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Mol Cell Biol. 2012 Dec;32(23):4769-79. doi: 10.1128/MCB.00809-12. Epub 2012 Sep 24.

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

Author information

  • 1Institute of Biochemistry and Molecular Biology, ZBMZ, University of Freiburg, Freiburg, Germany.

Abstract

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.

PMID:
23007158
[PubMed - indexed for MEDLINE]
PMCID:
PMC3497599
Free PMC Article

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