Translational Control by the Ribosome-Associated Complex in the Unfolded Protein Response

Ribosome-associated chaperones are ubiquitous and highly conserved. There are two classes of ribosome-associated chaperones in eukaryotes, the nascent polypeptide-associated complex (NAC) and the ribosome-associated complex (RAC). Mammalian RAC consists of Hsp70L1, an Hsp70 chaperone homologue, and Mpp11, a DnaJ cofactor. RAC interacts with the nascent chain near the polypeptide exit tunnel and the decoding center on the 60S and 40S ribosomal subunits, respectively. Its unique position on the ribosome implies the coordinating role of de novo protein folding with translation. Deletion of RAC causes growth defects and sensitizes to osmotic, cold, and aminoglycoside stresses in yeast. Furthermore, studies have shown that Mpp11 is over-expressed in head and neck squamous cell cancer and leukemia. However, the function of RAC in stress responses and its role in oncogenesis remain obscure. The current hypothesis predicts that RAC supports co-translational folding of nascent cytosolic polypeptides. To directly test this hypothesis, I altered levels of RAC components and monitored the cytosolic heat shock response (HSR) and the unfolded protein response (UPR) in the ER, two stress pathways known to be activated by accumulation of misfolded proteins. Contrary to its presumptive role in cytosolic protein folding, the reduction of RAC expression did not activate the cytosolic HSR. Unexpectedly, reduction of RAC sensitizes cells to ER stress by selectively attenuating activation of the IRE1 branch of UPR. When RAC is reduced, Xbp1 mRNA splicing is inhibited upon ER stress. Consistent with this activity, ER stress induces changes in the subcellular distribution of RAC, which coincides with the localization of Xbp1 mRNA. Mechanistically, reduction of RAC affects the pathway at a very early step, as IRE1 self-association is inhibited. Additionally, this study shows that the reduction of RAC enhances cellular mRNA translation, including Xbp1 mRNA translation. Interestingly, reduction of Pelo, a protein involved in recognizing stalled ribosomes, counters the inhibition of Xbp1 mRNA splicing, and IRE1 foci formation due to RAC knockdown. Collectively, these results suggest that RAC plays a central role in the IRE1 branch of the UPR tuning IRE1 clustering and mRNA translation.