Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

Understanding the biosynthesis of human IgM SAM-6 through a combinatorial expression of mutant subunits that affect product assembly and secretion

Polymeric IgMs are secreted from plasma cells abundantly despite their structural complexity and intricate multimerization steps. To gain insights into IgM's assembly mechanics that underwrite such high-level secretion, we characterized the biosynthetic process of a natural human IgM, SAM-6, using a heterologous HEK293(6E) cell platform that allowed the production of IgMs both in hexameric and pentameric forms in a controlled fashion. By creating a series of mutant subunits that differentially disrupt secretion, folding, and specific inter-chain disulfide bond formation, we assessed their effects on various aspects of IgM biosynthesis in 57 different subunit chain combinations, both in hexameric and pentameric formats. The mutations caused a spectrum of changes in steady-state subcellular subunit distribution, ER-associated inclusion body formation, intracellular subunit detergent solubility, covalent assembly, secreted IgM product quality, and secretion output. Some mutations produced differential effects on product quality depending on whether the mutation was introduced to hexameric IgM or pentameric IgM. Through this systematic combinatorial approach, we consolidate diverse overlapping knowledge on IgM biosynthesis for both hexamers and pentamers, while unexpectedly revealing that the loss of certain inter-chain disulfide bonds, including the one between μHC and λLC, is tolerated in polymeric IgM assembly and secretion. The findings highlight the differential roles of underlying non-covalent protein-protein interactions in hexamers and pentamers when orchestrating the initial subunit interactions and maintaining the polymeric IgM product integrity during ER quality control steps, secretory pathway trafficking, and secretion.

DR5 disulfide bonding as a sensor and effector of protein folding stress

New agents are needed that selectively kill cancer cells without harming normal tissues. The TRAIL ligand and its receptors, DR5 and DR4, exhibit cancer-selective toxicity, but TRAIL analogs or agonistic antibodies targeting these receptors have not received FDA approval for cancer therapy. Small molecules for activating DR5 or DR4 independently of protein ligands may bypass some of the pharmacological limitations of these protein drugs. Previously described Disulfide bond Disrupting Agents (DDAs) activate DR5 by altering its disulfide bonding through inhibition of the Protein Disulfide Isomerases (PDIs) ERp44, AGR2, and PDIA1. Work presented here extends these findings by showing that disruption of single DR5 disulfide bonds causes high-level DR5 expression, disulfide-mediated clustering, and activation of Caspase 8-Caspase 3 mediated pro-apoptotic signaling. Recognition of the extracellular domain of DR5 by various antibodies is strongly influenced by the pattern of DR5 disulfide bonding, which has important implications for the use of agonistic DR5 antibodies for cancer therapy. Disulfide-defective DR5 mutants do not activate the ER stress response or stimulate autophagy, indicating that these DDA-mediated responses are separable from DR5 activation and pro-apoptotic signaling. Importantly, other ER stressors, including Thapsigargin and Tunicamycin also alter DR5 disulfide bonding in various cancer cell lines and in some instances, DR5 mis-disulfide bonding is potentiated by overriding the Integrated Stress Response (ISR) with inhibitors of the PERK kinase or the ISR inhibitor ISRIB. These observations indicate that the pattern of DR5 disulfide bonding functions as a sensor of ER stress and serves as an effector of proteotoxic stress by driving extrinsic apoptosis independently of extracellular ligands.

Mammalian cargo receptors for endoplasmic reticulum-to-Golgi transport: mechanisms and interactions

Proteins that are destined to enter the secretory pathway are synthesized on the rough endoplasmic reticulum (ER) and then translocated into the ER lumen, where they undergo posttranslational modifications, folding, and assembly. After passing a quality control system, the cargo proteins are packaged into coat protein complex II (COPII) vesicles to exit the ER. In metazoans, most COPII subunits have multiple paralogs, enabling COPII vesicles the flexibility to transport a diverse range of cargo. The cytoplasmic domains of transmembrane proteins can interact with SEC24 subunits of COPII to enter the ER exit sites. Some transmembrane proteins may also act as cargo receptors that bind soluble secretory proteins within the ER lumen, enabling them to enter COPII vesicles. The cytoplasmic domains of cargo receptors also contain coat protein complex I binding motifs that allow for their cycling back to the ER after unloading their cargo in the ER-Golgi intermediate compartment and cis-Golgi. Once unloaded, the soluble cargo proteins continue maturation through the Golgi before reaching their final destinations. This review provides an overview of receptor-mediated transport of secretory proteins from the ER to the Golgi, with a focus on the current understanding of two mammalian cargo receptors: the LMAN1-MCFD2 complex and SURF4, and their roles in human health and disease.

Inhibitors of ERp44, PDIA1, and AGR2 induce disulfide-mediated oligomerization of Death Receptors 4 and 5 and cancer cell death

Breast cancer mortality remains unacceptably high, indicating a need for safer and more effective therapeutic agents. Disulfide bond Disrupting Agents (DDAs) were previously identified as a novel class of anticancer compounds that selectively kill cancers that overexpress the Epidermal Growth Factor Receptor (EGFR) or its family member HER2. DDAs kill EGFR+ and HER2+ cancer cells via the parallel downregulation of EGFR, HER2, and HER3 and activation/oligomerization of Death Receptors 4 and 5 (DR4/5). However, the mechanisms by which DDAs mediate these effects are unknown. Affinity purification analyses employing biotinylated-DDAs reveal that the Protein Disulfide Isomerase (PDI) family members AGR2, PDIA1, and ERp44 are DDA target proteins. Further analyses demonstrate that shRNA-mediated knockdown of AGR2 and ERp44, or expression of ERp44 mutants, enhance basal DR5 oligomerization. DDA treatment of breast cancer cells disrupts PDIA1 and ERp44 mixed disulfide bonds with their client proteins. Together, the results herein reveal DDAs as the first small molecule, active site inhibitors of AGR2 and ERp44, and demonstrate roles for AGR2 and ERp44 in regulating the activity, stability, and localization of DR4 and DR5, and activation of Caspase 8.

ERp44 is required for endocardial cushion development by regulating VEGFA secretion in myocardium

OBJECTIVES

Endocardial cushions are precursors of the valve septum complex that separates the four heart chambers. Several genes have been implicated in the development of endocardial cushions. Specifically, ERp44 has been found to play a role in the early secretory pathway, but its function in heart development has not been well studied.

MATERIALS AND METHODS

In this study, we established conditional and tissue-specific knockout mouse models. The morphology, survival rate, the development of heart and endocardial cushion were under evaluation. The relationship between ERp44 and VEGFA was investigated by transcriptome, qPCR, WB, immunofluorescence and immunohistochemistry.

RESULTS

ERp44 knockout (KO) mice were smaller in size, and most mice died during early postnatal life. KO hearts exhibited the typical phenotypes of congenital heart diseases, such as abnormal heart shapes and severe septal and valvular defects. Similar phenotypes were found in cTNT-Cre+/- ; ERp44fl / fl mice, which indicated that myocardial ERp44 principally controls endocardial cushion formation. Further studies demonstrated that the deletion of ERp44 significantly decreased the proliferation of cushion cells and impaired the endocardial-mesenchymal transition (EndMT), which was followed by endocardial cushion dysplasia. Finally, we found that ERp44 was directly bound to VEGFA and controlled its release, further regulating EndMT.

CONCLUSION

We demonstrated that ERp44 plays a specific role in heart development. ERp44 contributes to the development of the endocardial cushion by affecting VEGFA-mediated EndMT.

Biogenesis of secretory immunoglobulin M requires intermediate non-native disulfide bonds and engagement of the protein disulfide isomerase ERp44

Antibodies of the immunoglobulin M (IgM) class represent the frontline of humoral immune responses. They are secreted as planar polymers in which flanking µ2 L2 "monomeric" subunits are linked by two disulfide bonds, one formed by the penultimate cysteine (C575) in the tailpiece of secretory µ chains (µs tp) and the second by C414 in the Cµ3. The latter bond is not present in membrane IgM. Here, we show that C575 forms a non-native, intra-subunit disulfide bond as a key step in the biogenesis of secretory IgM. The abundance of this unexpected intermediate correlates with the onset and extent of polymerization. The rearrangement of the C-terminal tails into a native quaternary structure is guaranteed by the engagement of protein disulfide isomerase ERp44, which attacks the non-native C575 bonds. The resulting conformational changes promote polymerization and formation of C414 disulfide linkages. This unusual assembly pathway allows secretory polymers to form without the risk of disturbing the role of membrane IgM as part of the B cell antigen receptor.

Emp47 and Vip36 are required for polarized growth and protein trafficking between ER and Golgi apparatus in opportunistic fungal pathogen Aspergillus fumigatus

In Aspergillus fumigatus, an opportunistic fungal pathogen causing fatal invasive aspergillosis, N-glycosylation is vital for polarized growth. To investigate its mechanism, two putative L-type lectin genes emp47 (AFUB_032470) and vip36 (AFUB_027870) were identified in A. fumigatus. Deletion of the emp47 or vip36 gene resulted in delayed germination and abnormal polarity. Also, the Δemp47 displayed an increased resistance to azoles whereas the Δvip36 showed an increased susceptibility to amphotericin B. Secretome analysis revealed that 205 proteins were differentially secreted in the Δemp47 and 145 of them were reduced, while 153 proteins displayed a differential secretion and 134 of them were increased in the Δvip36 as compared with that of the wild-type strain. Also, potential cargo glycoproteins of Emp47 and Vip36 were identified by comparative secretome analysis. Our results suggest that Emp47 is responsible for the transport of proteins from endoplasmic reticulum (ER) to Golgi, while Vip36 acts in protein retrieval from Golgi to ER.

A virtuous cycle operated by ERp44 and ERGIC-53 guarantees proteostasis in the early secretory compartment

The composition of the secretome depends on the combined action of cargo receptors that facilitate protein transport and sequential checkpoints that restrict it to native conformers. Acting after endoplasmic reticulum (ER)-resident chaperones, ERp44 retrieves its clients from downstream compartments. To guarantee efficient quality control, ERp44 should exit the ER as rapidly as its clients, or more. Here, we show that appending ERp44 to different cargo proteins increases their secretion rates. ERp44 binds the cargo receptor ER-Golgi intermediate compartment (ERGIC)-53 in the ER to negotiate preferential loading into COPII vesicles. Silencing ERGIC-53, or competing for its COPII binding with 4-phenylbutyrate, causes secretion of Prdx4, an enzyme that relies on ERp44 for intracellular localization. In more acidic, zinc-rich downstream compartments, ERGIC-53 releases its clients and ERp44, which can bind and retrieve non-native conformers via KDEL receptors. By coupling the transport of cargoes and inspector proteins, cells ensure efficiency and fidelity of secretion.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

The pivotal role of ERp44 in patrolling protein secretion

Interactions between protein ligands and receptors are the main language of intercellular communication; hence, how cells select proteins to be secreted or presented on the plasma membrane is a central concern in cell biology. A series of checkpoints are located along the secretory pathway, which ensure the fidelity of such protein signals (quality control). Proteins that pass the checkpoints operated in the endoplasmic reticulum (ER) by the binding immunoglobulin protein (BiP; also known as HSPA5 and GRP78) and the calnexin-calreticulin systems, must still overcome additional scrutiny in the ER-Golgi intermediate compartment (ERGIC) and the Golgi. One of the main players of this process in all metazoans is the ER-resident protein 44 (ERp44); by cycling between the ER and the Golgi, ERp44 controls the localization of key enzymes designed to act in the ER but that are devoid of suitable localization motifs. ERp44 also patrols the secretion of correctly assembled disulfide-linked oligomeric proteins. Here, we discuss the mechanisms driving ERp44 substrate recognition, with important consequences on the definition of 'thiol-mediated quality control'. We also describe how pH and zinc gradients regulate the functional cycle of ERp44, coupling quality control and membrane trafficking along the early secretory compartment.

Oxidoreductases in Glycoprotein Glycosylation, Folding, and ERAD

N-linked glycosylation and sugar chain processing, as well as disulfide bond formation, are among the most common post-translational protein modifications taking place in the endoplasmic reticulum (ER). They are essential modifications that are required for membrane and secretory proteins to achieve their correct folding and native structure. Several oxidoreductases responsible for disulfide bond formation, isomerization, and reduction have been shown to form stable, functional complexes with enzymes and chaperones that are involved in the initial addition of an N-glycan and in folding and quality control of the glycoproteins. Some of these oxidoreductases are selenoproteins. Recent studies also implicate glycan machinery–oxidoreductase complexes in the recognition and processing of misfolded glycoproteins and their reduction and targeting to ER-associated degradation. This review focuses on the intriguing cooperation between the glycoprotein-specific cell machineries and ER oxidoreductases, and highlights open questions regarding the functions of many members of this large family.

A CLN6-CLN8 complex recruits lysosomal enzymes at the ER for Golgi transfer

Lysosomal enzymes are synthesized in the endoplasmic reticulum (ER) and transferred to the Golgi complex by interaction with the Batten disease protein CLN8 (ceroid lipofuscinosis, neuronal, 8). Here we investigated the relationship of this pathway with CLN6, an ER-associated protein of unknown function that is defective in a different Batten disease subtype. Experiments focused on protein interaction and trafficking identified CLN6 as an obligate component of a CLN6-CLN8 complex (herein referred to as EGRESS: ER-to-Golgi relaying of enzymes of the lysosomal system), which recruits lysosomal enzymes at the ER to promote their Golgi transfer. Mutagenesis experiments showed that the second luminal loop of CLN6 is required for the interaction of CLN6 with the enzymes but dispensable for interaction with CLN8. In vitro and in vivo studies showed that CLN6 deficiency results in inefficient ER export of lysosomal enzymes and diminished levels of the enzymes at the lysosome. Mice lacking both CLN6 and CLN8 did not display aggravated pathology compared with the single deficiencies, indicating that the EGRESS complex works as a functional unit. These results identify CLN6 and the EGRESS complex as key players in lysosome biogenesis and shed light on the molecular etiology of Batten disease caused by defects in CLN6.

Efficient N-Glycosylation of the Heavy Chain Tailpiece Promotes the Formation of Plant-Produced Dimeric IgA

Production of monomeric IgA in mammalian cells and plant expression systems such as Nicotiana benthamiana is well-established and can be achieved by co-expression of the corresponding light and heavy chains. In contrast, the assembly of dimeric IgA requires the additional expression of the joining chain and remains challenging especially in plant-based systems. Here, we examined factors affecting the assembly and expression of HER2 binding dimeric IgA1 and IgA2m(2) variants transiently produced in N. benthamiana. While co-expression of the joining chain resulted in efficient formation of dimeric IgAs in HEK293F cells, a mixture of monomeric, dimeric and tetrameric variants was detected in plants. Mass-spectrometric analysis of site-specific glycosylation revealed that the N-glycan profile differed between monomeric and dimeric IgAs in the plant expression system. Co-expression of a single-subunit oligosaccharyltransferase from the protozoan Leishmania major in N. benthamiana increased the N-glycosylation occupancy at the C-terminal heavy chain tailpiece and changed the ratio of monomeric to dimeric IgAs. Our data demonstrate that N-glycosylation engineering is a suitable strategy to promote the formation of dimeric IgA variants in plants.

Intermediate compartment (IC): from pre-Golgi vacuoles to a semi-autonomous membrane system

Despite its discovery more than three decades ago and well-established role in protein sorting and trafficking in the early secretory pathway, the intermediate compartment (IC) has remained enigmatic. The prevailing view is that the IC evolved as a specialized organelle to mediate long-distance endoplasmic reticulum (ER)–Golgi communication in metazoan cells, but is lacking in other eukaryotes, such as plants and fungi. However, this distinction is difficult to reconcile with the high conservation of the core machineries that regulate early secretory trafficking from yeast to man. Also, it has remained unclear whether the pleiomorphic IC components—vacuoles, tubules and vesicles—represent transient transport carriers or building blocks of a permanent pre-Golgi organelle. Interestingly, recent studies have revealed that the IC maintains its compositional, structural and spatial properties throughout the cell cycle, supporting a model that combines the dynamic and stable aspects of the organelle. Moreover, the IC has been assigned novel functions, such as cell signaling, Golgi-independent trafficking and autophagy. The emerging permanent nature of the IC and its connections with the centrosome and the endocytic recycling system encourage reconsideration of its relationship with the Golgi ribbon, role in Golgi biogenesis and ubiquitous presence in eukaryotic cells.

ERP44 inhibits human lung cancer cell migration mainly via IP3R2

Cancer cell migration is involved in tumour metastasis. However, the relationship between calcium signalling and cancer migration is not well elucidated. In this study, we used the human lung adenocarcinoma A549 cell line to examine the role of endoplasmic reticulum protein 44 (ERP44), which has been reported to regulate calcium release inside of the endoplasmic reticulum (ER), in cell migration. We found that the inositol 1,4,5-trisphosphate receptors (IP3Rs/ITPRs) inhibitor 2-APB significantly inhibited A549 cell migration by inhibiting cell polarization and pseudopodium protrusion, which suggests that Ca2+ is necessary for A549 cell migration. Similarly, the overexpression of ERP44 reduced intracellular Ca2+ release via IP3Rs, altered cell morphology and significantly inhibited the migration of A549 cells. These phenomena were primarily dependent on IP3R2 because wound healing in A549 cells with IP3R2 rather than IP3R1 or IP3R3 siRNA was markedly inhibited. Moreover, the overexpression of ERP44 did not affect the migration of the human neuroblastoma cell line SH-SY5Y, which mainly expresses IP3R1. Based on the above observations, we conclude that ERP44 regulates A549 cell migration mainly via an IP3R2-dependent pathway.

Synthetic peptides designed to modulate adiponectin assembly improve obesity-related metabolic disorders

BACKGROUND AND PURPOSE

Adiponectin, an adipokine possessing profound insulin-sensitizing and anti-inflammatory properties, is a potent biotherapeutic agent . The trimeric adiponectin subunit assembles into hexameric and functionally important higher molecular weight (HMW) forms, controlled by the endoplasmic reticulum protein 44 (ERp44). Obesity-induced ER stress decreases the HMW form in serum, contributing to the development of insulin resistance and Type 2 diabetes. In this study, a panel of synthetic peptides, designed to target ERp44-adiponectin interactions, were tested for their effects on circulating levels of HMW adiponectin.

EXPERIMENTAL APPROACH

Peptides derived from the ERp44 binding region of adiponectin and immunoglobulin IgM were synthesized with or without a cell-penetrating sequence. Cultures of 3T3-L1 adipocytes were incubated with the peptides for assessing the assembly and secretion of HMW adiponectin. Mice given standard chow or a high-fat diet were treated acutely or chronically, with the peptides to investigate the therapeutic effects on insulin sensitivity and energy metabolism.

RESULTS

The designed peptides interfered with ERp44-adiponectin interactions and modulated adiponectin assembly and release from adipocytes. In particular, IgM-derived peptides facilitated the release of endogenous adiponectin (especially the HMW form) from adipose tissue, enhanced its circulating level and the ratio of HMW-to-total-adiponectin in obese mice. Long-term treatment of mice fed with high-fat diet by IgM-derived peptides reduced the circulating lipid levels and improved insulin sensitivity.

CONCLUSIONS AND IMPLICATIONS

Targeting ERp44-adiponectin interactions with short peptides represents an effective strategy to treat of obesity-related metabolic disorders, such as insulin resistance and Type 2 diabetes.

A peptide extension dictates IgM assembly

Professional secretory cells can produce large amounts of high-quality complex molecules, including IgM antibodies. Owing to their multivalency, polymeric IgM antibodies provide an efficient first-line of defense against pathogens. To decipher the mechanisms of IgM assembly, we investigated its biosynthesis in living cells and faithfully reconstituted the underlying processes in vitro. We find that a conserved peptide extension at the C-terminal end of the IgM heavy (Ig-μ) chains, termed the tailpiece, is necessary and sufficient to establish the correct geometry. Alanine scanning revealed that hydrophobic amino acids in the first half of the tailpiece contain essential information for generating the correct topology. Assembly is triggered by the formation of a disulfide bond linking two tailpieces. This induces conformational changes in the tailpiece and the adjacent domain, which drive further polymerization. Thus, the biogenesis of large and topologically challenging IgM complexes is dictated by a local conformational switch in a peptide extension.

Two novel effectors of trafficking and maturation of the yeast plasma membrane H+ -ATPase

The endoplasmic reticulum (ER) is the entry site of proteins into the endomembrane system. Proteins exit the ER via coat protein II (COPII) vesicles in a selective manner, mediated either by direct interaction with the COPII coat or aided by cargo receptors. Despite the fundamental role of such receptors in protein sorting, only a few have been identified. To further define the machinery that packages secretory cargo and targets proteins from the ER to Golgi membranes, we used multiple systematic approaches, which revealed 2 uncharacterized proteins that mediate the trafficking and maturation of Pma1, the essential yeast plasma membrane proton ATPase. Ydl121c (Exp1) is an ER protein that binds Pma1, is packaged into COPII vesicles, and whose deletion causes ER retention of Pma1. Ykl077w (Psg1) physically interacts with Exp1 and can be found in the Golgi and coat protein I (COPI) vesicles but does not directly bind Pma1. Loss of Psg1 causes enhanced degradation of Pma1 in the vacuole. Our findings suggest that Exp1 is a Pma1 cargo receptor and that Psg1 aids Pma1 maturation in the Golgi or affects its retrieval. More generally our work shows the utility of high content screens in the identification of novel trafficking components.

A Non-Classical Member of the Protein Disulfide Isomerase Family, PDI7 of Arabidopsis thaliana, Localizes to the cis-Golgi and Endoplasmic Reticulum Membranes

Members of the protein disulfide isomerase (PDI)-C subfamily are chimeric proteins containing the thioredoxin (Trx) domain of PDIs, and the conserved N- and C-terminal Pfam domains of Erv41p/Erv46p-type cargo receptors. They are unique to plants and chromalveolates. The Arabidopsis genome encodes three PDI-C isoforms: PDI7, PDI12 and PDI13. Here we demonstrate that PDI7 is a 65 kDa integral membrane glycoprotein expressed throughout many Arabidopsis tissues. Using a PDI7-specific antibody, we show through immunoelectron microscopy that PDI7 localizes to the endoplasmic reticulum (ER) and Golgi membranes in wild-type root tip cells, and was also detected in vesicles. Tomographic modeling of the Golgi revealed that PDI7 was confined to the cis-Golgi, and accumulated primarily at the cis-most cisterna. Shoot apical meristem cells from transgenic plants overexpressing PDI7 exhibited a dramatic increase in anti-PDI7 labeling at the cis-Golgi. When N- or C-terminal fusions between PDI7 and the green fluorescent protein variant, GFP(S65T), were expressed in mesophyll protoplasts, the fusions co-localized with the ER marker, ER-mCherry. However, when GFP(S65T) was positioned internally within PDI7 (PDI7-GFPint), the fusion strongly co-localized with the cis-Golgi marker, mCherry-SYP31, and faintly labeled the ER. In contrast to the Golgi-resident fusion protein (Man49-mCherry), PDI7-GFPint did not redistribute to the ER after brefeldin A treatment. Protease protection experiments indicated that the Trx domain of PDI7 is located within the ER/Golgi lumen. We propose a model where PDI-C isoforms function as cargo receptors for proteins containing exposed cysteine residues, cycling them from the Golgi back to the ER.

Roles of N-glycans in the polymerization-dependent aggregation of mutant Ig-μ chains in the early secretory pathway

The polymeric structure of secretory IgM allows efficient antigen binding and complement fixation. The available structural models place the N-glycans bound to asparagines 402 and 563 of Ig-μ chains within a densely packed core of native IgM. These glycans are found in the high mannose state also in secreted IgM, suggesting that polymerization hinders them to Golgi processing enzymes. Their absence alters polymerization. Here we investigate their role following the fate of aggregation-prone mutant μ chains lacking the Cμ1 domain (μ∆). Our data reveal that μ∆ lacking 563 glycans (μ∆5) form larger intracellular aggregates than μ∆ and are not secreted. Like μ∆, they sequester ERGIC-53, a lectin previously shown to promote polymerization. In contrast, μ∆ lacking 402 glycans (μ∆4) remain detergent soluble and accumulate in the ER, as does a double mutant devoid of both (μ∆4–5). These results suggest that the two C-terminal Ig-μ glycans shape the polymerization-dependent aggregation by engaging lectins and acting as spacers in the alignment of individual IgM subunits in native polymers.

The T cell IFT20 interactome reveals new players in immune synapse assembly

Sustained signalling at the immune synapse (IS) requires the synaptic delivery of recycling endosome-associated T cell antigen receptors (TCRs). IFT20, a component of the intraflagellar transport system, controls TCR recycling to the IS as a complex with IFT57 and IFT88. Here, we used quantitative mass spectrometry to identify additional interaction partners of IFT20 in Jurkat T cells. In addition to IFT57 and IFT88, the analysis revealed new binding partners, including IFT54 (also known as TRAF3IP1), GMAP-210 (also known as TRIP11), Arp2/3 complex subunit-3 (ARPC3), COP9 signalosome subunit-1 (CSN1, also known as GPS1) and ERGIC-53 (also known as LMAN1). A direct interaction between IFT20 and both IFT54 and GMAP-210 was confirmed in pulldown assays. Confocal imaging of antigen-specific conjugates using T cells depleted of these proteins by RNA interference showed that TCR accumulation and phosphotyrosine signalling at the IS were impaired in the absence of IFT54, ARPC3 or ERGIC-53. Similar to in IFT20-deficient T cells, this defect resulted from a reduced ability of endosomal TCRs to polarize to the IS despite a correct translocation of the centrosome towards the antigen-presenting cell contact. Our data underscore the traffic-related role of an IFT20 complex that includes components of the intracellular trafficking machinery in IS assembly.

Protein sorting at the ER-Golgi interface

In this review, Gomez-Navarro and Miller summarize the principles of cargo sorting by the vesicle traffic machinery and consider the diverse mechanisms by which cargo proteins are selected and captured into different transport vesicles.

Protein traffic is of critical importance for normal cellular physiology. In eukaryotes, spherical transport vesicles move proteins and lipids from one internal membrane-bound compartment to another within the secretory pathway. The process of directing each individual protein to a specific destination (known as protein sorting) is a crucial event that is intrinsically linked to vesicle biogenesis. In this review, we summarize the principles of cargo sorting by the vesicle traffic machinery and consider the diverse mechanisms by which cargo proteins are selected and captured into different transport vesicles. We focus on the first two compartments of the secretory pathway: the endoplasmic reticulum and Golgi. We provide an overview of the complexity and diversity of cargo adaptor function and regulation, focusing on recent mechanistic discoveries that have revealed insight into protein sorting in cells.

Cargo Capture and Bulk Flow in the Early Secretory Pathway

Transport of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi complex is highly selective. As a general rule, such transport is limited to soluble and membrane-associated secretory proteins that have reached properly folded and assembled conformations. To secure the efficiency, fidelity, and control of this crucial transport step, cells use a combination of mechanisms. The mechanisms are based on selective retention of proteins in the ER to prevent uptake into transport vesicles, on selective capture of proteins in COPII carrier vesicles, on inclusion of proteins in these vesicles by default as part of fluid and membrane bulk flow, and on selective retrieval of proteins from post-ER compartments by retrograde vesicle transport.

Knockdown of ERp44 leads to apoptosis via activation of ER stress in HeLa cells

ERp44, an endoplasmic reticulum (ER) resident protein, regulates intracellular Ca(2+) release and involves in the maturation of many proteins in mammalian cells. In this study, we investigated the effects and mechanism of ERp44 on cell apoptosis by using ERp44 knockdown stable HeLa cell lines. We found that ERp44 knockdown resulted in increases in cell apoptosis rate more than one fold higher than that of control; using serum starvation, caspase-3 protein level was significantly up-regulated in ERp44 knockdown cells compared to the control cells. Furthermore, we demonstrated that in response to serum starvation, the protein levels of CHOP and GRP78 were also largely raised in ERp44 knockdown cells. Moreover, caspase-12 was activated, which suggested cell apoptosis was induced by ER stress. Taken together, our results indicate that knockdown of ERp44 leads to cell apoptosis through the activation of ER stress.

Proteostasis and "redoxtasis" in the secretory pathway: Tales of tails from ERp44 and immunoglobulins

In multicellular organisms, some cells are given the task of secreting huge quantities of proteins. To comply with their duty, they generally equip themselves with a highly developed endoplasmic reticulum (ER) and downstream organelles in the secretory pathway. These professional secretors face paramount proteostatic challenges in that they need to couple efficiency and fidelity in their secretory processes. On one hand, stringent quality control (QC) mechanisms operate from the ER onward to check the integrity of the secretome. On the other, the pressure to secrete can be overwhelming, as for instance on antibody-producing cells during infection. Maintaining homeostasis is particularly hard when the products to be released contain disulfide bonds, because oxidative folding entails production of reactive oxygen species. How are redox homeostasis ("redoxtasis") and proteostasis maintained despite the massive fluxes of cargo proteins traversing the pathway? Here we describe recent findings on how ERp44, a multifunctional chaperone of the secretory pathway, can modulate these processes integrating protein QC, redoxtasis, and calcium signaling.

The back and forth of cargo exit from the endoplasmic reticulum

Endoplasmic reticulum (ER) to Golgi trafficking is an essential step in sorting mature, correctly folded, processed and assembled proteins (cargo) from immature proteins and ER-resident proteins. However, the mechanisms governing trafficking selectivity, specificity and regulation are not yet fully understood. To date, three complementary mechanisms have been described that enable regulation of this trafficking step: ER retention of immature proteins in the ER; selective uptake of fully mature proteins into Golgi-bound vesicles; and retrieval from the Golgi of immature cargo that has erroneously exited the ER. Together, these three mechanisms allow incredible specificity and enable the cell to carry out protein quality control and regulate protein processing, oligomerization and expression. This review will focus on the current knowledge of selectivity mechanisms acting during the ER-to-Golgi sorting step and their significance in health and disease. The review will also highlight several key questions that have remained unanswered and discuss the future frontiers.

Progressive quality control of secretory proteins in the early secretory compartment by ERp44

ERp44 is a pH-regulated chaperone of the secretory pathway. In the acidic milieu of the Golgi, its C-terminal tail changes conformation, simultaneously exposing the substrate-binding site for cargo capture and the RDEL motif for ER retrieval through interactions with cognate receptors. Protonation of cysteine 29 in the active site allows tail movements in vitro and in vivo. Here, we show that conserved histidine residues in the C-terminal tail also regulate ERp44 in vivo. Mutants lacking these histidine residues retain substrates more efficiently. Surprisingly, they are also O-glycosylated and partially secreted. Co-expression of client proteins prevents secretion of the histidine mutants, forcing tail opening and RDEL accessibility. Client-induced RDEL exposure allows retrieval of proteins from distinct stations along the secretory pathway, as indicated by the changes in O-glycosylation patterns upon overexpression of different partners. The ensuing gradients might help to optimize folding and assembly of different cargoes. Endogenous ERp44 is O-glycosylated and secreted by human primary endometrial cells, suggesting possible pathophysiological roles of these processes.

Identification of co-expressed gene signatures in mouse B1, marginal zone and B2 B-cell populations

In mice, three major B-cell subsets have been identified with distinct functionalities: B1 B cells, marginal zone B cells and follicular B2 B cells. Here, we used the growing body of publicly available transcriptomics data to create an expression atlas of 84 gene expression microarray data sets of distinct mouse B-cell subsets. These data were subjected to network-based cluster analysis using BioLayout Express(3D). Using this analysis tool, genes with related functions clustered together in discrete regions of the network graph and enabled the identification of transcriptional networks that underpinned the functional activity of distinct cell populations. Some gene clusters were expressed highly by most of the cell populations included in this analysis (such as those with activity related to house-keeping functions). Others contained genes with expression patterns specific to distinct B-cell subsets. While these clusters contained many genes typically associated with the activity of the cells they were specifically expressed in, many novel B-cell-subset-specific candidate genes were identified. A large number of uncharacterized genes were also represented in these B-cell lineage-specific clusters. Further analysis of the activities of these uncharacterized candidate genes will lead to the identification of novel B-cell lineage-specific transcription factors and regulators of B-cell function. We also analysed 36 microarray data sets from distinct human B-cell populations. These data showed that mouse and human germinal centre B cells shared similar transcriptional features, whereas mouse B1 B cells were distinct from proposed human B1 B cells.

Missing links in antibody assembly control

Fidelity of the humoral immune response requires that quiescent B lymphocytes display membrane bound immunoglobulin M (IgM) on B lymphocytes surface as part of the B cell receptor, whose function is to recognize an antigen. At the same time B lymphocytes should not secrete IgM until recognition of the antigen has occurred. The heavy chains of the secretory IgM have a C-terminal tail with a cysteine instead of a membrane anchor, which serves to covalently link the IgM subunits by disulfide bonds to form “pentamers” or “hexamers.” By virtue of the same cysteine, unassembled secretory IgM subunits are recognized and retained (via mixed disulfide bonds) by members of the protein disulfide isomerase family, in particular ERp44. This so-called “thiol-mediated retention” bars assembly intermediates from prematurely leaving the cell and thereby exerts quality control on the humoral immune response. In this essay we discuss recent findings on how ERp44 governs such assembly control in a pH-dependent manner, shuttling between the cisGolgi and endoplasmic reticulum, and finally on how pERp1/MZB1, possibly as a co-chaperone of GRP94, may help to overrule the thiol-mediated retention in the activated B cell to give way to antibody secretion.

B cell activating factor inhibition impairs bacterial immunity by reducing T cell-independent IgM secretion

B cell activating factor of the tumor necrosis factor family (BAFF) is an essential survival factor for B cells and has been shown to regulate T cell-independent (TI) IgM production. During Ehrlichia muris infection, TI IgM secretion in the spleen was BAFF dependent, and antibody-mediated BAFF neutralization led to an impairment of IgM-mediated host defense. The failure of TI plasmablasts to secrete IgM was not a consequence of alterations in their generation, survival, or early differentiation, since all occurred normally in infected mice following BAFF neutralization. Gene expression characteristic of plasma cell differentiation was also unaffected by BAFF neutralization in vivo, and except for CD138, plasmablast cell surface marker expression was unaffected. IgM was produced, since it was detected intracellularly, and impaired secretion was not due to a failure to express the IgM secretory exon. Addition of BAFF to plasmablasts in vitro rescued IgM secretion, suggesting that BAFF signaling can directly regulate secretory processes. Our findings indicate that BAFF signaling can modulate TI host defense by acting at a late stage in B cell differentiation, via its regulation of terminal plasmablast differentiation and/or IgM secretion.

High-resolution structures of the IgM Fc domains reveal principles of its hexamer formation

IgM is the first antibody produced during the humoral immune response. Despite its fundamental role in the immune system, IgM is structurally only poorly described. In this work we used X-ray crystallography and NMR spectroscopy to determine the atomic structures of the constant IgM Fc domains (Cµ2, Cµ3, and Cµ4) and to address their roles in IgM oligomerization. Although the isolated domains share the typical Ig fold, they differ substantially in dimerization properties and quaternary contacts. Unexpectedly, the Cµ4 domain and its C-terminal tail piece are responsible and sufficient for the specific polymerization of Cµ4 dimers into covalently linked hexamers of dimers. Based on small angle X-ray scattering data, we present a model of the ring-shaped Cµ4 structure, which reveals the principles of IgM oligomerization.

Cell biology of the endoplasmic reticulum and the Golgi apparatus through proteomics

Enriched endoplasmic reticulum (ER) and Golgi membranes subjected to mass spectrometry have uncovered over a thousand different proteins assigned to the ER and Golgi apparatus of rat liver. This, in turn, led to the uncovering of several hundred proteins of poorly understood function and, through hierarchical clustering, showed that proteins distributed in patterns suggestive of microdomains in cognate organelles. This has led to new insights with respect to their intracellular localization and function. Another outcome has been the critical testing of the cisternal maturation hypothesis showing overwhelming support for a predominant role of COPI vesicles in the transport of resident proteins of the ER and Golgi apparatus (as opposed to biosynthetic cargo). Here we will discuss new insights gained and also highlight new avenues undertaken to further explore the cell biology of the ER and the Golgi apparatus through tandem mass spectrometry.

Production of H₂O₂ in the endoplasmic reticulum promotes in vivo disulfide bond formation

AIMS

Oxidative protein folding in the luminal compartment of endoplasmic reticulum (ER) is thought to be accompanied by the generation of H₂O₂, as side-product of disulfide bond formation. We aimed to examine the role of H₂O₂ produced in the lumen, which on one hand can lead to redox imbalance and hence can contribute to ER stress caused by overproduction of secretory proteins; on the other hand, as an excellent electron acceptor, H₂O₂ might serve as an additional pro-oxidant in physiological oxidative folding.

RESULTS

Stimulation of H₂O₂ production in the hepatic ER resulted in a decrease in microsomal GSH and protein-thiol contents and in a redox shift of certain luminal oxidoreductases in mice. The oxidative effect, accompanied by moderate signs of ER stress and reversible dilation of ER cisternae, was prevented by concomitant reducing treatment. The imbalance also affected the redox state of pyridine nucleotides in the ER. Antibody producing cells artificially engineered with powerful luminal H₂O₂ eliminating system showed diminished secretion of mature antibody polymers, while incomplete antibody monomers/dimers were accumulated and/or secreted.

INNOVATION

Evidence are provided by using in vivo models that hydrogen peroxide can promote disulfide bond formation in the ER.

CONCLUSION

The results indicate that local H₂O₂ production promotes, while quenching of H₂O₂ impairs disulfide formation. The contribution of H₂O₂ to disulfide bond formation previously observed in vitro can be also shown in cellular and in vivo systems.

Ero1α regulates Ca(2+) fluxes at the endoplasmic reticulum-mitochondria interface (MAM)

AIMS

The endoplasmic reticulum (ER) is involved in many functions, including protein folding, redox homeostasis, and Ca(2+) storage and signaling. To perform these multiple tasks, the ER is composed of distinct, specialized subregions, amongst which mitochondrial-associated ER membranes (MAM) emerge as key signaling hubs. How these multiple functions are integrated with one another in living cells remains unclear.

RESULTS

Here we show that Ero1α, a key controller of oxidative folding and ER redox homeostasis, is enriched in MAM and regulates Ca(2+) fluxes. Downregulation of Ero1α by RNA interference inhibits mitochondrial Ca(2+) fluxes and modifies the activity of mitochondrial Ca(2+) uniporters. The overexpression of redox active Ero1α increases passive Ca(2+) efflux from the ER, lowering [Ca(2+)](ER) and mitochondrial Ca(2+) fluxes in response to IP3 agonists.

INNOVATION

The unexpected observation that Ca(2+) fluxes are affected by either increasing or decreasing the levels of Ero1α reveals a pivotal role for this oxidase in the early secretory compartment and implies a strict control of its amounts.

CONCLUSIONS

Taken together, our results indicate that the levels, subcellular localization, and activity of Ero1α coordinately regulate Ca(2+) and redox homeostasis and signaling in the early secretory compartment.

Redox-dependent protein quality control in the endoplasmic reticulum: folding to degradation

SIGNIFICANCE

Nascent polypeptides entering the endoplasmic reticulum (ER) are co- and post-translationally modified by N-glycosylation and the oxidation/isomerization of cysteine residues followed by folding with the aid of molecular chaperones. Only properly folded proteins reach their final destination. The oxidative environment in the ER enables ER-resident oxidoreductases to facilitate disulfide bond formation, which stabilizes protein structures. ER oxidoreductases involve in both the productive folding of newly synthesized proteins and ER-associated degradation (ERAD) of misfolded proteins.

RECENT ADVANCES

The ER luminal event of ERAD is composed of three major steps: the recognition and segregation of terminally misfolded proteins from folding intermediates, unfolding of misfolded substrates by oxidoreductases that cleave the disulfide bonds to enable the translocation of the substrates through the retrotranslocation channel, and transport of substrates to be degraded to the dislocon channel. The factors required for these three critical steps have been found to form a supramolecular complex in the ER.

CRITICAL ISSUES

This complex comprises EDEM1, a lectin-like molecule that recognizes mannose-trimming and segregates the identified substrates from the productive folding pathway into the degradation pathway; ER DnaJ (ERdj)5, a reductase that resides in the ER and reduces disulfides in misfolded proteins; and immunoglobulin heavy chain binding protein (BiP), an heat shock protein (Hsp)70 family molecular chaperone that recruits substrates to the dislocon channel after dissociation from the EDEM1/ERdj5 complex coupled with ATP hydrolysis.

FUTURE DIRECTIONS

The importance of disulfide bond reduction in misfolded proteins for retrotranslocation through the dislocon channel will be discussed by comparing the function of ERdj5 with that of other oxidoreductases in the ER.

Molecular and structural basis for N-glycan-dependent determination of glycoprotein fates in cells

BACKGROUND

N-linked oligosaccharides operate as tags for protein quality control, consigning glycoproteins to different fates, i.e. folding in the endoplasmic reticulum (ER), vesicular transport between the ER and the Golgi complex, and ER-associated degradation of glycoproteins, by interacting with a panel of intracellular lectins in the early secretory pathway.

SCOPE OF REVIEW

This review summarizes the current state of knowledge regarding the molecular and structural basis for glycoprotein-fate determination in cells that is achieved through the actions of the intracellular lectins and its partner proteins.

MAJOR CONCLUSIONS

Cumulative frontal affinity chromatography (FAC) data demonstrated that the intracellular lectins exhibit distinct sugar-binding specificity profiles. The glycotopes recognized by these lectins as fate determinants are embedded in the triantennary structures of the high-mannose-type oligosaccharides and are exposed upon trimming of the outer glucose and mannose residues during the N-glycan processing pathway. Furthermore, recently emerged 3D structural data offer mechanistic insights into functional interplay between an intracellular lectin and its binding partner in the early secretory pathway.

GENERAL SIGNIFICANCE

Structural biology approaches in conjunction with FAC methods provide atomic pictures of the mechanisms behind the glycoprotein-fate determination in cells. This article is a part of a Special issue entitled: Glycoproteomics.

Protein folding and quality control in the ER

The endoplasmic reticulum (ER) uses an elaborate surveillance system called the ER quality control (ERQC) system. The ERQC facilitates folding and modification of secretory and membrane proteins and eliminates terminally misfolded polypeptides through ER-associated degradation (ERAD) or autophagic degradation. This mechanism of ER protein surveillance is closely linked to redox and calcium homeostasis in the ER, whose balance is presumed to be regulated by a specific cellular compartment. The potential to modulate proteostasis and metabolism with chemical compounds or targeted siRNAs may offer an ideal option for the treatment of disease.

Mice deficient in LMAN1 exhibit FV and FVIII deficiencies and liver accumulation of α1-antitrypsin

The type 1-transmembrane protein LMAN1 (ERGIC-53) forms a complex with the soluble protein MCFD2 and cycles between the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC). Mutations in either LMAN1 or MCFD2 cause the combined deficiency of factor V (FV) and factor VIII (FVIII; F5F8D), suggesting an ER-to-Golgi cargo receptor function for the LMAN1-MCFD2 complex. Here we report the analysis of LMAN1-deficient mice. Levels of plasma FV and FVIII, and platelet FV, are all reduced to ∼ 50% of wild-type in Lman1(-/-) mice, compared with the 5%-30% levels typically observed in human F5F8D patients. Despite previous reports identifying cathepsin C, cathepsin Z, and α1-antitrypsin as additional potential cargoes for LMAN1, no differences were observed between wild-type and Lman1(-/-) mice in the levels of cathepsin C and cathepsin Z in liver lysates or α1-antitrypsin levels in plasma. LMAN1 deficiency had no apparent effect on COPII-coated vesicle formation in an in vitro assay. However, the ER in Lman1(-/-) hepatocytes is slightly distended, with significant accumulation of α1-antitrypsin and GRP78. An unexpected, partially penetrant, perinatal lethality was observed for Lman1(-/-) mice, dependent on the specific inbred strain genetic background, suggesting a potential role for other, as yet unidentified LMAN1-dependent cargo proteins.

Molecular bases of cyclic and specific disulfide interchange between human ERO1alpha protein and protein-disulfide isomerase (PDI)

In the endoplasmic reticulum (ER) of human cells, ERO1α and protein-disulfide isomerase (PDI) constitute one of the major electron flow pathways that catalyze oxidative folding of secretory proteins. Specific and limited PDI oxidation by ERO1α is essential to avoid ER hyperoxidation. To investigate how ERO1α oxidizes PDI selectively among more than 20 ER-resident PDI family member proteins, we performed docking simulations and systematic biochemical analyses. Our findings reveal that a protruding β-hairpin of ERO1α specifically interacts with the hydrophobic pocket present in the redox-inactive PDI b'-domain through the stacks between their aromatic residues, leading to preferred oxidation of the C-terminal PDI a'-domain. ERO1α associated preferentially with reduced PDI, explaining the stepwise disulfide shuttle mechanism, first from ERO1α to PDI and then from oxidized PDI to an unfolded polypeptide bound to its hydrophobic pocket. The interaction of ERO1α with ERp44, another PDI family member protein, was also analyzed. Notably, ERO1α-dependent PDI oxidation was inhibited by a hyperactive ERp44 mutant that lacks the C-terminal tail concealing the substrate-binding hydrophobic regions. The potential ability of ERp44 to inhibit ERO1α activity may suggest its physiological role in ER redox and protein homeostasis.

Disulfide bonds in ER protein folding and homeostasis

Proteins that are expressed outside the cell must be synthesized, folded, and assembled in a way that ensures they can function in their designate location. Accordingly, these proteins are primarily synthesized in the endoplasmic reticulum (ER), which has developed a chemical environment more similar to that outside the cell. This organelle is equipped with a variety of molecular chaperones and folding enzymes that both assist the folding process, while at the same time exerting tight quality control measures that are largely absent outside the cell. A major post-translational modification of ER-synthesized proteins is disulfide bridge formation, which is catalyzed by the family of protein disulfide isomerases. As this covalent modification provides unique structural advantages to extracellular proteins, multiple pathways to disulfide bond formation have evolved. However, the advantages that disulfide bonds impart to these proteins come at a high cost to the cell. Very recent reports have shed light on how the cell can deal with or even exploit the side reactions of disulfide bond formation to maintain homeostasis of the ER and its folding machinery.

From antibodies to adiponectin: role of ERp44 in sizing and timing protein secretion

A large fraction of the proteome is synthesized and folded in the endoplasmic reticulum (ER), a multifunctional compartment also playing pivotal roles in Ca(2+) storage, redox homeostasis and signalling. From the ER, secretory proteins begin their journey towards their final destinations, the organelles of the exocytic and endocytic compartments, the plasma membrane or the extracellular space. Fidelity of protein-based intracellular communication is guaranteed by quality control (QC) mechanisms located at the ER-Golgi interface, which restrict forward transport to native proteins. QC is used also to time or shape the secretome. Furthermore, professional secretory cells face a problem of quantity, as well as quality of their protein products. This essay summarizes recent findings that identify ERp44 as a key regulator of protein secretion, Ca(2+) signalling and redox regulation.

Role of the lectin VIP36 in post-ER quality control of human alpha1-antitrypsin

The leguminous-type (L-type) lectin VIP36 localizes to the Golgi apparatus and cycles early in the secretory pathway. In vitro, VIP36 binds high-mannose glycans with a pH optimum of 6.5, a value similar to the luminal pH of the Golgi apparatus. Although the sugar-binding properties of VIP36 in vitro have been characterized in detail, the function of VIP36 in the intact cell remains unclear as no convincing glycoprotein cargo has been identified. Here, we used yellow fluorescent protein (YFP) fragment complementation to identify luminal interaction partners of VIP36. By screening a human liver cDNA library, we identified the glycoprotein alpha1-antitrypsin (alpha1-AT) as a cargo of VIP36. The VIP36/alpha1-AT complex localized to Golgi and endoplasmic reticulum (ER). In the living cell, VIP36 bound exclusively to the high-mannose form of alpha1-AT. The binding was increased when complex glycosylation was prevented by kifunensine and abolished when the glycosylation sites of alpha1-AT were inactivated by mutagenesis. Silencing VIP36 accelerated alpha1-AT transport, arguing against a role of VIP36 in anterograde traffic. The complex formed by VIP36 and alpha1-AT in the Golgi recycled back to the ER. The combined data are most consistent with a function of VIP36 in post-ER quality control of alpha1-AT.

Pathogenesis of ER storage disorders: modulating Russell body biogenesis by altering proximal and distal quality control

In many protein storage diseases, detergent-insoluble proteins accumulate in the early secretory compartment (ESC). Protein condensation reflects imbalances between entry into (synthesis/translocation) and exit from (secretion/degradation) ESC, and can be also a consequence of altered quality control (QC) mechanisms. Here we exploit the inducible formation of Russell bodies (RB), dilated ESC cisternae containing mutant Ig-micro chains, as a model to mechanistically dissect protein condensation. Depending on the presence or absence of Ig-L chains, mutant Ig-micro chains lacking their first constant domain (Ch1) accumulate in rough or smooth RB (rRB and sRB), dilations of the endoplasmic reticulum (ER) and ER-Golgi intermediate compartment (ERGIC), respectively, reflecting the proximal and distal QC stations in the stepwise biogenesis of polymeric IgM. Either weakening ERp44-dependent distal QC or facilitating ER-associated degradation (ERAD) inhibits RB formation. Overexpression of PDI or ERp44 inhibits muDeltaCh1 secretion. However, PDI inhibits while ERp44 promotes muDeltaCh1 condensation. Both Ero1alpha silencing and overexpression prevent RB formation, demonstrating a strict redox dependency of the phenomenon. Altogether, our findings identify key controllers of protein condensation along the ESC as potential targets to handle certain storage disorders.

Physiology and pathology of proteostasis in the early secretory compartment

The endoplasmic reticulum (ER), the port of entry for proteins into the secretory pathway, is a multifunctional organelle emerging as a central integrator of numerous signalling pathways. The mechanisms that control proteostasis are integral part of this signalling network, providing cues for morphological and functional cell remodelling, proliferation, inflammation and cell death. The complexity of ER responses is exploited during physiological and pathological tissue development, cell differentiation and lifespan control. This essay outlines some of the mechanisms that link proteostasis within the early secretory compartment to signalling in development and disease.