A role for the ribosome-associated complex in activation of the IRE1 branch of UPR.

I-Hui Wu,Qian Yang,Jae Seok Yoon,Yi Liu,William Skach,Philip Thomas

doi:10.1016/j.celrep.2021.109217

Abstract

The ubiquitous ribosome-associated complex (RAC) is a chaperone that spans ribosomes, making contacts near both the polypeptide exit tunnel and the decoding center, a position prime for sensing and coordinating translation and folding. Loss of RAC is known to result in growth defects and sensitization to translational and osmotic stresses. However, the physiological substrates of RAC and the mechanism(s) by which RAC is involved in responding to specific stresses in higher eukaryotes remain obscure. The data presented here uncover an essential function of mammalian RAC in the unfolded protein response (UPR). Knockdown of RAC sensitizes mammalian cells to endoplasmic reticulum (ER) stress and selectively interferes with IRE1 branch activation. Higher-order oligomerization of the inositol-requiring enzyme 1α (IRE1α) kinase/endoribonuclease depends upon RAC. These results reveal a surveillance function for RAC in the UPR, as follows: modulating IRE1α clustering as required for endonuclease activation and splicing of the substrate Xbp1 mRNA.

Highlights

Within the protein-dense interior of the cell, free molecular chaperones maintain protein homeostasis by facilitating post-translational folding and degradation of misfolded proteins under a wide range of environmental stresses (Balchin et al, 2016; Bukau et al, 2006; Hartl and Hayer-Hartl, 2009; Rosenzweig et al, 2019)
We reduced levels of ribosome-associated complex (RAC) by transient transfection of small interfering RNA pools against Hsp70L1, a RAC component, for 48 h in HeLa cells and monitored the cytosolic heat shock response (HSR)
As previously observed in yeast (Gautschi et al, 2001), reduction in Hsp70L1 led to a loss of its partner Mpp11 in HeLa cells (Figure 1A), suggesting it must be in complex with Hsp70L1 for stability

Summary

Introduction

Within the protein-dense interior of the cell, free molecular chaperones maintain protein homeostasis by facilitating post-translational folding and degradation of misfolded proteins under a wide range of environmental stresses (Balchin et al, 2016; Bukau et al, 2006; Hartl and Hayer-Hartl, 2009; Rosenzweig et al, 2019). Ribosome-associated chaperones, in addition to their probable roles in co-translational folding and degradation of defective nascent chains, occupy a position that enables them to preemptively influence the production of the protein (Deuerling et al, 2019; Kramer et al, 2009; Pechmann et al, 2013; Preissler and Deuerling, 2012; Zhang et al, 2017). As such, these complexes are at the forefront of quality control and homeostatic mechanisms. Recent cross-linking data indicate that Ssz can directly interact with nascent polypeptides prior to Hsp chaperone Ssb contacts, indicating that Ssz is in position to be an active chaperone for co-translational folding (Zhang et al, 2020)

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Discussion

Conclusion