Gerardo Z. Lederkremer

Glycoprotein folding, quality control and ER-associated degradation

Nascent N-linked glycoproteins possess a large oligosaccharide precursor, Glc(3)Man(9)GlcNAc(2), which is later sequentially trimmed. Recent studies help understand the code displayed by each structure produced by this trimming and its decoding by lectins. The calnexin folding cycle targets only monoglucosylated oligosaccharides. N-glycans of misfolded glycoproteins are then more extensively trimmed than once thought, being targeted for degradation by removal of three or four mannose residues. A high local concentration of endoplasmic reticulum (ER) mannosidase I in an ER-derived quality control compartment is mainly responsible for this trimming, with the possible participation of other mannosidases. The shortened chains, Man(5-6)GlcNAc(2), are recognized by the ubiquitination machinery-associated lectin OS9 but not by lectins that associate with properly folded glycoproteins en route to the Golgi that bind best to Man(8-9)GlcNAc(2).

Structure of the Sec23p/24p and Sec13p/31p complexes of COPII

COPII-coated vesicles carry proteins from the endoplasmic reticulum to the Golgi complex. This vesicular transport can be reconstituted by using three cytosolic components containing five proteins: the small GTPase Sar1p, the Sec23p/24p complex, and the Sec13p/Sec31p complex. We have used a combination of biochemistry and electron microscopy to investigate the molecular organization and structure of Sec23p/24p and Sec13p/31p complexes. The three-dimensional reconstruction of Sec23p/24p reveals that it has a bone-shaped structure, (17 nm in length), composed of two similar globular domains, one corresponding to Sec23p and the other to Sec24p. Sec13p/31p is a heterotetramer composed of two copies of Sec13p and two copies of Sec31p. It has an elongated shape, is 28–30 nm in length, and contains five consecutive globular domains linked by relatively flexible joints. Putting together the architecture of these Sec complexes with the interactions between their subunits and the appearance of the coat in COPII-coated vesicles, we present a model for COPII coat organization.

Endoplasmic reticulum-associated degradation of mammalian glycoproteins involves sugar chain trimming to Man6-5GlcNAc2.

Endoplasmic reticulum-associated degradation of misfolded or misprocessed glycoproteins in mammalian cells is prevented by inhibitors of class I α-mannosidases implicating mannose trimming from the precursor oligosaccharide Glc3Man9GlcNAc2 as an essential step in this pathway. However, the extent of mannose removal has not been determined. We show here that glycoproteins subject to endoplasmic reticulum-associated degradation undergo reglucosylation, deglucosylation, and mannose trimming to yield Man6GlcNAc2 and Man5GlcNAc2. These structures lack the mannose residue that is the acceptor of glucose transferred by UDP-Glc:glycoprotein glucosyltransferase. This could serve as a mechanism for removal of the glycoproteins from folding attempts catalyzed by cycles of reglucosylation and calnexin/calreticulin binding and result in targeting of these molecules for proteasomal degradation.

Palmitoylation is the switch that assigns calnexin to quality control or ER Ca2+ signaling

Summary The palmitoylation of calnexin serves to enrich calnexin on the mitochondria-associated membrane (MAM). Given a lack of information on the significance of this finding, we have investigated how this endoplasmic reticulum (ER)-internal sorting signal affects the functions of calnexin. Our results demonstrate that palmitoylated calnexin interacts with sarcoendoplasmic reticulum (SR) Ca2+ transport ATPase (SERCA) 2b and that this interaction determines ER Ca2+ content and the regulation of ER–mitochondria Ca2+ crosstalk. In contrast, non-palmitoylated calnexin interacts with the oxidoreductase ERp57 and performs its well-known function in quality control. Interestingly, our results also show that calnexin palmitoylation is an ER-stress-dependent mechanism. Following a short-term ER stress, calnexin quickly becomes less palmitoylated, which shifts its function from the regulation of Ca2+ signaling towards chaperoning and quality control of known substrates. These changes also correlate with a preferential distribution of calnexin to the MAM under resting conditions, or the rough ER and ER quality control compartment (ERQC) following ER stress. Our results have therefore identified the switch that assigns calnexin either to Ca2+ signaling or to protein chaperoning.

Glycan regulation of ER-associated degradation through compartmentalization.

The internal environment of the eukaryotic cell is divided by membranes into various organelles, containing diverse functional subcompartments, which allow complex cellular life. The quality control of newly made secretory proteins relies on the ability of the endoplasmic reticulum (ER) to segregate and compartmentalize molecules at different folding states. These folding states are communicated by N-glycans present on most secretory proteins. In ER-associated degradation (ERAD), protein molecules that have been identified as terminally misfolded are sent for degradation at the cytosolic proteasomes after being dislocated from the ER to the cytosol. This review will focus on how misfolded glycoprotein molecules are segregated from their properly folded counterparts and targeted to ERAD. The pathway involves compartmentalization, which is intimately linked to differential N-glycan processing. Recent data suggests that these processes are very dynamic, and include transient assembly of ERAD machinery complexes.

Protein aggregation and ER stress.

Protein aggregation is a common feature of the protein misfolding or conformational diseases, among them most of the neurodegenerative diseases. These disorders are a major scourge, with scarce if any effective therapies at present. Recent research has identified ER stress as a major mechanism implicated in cytotoxicity in these diseases. Whether amyloid-β or tau in Alzheimers, α-synuclein in Parkinsons, huntingtin in Huntingtons disease or other aggregation-prone proteins in many other neurodegenerative diseases, there is a shared pathway of oligomerization and aggregation into amyloid fibrils. There is increasing evidence in recent years that the toxic species, and those that evoke ER stress, are the intermediate oligomeric forms and not the final amyloid aggregates. This review focuses on recent findings on the mechanisms and importance of the development of ER stress upon protein aggregation, especially in neurodegenerative diseases, and possible therapeutic approaches that are being examined. This article is part of a Special Issue entitled SI:ER stress.

Mammalian ER mannosidase I resides in quality control vesicles, where it encounters its glycoprotein substrates

ER mannosidase I (ERManI) was found recently in the Golgi. This result is found to arise artificially from membrane disturbance in immunofluorescence methods. ERManI is located in novel vesicles to which substrates traffic and that converge at the ER-derived quality control compartment under ER stress.

Transient arrest in proteasomal degradation during inhibition of translation in the unfolded protein response

The UPR (unfolded protein response) activates transcription of genes involved in proteasomal degradation. However, we found that in its early stages the UPR leads to a transient inhibition of proteasomal disposal of cytosolic substrates (p53 and p27kip1) and of those targeted to ER (endoplasmic reticulum)-associated degradation (uncleaved precursor of asialoglycoprotein receptor H2a). Degradation resumed soon after the protein synthesis arrest that occurs in early UPR subsided. Consistent with this, protein synthesis inhibitors blocked ubiquitin/proteasomal degradation. Ubiquitination was inhibited during the translation block, suggesting short-lived E3 ubiquitin ligases as candidate depleted proteins. This was indeed the case for p53 whose E3 ligase, Mdm2 (murine double minute 2), when overexpressed, restored the degradation, whereas a mutant Mdm2 in its acidic domain restored the ubiquitination but did not completely restore the degradation. Inhibition of proteasomal degradation early in UPR may prevent depletion of essential short-lived factors during the translation arrest. Stabilization of p27 through this mechanism may explain the cell cycle arrest in G1 when translation is blocked by inhibitors or by the UPR.

Masking of an Endoplasmic Reticulum Retention Signal by Its Presence in the Two Subunits of the Asialoglycoprotein Receptor

Human asialoglycoprotein receptor H1 and H2b subunits assemble into a hetero-oligomer that travels to the cell surface. The H2a variant on the other hand is a precursor of a cleaved soluble form that is secreted. Uncleaved H2a precursor molecules cannot exit the endoplasmic reticulum (ER), a lumenal juxtamembrane pentapeptide being responsible for their retention. Insertion of this pentapeptide into H1 (H1i5) causes its complete ER retention but not fast degradation as happens to H2a. Cotransfection of H2a elicited, by heterodimerization, the Golgi processing of H1i5 and its surface expression. This occurred to a much lesser extent by cotransfection of H2b. Likewise, coexpression of H1i5 and not H1 stabilized H2a and caused its export to the cell surface. Homodimerization of molecules containing the pentapeptide did not cancel the retention. Thus, only when the pentapeptide is present in both subunits is the ER retention efficiently abrogated. The results show the unexpected finding that identical ER retention signals present in two associated chains can mask and cancel each others effect. This could have important implications as similar abrogation of ER retention of other proteins could eventually be obtained by engineering and coexpressing an associated protein containing the same retention signal.

The E3 Ubiquitin Ligases HRD1 and SCFFbs2 Recognize the Protein Moiety and Sugar Chains, Respectively, of an ER‐Associated Degradation Substrate

Several E3 ubiquitin ligases have been identified that participate in endoplasmic reticulum-associated degradation (ERAD) of misfolded membrane and secretory proteins. Sometimes two ligases were shown to share the same substrate. However, it is not known whether they recognize the same structural determinants and constitute alternative pathways for ubiquitination or if they can act sequentially on different determinants on the substrate. Here we have analyzed in cells in vivo interactions of the recently identified mammalian HRD1 and SCFFbs2 with a model ERAD substrate, uncleaved precursor of asialoglycoprotein receptor H2a, and their involvement in its degradation. As Fbs2 had been shown to bind sugar chains, we studied the influence of glycosylation of the substrate by analyzing the effect of dominant negative mutants of HRD1 and Fbs2 on ERAD of H2a and of a mutant lacking all three of its glycosylation sites. Both HRD1 and Fbs2 were found to associate with H2a, and their mutants inhibited its degradation, but only dominant negative HRD1 blocked the degradation of the unglycosylated version of H2a. This suggests a cooperative action of the two ligases that target the glycans (Fbs2) and a protein motif (HRD1). In addition, our results suggest the need for retrotranslocation of the luminal glycosylated domain of the substrate prior to interaction with cytosolic Fbs2.