The Role of p58^IPK in Protecting the Stressed Endoplasmic Reticulum

Abstract

The preemptive quality control (pQC) pathway protects cells from acute endoplasmic reticulum (ER) stress by attenuating translocation of nascent proteins despite their targeting to translocons at the ER membrane. Here, we investigate the hypothesis that the DnaJ protein p58 IPK plays an essential role in this process via HSP70 recruitment to the cytosolic face of translocons for extraction of translocationally attenuated nascent chains. Our analyses revealed that the heightened stress sensitivity of p58؊/؊ cells was not due to an impairment of the pQC pathway or elevated ER substrate burden during acute stress. Instead, the lesion was in the protein processing capacity of the ER lumen, where p58 IPK was found to normally reside in association with BiP. ER lumenal p58 IPK could be coimmunoprecipitated with a newly synthesized secretory protein in vitro and stimulated protein maturation upon overexpression in cells. These results identify a previously unanticipated location for p58 IPK in the ER lumen where its putative function as a cochaperone explains the stress-sensitivity phenotype of knockout cells and mice.

Protein Folding and Quality Control in the ER

“…ERdj6, designated p58 IPK , was initially reported to negatively regulate PKR and PERK phosphorylation in the cytosol (Gale et al 1998;Yan et al 2002). However, it was later determined that ERdj6 is also localized to the ER by an ER-targeting signal and that ERdj6 binds to BiP through its J-domain or the tetratricopeptide (TPR) repeat domain and functions as a cochaperone (Rutkowski et al 2007;Petrova et al 2008). ERdj6 is most probably involved in the productive folding of newly synthesized proteins in the ER lumen.…”

ERdj4 and ERdj5 Are Required for Endoplasmic Reticulum-associated Protein Degradation of Misfolded Surfactant Protein C

“…Interestingly, p58 IPK , a DnaJ protein recently reported to be localized to the ER lumen (Rutkowski et al, 2007), was also robustly increased in cells expressing mutant SP-C ( Table 1), suggesting that it may also be involved in ERAD of SP-C. BiP, ERdj4, and ERdj5 coprecipitated with misfolded SP-C and remained associated with the substrate until it was retrotranslocated to the cytosol. Inhibition of ERdj4 and ERdj5 expression resulted in ER retention and accumulation of misfolded SP-C. Inhibition of ERdj4 and ERdj5 did produce a minor increase in the amount of wild-type proprotein; however, this effect is consistent with ERAD-mediated turnover of a small amount of unfolded/misfolded wild-type protein, as reported for many newly synthesized proteins (Schubert et al, 2000;Rutkowski et al, 2006).…”

Emerging features of ER resident J-proteins in plants

“…19 P58 IPK was later discovered to also be an ER resident protein that binds to unfolded proteins and acts as a co-chaperone for BiP. 20,21 Although P58 IPK binds to several unfolded proteins, modulation of P58 IPK level did not affect degradation of the unfolded proteins. 22 Binding of P58 IPK to unfolded proteins is BiP-independent, and BiP enhances release of P58 IPK from the unfolded proteins, implicating that P58 IPK may reduce the burden of unfolded proteins by upregulating protein folding in the ER lumen.…”