
  • Proteolysis
  • RNA, Fungal
  • Ribosomes
  • Saccharomyces cerevisiae
  • Saccharomyces cerevisiae Proteins