[Abstract unavailable

Membrane and secretory proteins cotranslationally enter and are folded in the endoplasmic reticulum (ER). Misfolded or unassembled proteins are discarded by a process known as ER-associated degradation (ERAD), which involves their retrotranslocation into the cytosol. ERAD substrates frequently contain disulfide bonds that must be cleaved before their retrotranslocation. Here, we found that an ER-resident protein ERdj5 had a reductase activity, cleaved the disulfide bonds of misfolded proteins, and accelerated ERAD through its physical and functional associations with EDEM (ER degradation-enhancing alpha-mannosidase-like protein) and an ER-resident chaperone BiP. Thus, ERdj5 is a member of a supramolecular ERAD complex that recognizes and unfolds misfolded proteins for their efficient retrotranslocation.

ER-associated degradation (ERAD) is an ER quality-control process that eliminates terminally misfolded proteins. ERdj5 was recently discovered to be a key ER-resident PDI family member protein that accelerates ERAD by reducing incorrect disulfide bonds in misfolded glycoproteins recognized by EDEM1. We here solved the crystal structure of full-length ERdj5, thereby revealing that ERdj5 contains the N-terminal J domain and six tandem thioredoxin domains that can be divided into the N- and C-terminal clusters. Our systematic biochemical analyses indicated that two thioredoxin domains that constitute the C-terminal cluster form the highly reducing platform that interacts with EDEM1 and reduces EDEM1-recruited substrates, leading to their facilitated degradation. The pulse-chase experiment further provided direct evidence for the sequential movement of an ERAD substrate from calnexin to the downstream EDEM1-ERdj5 complex, and then to the retrotranslocation channel, probably through BiP. We present a detailed molecular view of how ERdj5 mediates ERAD in concert with EDEM1.

The folding of secretory and membrane proteins takes place in the endoplasmic reticulum (ER). The quality of the proteins folded in the ER is carefully monitored by an ER quality control mechanism that allows only correctly folded proteins to be transported to their final destination, and misfolded or unassembled proteins to be retained in the ER and subsequently degraded in a process termed 'ER-associated degradation' (ERAD). The ERAD pathway is conserved from yeast to mammals, and plays an essential role in the maintenance of ER homeostasis, as well as in the prevention of various diseases that arise from the accumulation of aberrant proteins in the ER. In the ERAD pathway, molecular chaperones and lectin-like proteins are involved in the identification of misfolded proteins, ER-resident reductases cleave disulfide bonds in these proteins to facilitate retrograde transport to the cytosol and AAA(+) adenosine triphosphatase withdraws them from the retrotranslocation channel to the cytosol where they are degraded by the ubiquitin/proteasome system. The possible mechanisms that underlie ERAD and the various factors involved in this process are discussed in this article.

Hsp70 chaperone systems are very versatile machines present in nearly all living organisms and in nearly all intracellular compartments. They function in many fundamental processes through their facilitation of protein (re)folding, trafficking, remodeling, disaggregation, and degradation. Hsp70 machines are regulated by co-chaperones. J-domain containing proteins (JDPs) are the largest family of Hsp70 co-chaperones and play a determining role functionally specifying and directing Hsp70 functions. Many features of JDPs are not understood; however, a number of JDP experts gathered at a recent CSSI-sponsored workshop in Gdansk (Poland) to discuss various aspects of J-domain protein function, evolution, and structure. In this report, we present the main findings and the consensus reached to help direct future developments in the field of Hsp70 research.

Calcium ion (Ca 2+ ) is an important second messenger that regulates numerous cellular functions. Intracellular Ca 2+ concentration ([Ca 2+ ]i) is strictly controlled by Ca 2+ channels and pumps on the endoplasmic reticulum (ER) and plasma membranes. The ER calcium pump, sarco/endoplasmic reticulum calcium ATPase (SERCA), imports Ca 2+ from the cytosol into the ER in an ATPase activitydependent manner. The activity of SERCA2b, the ubiquitous isoform of SERCA, is negatively regulated by disulfide bond formation between two luminal cysteines. Here, we show that ERdj5, a mammalian ER disulfide reductase, which we reported to be involved in the ER-associated degradation of misfolded proteins, activates the pump function of SERCA2b by reducing its luminal disulfide bond.

Glycosylated and nonglycosylated proteins misfolded in the ER are degraded by discrete but interchangeable pathways in the ERAD system. Disulfide reductase ERdj5 plays a central role in both pathways through the complex formation with EDEM and/or BiP. The nonglycoprotein ERAD pathway serves as a backup system under ER stress conditions.

Background: Although knockdown of heat shock protein 47 (Hsp47) attenuates liver fibrosis, the underlying molecular mechanism is unknown. Results: Deletion of Hsp47 caused activated hepatic stellate cells (HSCs) to undergo ER stress-mediated apoptosis when autophagy was inhibited. Conclusion: ER stress-induced apoptosis may underlie the clearance of collagen-producing HSCs. Significance: Hsp47 could be an attractive therapeutic target for fibrosis treatment.