The requirement for molecular chaperones during endoplasmic reticulum-associated protein degradation demonstrates that protein export and import are mechanistically distinct

Export of a cysteine-free misfolded secretory protein from the endoplasmic reticulum for degradation requires interaction with protein disulfide isomerase

Protein disulfide isomerase (PDI) interacts with secretory proteins, irrespective of their thiol content, late during translocation into the ER; thus, PDI may be part of the quality control machinery in the ER. We used yeast pdi1 mutants with deletions in the putative peptide binding region of the molecule to investigate its role in the recognition of misfolded secretory proteins in the ER and their export to the cytosol for degradation. Our pdi1 deletion mutants are deficient in the export of a misfolded cysteine-free secretory protein across the ER membrane to the cytosol for degradation, but ER-to-Golgi complex transport of properly folded secretory proteins is only marginally affected. We demonstrate by chemical cross-linking that PDI specifically interacts with the misfolded secretory protein and that mutant forms of PDI have a lower affinity for this protein. In the ER of the pdi1 mutants, a higher proportion of the misfolded secretory protein remains associated with BiP, and in export-deficient sec61 mutants, the misfolded secretory protein remain bounds to PDI. We conclude that the chaperone PDI is part of the quality control machinery in the ER that recognizes terminally misfolded secretory proteins and targets them to the export channel in the ER membrane.

Integrin subunits (beta)1C-1 and (beta)1C-2 expressed in GD25T cells are retained and degraded intracellularly rather than localised to the cell surface

We have previously identified the integrin (beta)1C-2 and characterised the distribution of (beta)1C-1 and (beta)1C-2 transcripts in various cell lines and normal cells. In this study we have investigated the expression of the two (beta)1C-variants in integrin (beta)1 deficient mouse GD25T cells. After stable transfection of the GD25T cells with cDNAs coding for (beta)1A, (beta)1C-1 and (beta)1C-2, the cell surface expression of the (beta)1C-1 and (beta)1C-2 variants was found to be very low while the (beta)1A variant was expressed at high levels. Northern blot analysis showed that the level of (beta)1-transcript in the (beta)1C-1 and (beta)1C-2 clones was equal or higher than in the (beta)1A clones. Metabolic labelling and deglycosylation by endoglycosidase H treatment clearly demonstrated that the majority of the (beta)1C-1 and (beta)1C-2 chains did not become maturely glycosylated, nor did they dimerize with (alpha) subunits. After 20 hours of chase, the labelled (beta)1C-1 and (beta)1C-2 chains had been gradually degraded, whereas immature (beta)1A was converted into the maturely glycosylated form during the same period of time. Immunostaining showed intracellular (beta)1 localisation in the (beta)1C-1 and (beta)1C-2 expressing clones, while in the (beta)1A expressing clones the (beta)1 chains were mainly localised to focal adhesion sites and along fibronectin fibres. Taken together, we have shown that expression of both integrin (beta)1C-1 and (beta)1C-2 in GD25T cells result in very low cell surface expression compared with the normal (beta)1A isoform. Instead, both (beta)1C-1 and (beta)1C-2 chains remain in the endoplasmic reticulum until they are intracellularly degraded.

Setting the standards: quality control in the secretory pathway

A variety of quality control mechanisms operate in the endoplasmic reticulum and in downstream compartments of the secretory pathway to ensure the fidelity and regulation of protein expression during cell life and differentiation. As a rule, only proteins that pass a stringent selection process are transported to their target organelles and compartments. If proper maturation fails, the aberrant products are degraded. Quality control improves folding efficiency by retaining proteins in the special folding environment of the endoplasmic reticulum, and it prevents harmful effects that could be caused by the deployment of incompletely folded or assembled proteins.

Surfing the Sec61 channel: bidirectional protein translocation across the ER membrane

Misfolded secretory and transmembrane proteins are retained in the endoplasmic reticulum (ER) and subsequently degraded. Degradation is primarily mediated by cytosolic proteasomes and thus requires retrograde transport out of the ER back to the cytosol. The available evidence suggests that the protein-conducting channel formed by the Sec61 complex is responsible for both forward and retrograde transport of proteins across the ER membrane. For transmembrane proteins, retrograde transport can be viewed as a reversal of integration of membrane proteins into the ER membrane. Retrograde transport of soluble proteins through the Sec61 channel after signal-peptide cleavage, however, must be mechanistically distinct from signal-peptide-mediated import into the ER through the same channel.

Although calnexin is essential in S. pombe, its highly conserved central domain is dispensable for viability

In mammalian cells, the calnexin/calreticulin chaperones play a key role in glycoprotein folding and its control within the endoplasmic reticulum (ER), by interacting with folding intermediates via their monoglucosylated glycans. This lectin activity has been mapped in mammalian calnexin/calreticulin chaperones to the central region, which is a highly conserved feature of calnexin/calreticulin molecules across species. The central domain has also been implicated in Ca(2+) binding, and it has been proposed to be involved in the regulation of calcium homeostasis in the ER. Herein, we show that although the Schizosaccharomyces pombe calnexin is essential for viability, cells lacking its 317-amino-acid highly conserved central region are viable under normal growth conditions. However, the central region appears to be necessary for optimal growth under high ER-stress, suggesting that this region is important under extreme folding situations (such as DTT and temperature). The minimal length of calnexin required for viability spans the C-terminal 123 residues. Furthermore, cells with the central domain of the protein deleted were affected in their morphology at 37 degrees C, probably due to a defect in cell wall synthesis, although these mutant cells exhibited the same calcium tolerance as wild-type cells at 30 degrees C.

Genetic interactions of Hrd3p and Der3p/Hrd1p with Sec61p suggest a retro-translocation complex mediating protein transport for ER degradation

The endoplasmic reticulum contains a quality control system that subjects misfolded or unassembled secretory proteins to rapid degradation via the cytosolic ubiquitin proteasome system. This requires retrograde protein transport from the endoplasmic reticulum back to the cytosol. The Sec61 pore, the central component of the protein import channel into the endoplasmic reticulum, was identified as the core subunit of the retro-translocon as well. As import of mutated proteins into the endoplasmic reticulum lumen is successfully terminated, a new targeting mechanism must exist that mediates re-entering of misfolded proteins into the Sec61 pore from the lumenal side de novo. The previously identified proteins Der3p/Hrd1p and, as we show here, Hrd3p of the yeast Saccharomyces cerevisiae, are localised in the endoplasmic reticulum membrane and are essential for the degradation of several substrates of the endoplasmic reticulum degradation machinery. Based on genetic studies we demonstrate that they functionally interact with each other and with Sec61p, probably establishing the central part of the retro-translocon. In the absence of Hrd3p, the otherwise stable protein Der3p/Hrd1p becomes rapidly degraded. This depends on a functional ubiquitin proteasome system and the presence of substrate molecules of the endoplasmic reticulum degradation system. When overexpressed, Der3p/Hrd1p accelerates CPY* degradation in Delta(hrd3) cells. Our data suggest a recycling process of Der3p/Hrd1p through Hrd3p. The retro-translocon seems to be build up at least by the Sec61 pore, Der3p/Hrd1p and Hrd3p and mediates both retrograde transport and ubiquitination of substrate molecules.

Role and regulation of the ER chaperone BiP

BiP, an HSP70 molecular chaperone located in the lumen of the endoplasmic reticulum (ER), binds newly-synthesized proteins as they are translocated into the ER and maintains them in a state competent for subsequent folding and oligomerization. BiP is also an essential component of the translocation machinery, as well as playing a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome. BiP is an abundant protein under all growth conditions, but its synthesis is markedly induced under conditions that lead to the accumulation of unfolded polypeptides in the ER. This attribute provides a marker for disease states that result from misfolding of secretory and transmembrane proteins.

ER protein quality control and proteasome-mediated protein degradation

A variety of mutant polypeptides that are associated with human disease are targeted for degradation by an endoplasmic reticulum (ER) quality control system. In addition, physiological signals and viral gene products can target the degradation of several ER resident proteins and secreted proteins passing through the ER. Although the mechanism of protein quality control and the site of degradation were obscure, recent data indicate that degradation requires the cytosolic proteasome. Biochemical and genetic analyses have indicated that both lumenal and integral membrane proteins are selected for proteolysis and exported to the cytosol by a process that in several cases requires ER associated molecular chaperones.

The yeast Hsp110 family member, Sse1, is an Hsp90 cochaperone

In eukaryotes, production of the diverse repertoire of molecular chaperones during normal growth and in response to stress is governed by the heat shock transcription factor HSF. The HSC82 and HSP82 genes, encoding isoforms of the yeast Hsp90 molecular chaperone, were recently identified as targets of the HSF carboxyl-terminal activation domain (CTA), whose expression is required for cell cycle progression during prolonged heat stress conditions. In the present study, we have identified additional target genes of the HSF CTA, which include nearly all of the heat shock-inducible members of the Hsp90 chaperone complex, demonstrating coordinate regulation of these components by HSF. Heat shock induction of SSE1, encoding a member of the Hsp110 family of heat shock proteins, was also dependent on the HSF CTA. Disruption of SSE1 along with STI1, encoding an established subunit of the Hsp90 chaperone complex, resulted in a severe synthetic growth phenotype. Sse1 associated with partially purified Hsp90 complexes and deletion of the SSE1 gene rendered cells susceptible to the Hsp90 inhibitors macbecin and geldanamycin, suggesting functional interaction between Sse1 and Hsp90. Sse1 is required for function of the glucocorticoid receptor, a model substrate of the Hsp90 chaperone machinery, and Hsp90-based repression of HSF under nonstress conditions. Taken together, these data establish Sse1 as an integral new component of the Hsp90 chaperone complex in yeast.

Molecular characterization of a novel mammalian DnaJ-like Sec63p homolog

We identified a human cDNA sequence encoding a polypeptide of 760 amino acids that shares 53% homology and 25.6% identity with the yeast DnaJ-like endoplasmic reticulum (ER) translocon component Sec63p. Three epitope-specific antisera revealed a protein of an apparent molecular mass of 83 kDa, both in human cell extracts and in dog pancreatic microsomes. Biochemical analyses show that it is an integral membrane protein of the rough ER, which has the DnaJ domain located in the ER lumen. The novel Sec63 protein could thus represent a key component of the mammalian ER protein translocation machinery.

Retrograde protein translocation: ERADication of secretory proteins in health and disease

Eukaryotic cells have a complex degradation machinery that eliminates misfolded or unassembled secretory proteins from the endoplasmic reticulum (ER). The proteins are retained in an ER/pre-Golgi compartment and then hydrolysed by the cytosolic ubiquitin-proteasome system. This requires retrograde translocation of proteins from the ER back to the cytoplasm, which is mediated by Sec61, the central component of the ER protein-import channel. This proteolytic pathway prevents a potentially lethal aggregation of secretory proteins; however, several viruses misuse it to escape detection, and bacterial and plant toxins might also exploit it. Underactive or overactive ER degradation machinery contributes to the pathogenesis of several severe human diseases.