RAC/Ssb is required for translational repression induced by polylysine segments within nascent chains
24.09.2012
Chiabudini M, Conz C, Reckmann F, Rospert S
Mol Cell Biol. 2012; 32(23):4769-79
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 yeast, 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 was 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 expression level of nonstop transcripts in vivo.
