title: Ribosome-associated complex and Ssb are required for translational repression induced by polylysine segments within nascent chains.
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.
authors.affiliation: Institute of Biochemistry and Molecular Biology, ZBMZ, University of Freiburg, Freiburg, Germany.
authors.forename: Marco
authors.initials: M
authors.lastname: Chiabudini
authors.forename: Charlotte
authors.initials: C
authors.lastname: Conz
authors.forename: Friederike
authors.initials: F
authors.lastname: Reckmann
authors.forename: Sabine
authors.initials: S
authors.lastname: Rospert
mesh_terms: D000251:Adenosine Triphosphatases / Q000235:genetics / Q000378:metabolism*; D015966:Gene Expression Regulation, Fungal; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D018832:Molecular Chaperones / Q000235:genetics / Q000378:metabolism*; D059365:Nonsense Mediated mRNA Decay; D011107:Polylysine / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism; D017510:Protein Folding; D055550:Protein Stability; D059748:Proteolysis; D012331:RNA, Fungal / Q000235:genetics / Q000378:metabolism; D012270:Ribosomes / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D018840:HSP70 Heat-Shock Proteins; D018832:Molecular Chaperones; D012331:RNA, Fungal; C095192:SSB1 protein, S cerevisiae; C424445:SSZ1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C077019:ZUO1 protein, S cerevisiae; D011107:Polylysine; D046988:Proteasome Endopeptidase Complex; D000251:Adenosine Triphosphatases; C494470:SSB2 protein, S cerevisiae