Mechanisms of substrate processing during ER-associated protein degradation.

Maintaining proteome integrity is essential for long-term viability of all organisms and is overseen by intrinsic quality control mechanisms. The secretory pathway of eukaryotes poses a challenge for such quality assurance as proteins destined for secretion enter the endoplasmic reticulum (ER) and become spatially segregated from the cytosolic machinery responsible for disposal of aberrant (misfolded or otherwise damaged) or superfluous polypeptides. The elegant solution provided by evolution is ER-membrane-bound ubiquitylation machinery that recognizes misfolded or surplus proteins or by-products of protein biosynthesis in the ER and delivers them to 26S proteasomes for degradation.

Thioester and Oxyester Linkages in the Ubiquitin System.

The traditional textbook describes ubiquitylation as the conjugation of ubiquitin to a target by forming a covalent bond connecting ubiquitin's carboxy-terminal glycine residue with an acceptor amino acid like lysine or amino-terminal methionine in the substrate protein. While this adequately depicts a significant fraction of cellular ubiquitylation processes, a growing number of ubiquitin modifications do not follow this rule. Recent data demonstrate that ubiquitin can also be efficiently attached to other amino acids, such as cysteine, serine, and threonine, via ester bonding.

The UBA domain of conjugating enzyme Ubc1/Ube2K facilitates assembly of K48/K63-branched ubiquitin chains.

The assembly of a specific polymeric ubiquitin chain on a target protein is a key event in the regulation of numerous cellular processes. Yet, the mechanisms that govern the selective synthesis of particular polyubiquitin signals remain enigmatic. The homologous ubiquitin-conjugating (E2) enzymes Ubc1 (budding yeast) and Ube2K (mammals) exclusively generate polyubiquitin linked through lysine 48 (K48).

Serum amyloid A1 mediates myotube atrophy via Toll-like receptors.

Background: Critically ill patients frequently develop muscle atrophy and weakness in the intensive-care-unit setting [intensive care unit-acquired weakness (ICUAW)]. Sepsis, systemic inflammation, and acute-phase response are major risk factors. We reported earlier that the acute-phase protein serum amyloid A1 (SAA1) is increased and accumulates in muscle of ICUAW patients, but its relevance was unknown.

Structural investigation of glycan recognition by the ERAD quality control lectin Yos9.

Yos9 is an essential component of the endoplasmic reticulum associated protein degradation (ERAD) system that is responsible for removing terminally misfolded proteins from the ER lumen and mediating proteasomal degradation in the cytosol. Glycoproteins that fail to attain their native conformation in the ER expose a distinct oligosaccharide structure, a terminal α1,6-linked mannose residue, that is specifically recognized by the mannose 6-phoshate receptor homology (MRH) domain of Yos9. We have determined the structure of the MRH domain of Yos9 in its free form and complexed with 3α, 6α-mannopentaose.

Sequential Poly-ubiquitylation by Specialized Conjugating Enzymes Expands the Versatility of a Quality Control Ubiquitin Ligase.

The Doa10 quality control ubiquitin (Ub) ligase labels proteins with uniform lysine 48-linked poly-Ub (K48-pUB) chains for proteasomal degradation. Processing of Doa10 substrates requires the activity of two Ub conjugating enzymes. Here we show that the non-canonical conjugating enzyme Ubc6 attaches single Ub molecules not only to lysines but also to hydroxylated amino acids.

A Complex of Htm1 and the Oxidoreductase Pdi1 Accelerates Degradation of Misfolded Glycoproteins.

A quality control system in the endoplasmic reticulum (ER) efficiently discriminates polypeptides that are in the process of productive folding from conformers that are trapped in an aberrant state. Only the latter are transported into the cytoplasm and degraded in a process termed ER-associated protein degradation (ERAD). In the ER, an enzymatic cascade generates a specific N-glycan structure of seven mannosyl and two N-acetylglucosamine residues (Man7GlcNAc2) on misfolded glycoproteins to facilitate their disposal.

Rsp5/Nedd4 clears cells of heat-damaged proteins.

Protein quality control systems protect cells from proteotoxicity caused by the accumulation of aberrantly folded polypeptides. The Rsp5 ubiquitin ligase (mammalian homologue Nedd4) is now identified as a major constituent of a clearance pathway that degrades misfolded cytosolic proteins after exposure to heat.

The interplay of Hrd3 and the molecular chaperone system ensures efficient degradation of malfolded secretory proteins.

Misfolded proteins of the secretory pathway are extracted from the endoplasmic reticulum (ER), polyubiquitylated by a protein complex termed the Hmg-CoA reductase degradation ligase (HRD-ligase), and degraded by cytosolic 26S proteasomes. This process is termed ER-associated protein degradation (ERAD). We previously showed that the membrane protein Der1, which is a subunit of the HRD-ligase, is involved in the export of aberrant polypeptides from the ER.

Hexosamine pathway metabolites enhance protein quality control and prolong life.

Aging entails a progressive decline in protein homeostasis, which often leads to age-related diseases. The endoplasmic reticulum (ER) is the site of protein synthesis and maturation for secreted and membrane proteins. Correct folding of ER proteins requires covalent attachment of N-linked glycan oligosaccharides.

Der1 promotes movement of misfolded proteins through the endoplasmic reticulum membrane.

Misfolded proteins of the secretory pathway are extracted from the endoplasmic reticulum (ER), polyubiquitylated by a protein complex termed the Hmg-CoA reductase degradation ligase (HRD ligase) and degraded by cytosolic 26S proteasomes. The movement of these proteins through the lipid bilayer is assumed to occur via a protein-conducting channel of unknown nature. We show that the integral membrane protein Der1 oligomerizes, which relies on its interaction with the scaffolding protein Usa1.

ERAD ubiquitin ligases: multifunctional tools for protein quality control and waste disposal in the endoplasmic reticulum.

In eukaryotic cells terminally misfolded proteins of the secretory pathway are retarded in the endoplasmic reticulum (ER) and subsequently degraded in a ubiquitin-proteasome-dependent manner. This highly conserved process termed ER-associated protein degradation (ERAD) ensures homeostasis in the secretory pathway by disposing faulty polypeptides and preventing their deleterious accumulation and eventual aggregation in the cell. The focus of this paper is the functional description of membrane-bound ubiquitin ligases, which are involved in all critical steps of ERAD.

Protein dislocation from the ER.

Protein folding within the endoplasmic reticulum (ER) of eukaryotic cells is erroneous and often results in the formation of terminally malfolded species. A quality control system retards such molecules in the ER and eventually initiates their dislocation into the cytosol for proteolysis by 26S proteasomes. This process is termed ER associated protein degradation (ERAD).

Usa1 functions as a scaffold of the HRD-ubiquitin ligase.

Protein quality control in the endoplasmic reticulum is of central importance for cellular homeostasis in eukaryotes. Crucial for this process is the HRD-ubiquitin ligase (HMG-CoA reductase degradation), which singles out terminally misfolded proteins and routes them for degradation to cytoplasmic 26S-proteasomes. Certain functions of this enzyme complex are allocated to defined subunits.

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.

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.

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.

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.

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.

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.

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.

Cardiovascular risk factors and atherosclerosis in young males: ARMY study (Atherosclerosis Risk-Factors in Male Youngsters).

Background: Necropsy studies suggest that atherosclerosis begins in childhood, but in vivo confirmation of this concept is sparse and limited to selected population samples. Furthermore, new risk concepts of atherosclerosis focusing on inflammation, infections, and immunity have not yet been evaluated in this age group.

Methods And Results: This study was conducted in a sample of 141 17- to 18-year-old white males homogenous in age and sex.

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.

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.

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.