Biomedical subjects

Ernst Jarosch

Publications and source records attributed to Ernst Jarosch.

11 recordsLinked to original sources

PubMed2006

A complex of Yos9p and the HRD ligase integrates endoplasmic reticulum quality control into the degradation machinery.

A quality-control system surveys the lumen of the endoplasmic reticulum for terminally misfolded proteins. Polypeptides singled out by this system are ultimately degraded by the cytosolic ubiquitin-proteasome pathway. Key components of both the endoplasmic reticulum quality-control system and the degradation machinery have been identified, but a connection between the two systems has remained elusive. Here, we report an association between the endoplasmic reticulum quality-control lectin Yos9p and Hrd3p, a component of the ubiquitin-proteasome system that links these pathways. We identify designated regions in the luminal domain of Hrd3p that interact with Yos9p and the ubiquitin ligase Hrd1p. Binding of misfolded proteins occurs through Hrd3p, suggesting that Hrd3p recognises proteins that deviate from their native conformation, whereas Yos9p ensures that only terminally misfolded polypeptides are degraded.

Blotting, Western↗

PubMed2006

The Hrd1p ligase complex forms a linchpin between ER-lumenal substrate selection and Cdc48p recruitment.

Misfolded proteins of the endoplasmic reticulum (ER) are targeted to the cytoplasm for proteasomal degradation. Key components of this process are ER membrane-bound ubiquitin ligases. These ligases associate with the cytoplasmic AAA-ATPase Cdc48p/p97, which is thought to support the release of malfolded proteins from the ER. Here, we characterize a yeast protein complex containing the ubiquitin ligase Hrd1p and the ER membrane proteins Hrd3p and Der1p. Hrd3p binds malfolded proteins in the ER lumen enabling their delivery to downstream components. Therefore, we propose that Hrd3p acts as a substrate recruitment factor for the Hrd1p ligase complex. Hrd3p function is also required for the association of Cdc48p with Hrd1p. Moreover, our data demonstrate that recruitment of Cdc48p depends on substrate processing by the Hrd1p ligase complex. Thus, the Hrd1p ligase complex unites substrate selection in the ER lumen and polyubiquitination in the cytoplasm and links these processes to the release of ER proteins via the Cdc48p complex.

Adenosine Triphosphatases↗

PubMed2005

Ubx2 links the Cdc48 complex to ER-associated protein degradation.

Endoplasmic reticulum (ER)-associated protein degradation requires the dislocation of selected substrates from the ER to the cytosol for proteolysis via the ubiquitin-proteasome system. The AAA ATPase Cdc48 (known as p97 or VCP in mammals) has a crucial, but poorly understood role in this transport step. Here, we show that Ubx2 (Sel1) mediates interaction of the Cdc48 complex with the ER membrane-bound ubiquitin ligases Hrd1 (Der3) and Doa10. The membrane protein Ubx2 contains a UBX domain that interacts with Cdc48 and an additional UBA domain. Absence of Ubx2 abrogates breakdown of ER proteins but also that of a cytosolic protein, which is ubiquitinated by Doa10. Intriguingly, our results suggest that recruitment of Cdc48 by Ubx2 is essential for turnover of both ER and non-ER substrates, whereas the UBA domain of Ubx2 is specifically required for ER proteins only. Thus, a complex comprising the AAA ATPase, a ubiquitin ligase and the recruitment factor Ubx2 has a central role in ER-associated proteolysis.

Adenosine Triphosphatases↗

PubMed2005

Structure of palmitoylated BET3: insights into TRAPP complex assembly and membrane localization.

BET3 is a component of TRAPP, a complex involved in the tethering of transport vesicles to the cis-Golgi membrane. The crystal structure of human BET3 has been determined to 1.55-A resolution. BET3 adopts an alpha/beta-plait fold and forms dimers in the crystal and in solution, which predetermines the architecture of TRAPP where subunits are present in equimolar stoichiometry. A hydrophobic pocket within BET3 buries a palmitate bound through a thioester linkage to cysteine 68. BET3 and yeast Bet3p are palmitoylated in recombinant yeast cells, the mutant proteins BET3 C68S and Bet3p C80S remain unmodified. Both BET3 and BET3 C68S are found in membrane and cytosolic fractions of these cells; in membrane extractions, they behave like tightly membrane-associated proteins. In a deletion strain, both Bet3p and Bet3p C80S rescue cell viability. Thus, palmitoylation is neither required for viability nor sufficient for membrane association of BET3, which may depend on protein-protein contacts within TRAPP or additional, yet unidentified modifications of BET3. A conformational change may facilitate palmitoyl extrusion from BET3 and allow the fatty acid chain to engage in intermolecular hydrophobic interactions.

Amino Acid Sequence↗

PubMed2005

Pardon me--no access without ubiquitin.

Spatial separation of ubiquitin conjugation pathways contributes to target-specific ubiquitination. Recently, Plafker et al. reported that importin 11-dependent nuclear import of the ubiquitin-conjugating enzyme UbcM2 occurs only if the latter is charged with ubiquitin. This interesting finding describes a link between nuclear transport pathways and ubiquitin and reveals a novel mechanism for localizing components of the ubiquitin system within the cell.

Active Transport, Cell Nucleus↗

PubMed2005

ERAD: the long road to destruction.

Endoplasmic reticulum (ER)-associated protein degradation (ERAD) eliminates misfolded or unassembled proteins from the ER. ERAD targets are selected by a quality control system within the ER lumen and are ultimately destroyed by the cytoplasmic ubiquitin-proteasome system (UPS). The spatial separation between substrate selection and degradation in ERAD requires substrate transport from the ER to the cytoplasm by a process termed dislocation. In this review, we will summarize advances in various aspects of ERAD and discuss new findings on how substrate dislocation is achieved.

Animals↗

PubMed2004

Endoplasmic reticulum-associated protein degradation--one model fits all?

The endoplasmic reticulum (ER) is the eukaryotic organelle where most secretory proteins are folded for subsequent delivery to their site of action. Proper folding of newly synthesized proteins is monitored by a stringent ER quality control system. This system recognizes misfolded or unassembled proteins and prevents them from reaching their final destination. Instead, they are extracted from the ER, polyubiquitinated and degraded by the cytosolic proteasome. With the identification of novel components and substrates, a more and more complex picture of this process emerges in which distinct pathways target different sets of substrates for destruction.

Animals↗

PubMed2003

Endoplasmic reticulum-associated protein degradation.

Proteins that fail to fold properly as well as constitutive or regulated short-lived proteins of the endoplasmatic reticulum (ER) are subjected to proteolysis by cytosolic 26 S proteasomes. This process, termed ER-associated protein degradation (ERAD), has also been implicated in the generation of some important human disorders, for example, cystic fibrosis. To become accessible to the proteasome, ERAD substrates must first be retrogradely transported from the ER into the cytosol, in a process termed dislocation. Surprisingly, protein dislocation from the ER seems to require at least some components that also mediate import into this compartment. Moreover, polyubiquitination of ERAD substrates at the ER membrane as well as the cytoplasmic Cdc48p/Npl4p/Ufd1p complex were shown to contribute to this export reaction. In this article we will summarize our current knowledge on ERAD and discuss the possible function of certain components involved in this process.

Animals↗

PubMed2002

BiP binding keeps ATF6 at bay.

A study by, in this issue of Developmental Cell shows that transport to the Golgi complex and subsequent proteolytic activation of the stress-regulated transcription factor ATF6 is initiated by the dissociation of the ER chaperone BiP from ATF6. This demonstrates that BiP is a key element in sensing the folding capacity within the ER and provides mechanistic insights on how the activation of membrane-bound transcription factors can be regulated.

Activating Transcription Factor 6↗

PubMed2002

Protein dislocation from the endoplasmic reticulum--pulling out the suspect.

Proteins that fail to fold properly as well as constitutive or regulated short-lived proteins of the endoplasmic reticulum are subjected to proteolysis by cytosolic 26S proteasomes. This process is known as endoplasmic reticulum-associated protein degradation. In order to become accessible to the proteasome of this system substrates must first be retrogradely transported from the endoplasmic reticulum into the cytosol, in a process termed dislocation. This export step seems to be accompanied by polyubiquitination of such molecules. Surprisingly, protein dislocation from the endoplasmic reticulum seems to require at least some components that mediate import into this compartment. However, protein import and export display differences in the mechanism that provides the driving force and ensures directionality. Of special interest is the cytoplasmic Cdc48p/Npl4p/Ufd1p complex, which is required for the degradation of various endoplasmic reticulum-associated protein degradation substrates and seems to function in a step after polyubiquitination but before proteasomal digestion. In this review, we will summarize our knowledge on protein export during endoplasmic reticulum-associated protein degradation and discuss the possible function of certain components involved in this process.

Adenosine Triphosphatases↗

PubMed2002

Protein dislocation from the ER requires polyubiquitination and the AAA-ATPase Cdc48.

Endoplasmic reticulum (ER)-associated protein degradation by the ubiquitin-proteasome system requires the dislocation of substrates from the ER into the cytosol. It has been speculated that a functional ubiquitin proteasome pathway is not only essential for proteolysis, but also for the preceding export step. Here, we show that short ubiquitin chains synthesized on proteolytic substrates are not sufficient to complete dislocation; the size of the chain seems to be a critical determinant. Moreover, our results suggest that the AAA proteins of the 26S proteasome are not directly involved in substrate export. Instead, a related AAA complex Cdc48, is required for ER-associated protein degradation upstream of the proteasome.

Adenosine Triphosphatases↗