Secretory carrier membrane protein 2 regulates exocytic insertion of NKCC2 into the cell membrane

The renal-specific Na-K-2Cl co-transporter, NKCC2, plays a pivotal role in regulating body salt levels and blood pressure. NKCC2 mutations lead to type I Bartter syndrome, a life-threatening kidney disease. Regulation of NKCC2 trafficking behavior serves as a major mechanism in controlling NKCC2 activity across the plasma membrane. However, the identities of the protein partners involved in cell surface targeting of NKCC2 are largely unknown. To gain insight into these processes, we used a yeast two-hybrid system to screen a kidney cDNA library for proteins that interact with the NKCC2 C terminus. One binding partner we identified was SCAMP2 (secretory carrier membrane protein 2). Microscopic confocal imaging and co-immunoprecipitation assays confirmed NKCC2-SCAMP2 interaction in renal cells. SCAMP2 associated also with the structurally related co-transporter NCC, suggesting that the interaction with SCAMP2 is a common feature of sodium-dependent chloride co-transporters. Heterologous expression of SCAMP2 specifically decreased cell surface abundance as well as transport activity of NKCC2 across the plasma membrane. Co-immunolocalization experiments revealed that intracellularly retained NKCC2 co-localizes with SCAMP2 in recycling endosomes. The rate of NKCC2 endocytic retrieval, assessed by the sodium 2-mercaptoethane sulfonate cleavage assay, was not affected by SCAMP2. The surface-biotinylatable fraction of newly inserted NKCC2 in the plasma membrane was reduced by SCAMP2, demonstrating that SCAMP2-induced decrease in surface NKCC2 is due to decreased exocytotic trafficking. Finally, a single amino acid mutation, cysteine 201 to alanine, within the conserved cytoplasmic E peptide of SCAMP2, which is believed to regulate exocytosis, abolished SCAMP2-mediated down-regulation of the co-transporter. Taken together, these data are consistent with a model whereby SCAMP2 regulates NKCC2 transit through recycling endosomes and limits the cell surface targeting of the co-transporter by interfering with its exocytotic trafficking.     

Unraveling the function of Arabidopsis thaliana OS9 in the endoplasmic reticulum-associated degradation of glycoproteins

In the endoplasmic reticulum, immature polypeptides coincide with terminally misfolded proteins. Consequently, cells need a well-balanced quality control system, which decides about the fate of individual proteins and maintains protein homeostasis. Misfolded and unassembled proteins are sent for destruction via the endoplasmic reticulum-associated degradation (ERAD) machinery to prevent the accumulation of potentially toxic protein aggregates. Here, we report the identification of Arabidopsis thaliana OS9 as a component of the plant ERAD pathway. OS9 is an ER-resident glycoprotein containing a mannose-6-phosphate receptor homology domain, which is also found in yeast and mammalian lectins involved in ERAD. OS9 fused to the C-terminal domain of YOS9 can complement the ERAD defect of the corresponding yeast Δyos9 mutant. An A. thaliana OS9 loss-of-function line suppresses the severe growth phenotype of the bri1-5 and bri1-9 mutant plants, which harbour mutated forms of the brassinosteroid receptor BRI1. Co-immunoprecipitation studies demonstrated that OS9 associates with Arabidopsis SEL1L/HRD3, which is part of the plant ERAD complex and with the ERAD substrates BRI1-5 and BRI1-9, but only the binding to BRI1-5 occurs in a glycan-dependent way. OS9-deficiency results in activation of the unfolded protein response and reduces salt tolerance, highlighting the role of OS9 during ER stress. We propose that OS9 is a component of the plant ERAD machinery and may act specifically in the glycoprotein degradation pathway.     

Multiple Evolutionarily Conserved Di-leucine Like Motifs in the Carboxyl Terminus Control the Anterograde Trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the ¹⁰⁸¹LLV¹⁰⁸³ motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, ¹⁰³⁸LL¹⁰³⁹ and ¹⁰⁴⁸LI¹⁰⁴⁹, that are required for ER exit and surface expression of the co-transporter. Double mutations of ¹⁰³⁸LL¹⁰³⁹ or ¹⁰⁴⁸LI¹⁰⁴⁹ to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that ¹⁰³⁸AA¹⁰³⁹ and ¹⁰⁴⁸AA¹⁰⁴⁹ were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.     

A Highly Conserved Motif at the COOH Terminus Dictates Endoplasmic Reticulum Exit and Cell Surface Expression of NKCC2

Mutations in the apically located Na⁺-K⁺-2Cl⁻ co-transporter, NKCC2, lead to type I Bartter syndrome, a life-threatening kidney disorder, yet the mechanisms underlying the regulation of mutated NKCC2 proteins in renal cells have not been investigated. Here, we identified a trihydrophobic motif in the distal COOH terminus of NKCC2 that was required for endoplasmic reticulum (ER) exit and surface expression of the co-transporter. Indeed, microscopic confocal imaging showed that a naturally occurring mutation depriving NKCC2 of its distal COOH-terminal region results in the absence of cell surface expression. Biotinylation assays revealed that lack of cell surface expression was associated with abolition of mature complex-glycosylated NKCC2. Pulse-chase analysis demonstrated that the absence of mature protein was not caused by reduced synthesis or increased rates of degradation of mutant co-transporters. Co-immunolocalization experiments revealed that these mutants co-localized with the ER marker protein-disulfide isomerase, demonstrating that they are retained in the ER. Cell treatment with proteasome or lysosome inhibitors failed to restore the loss of complex-glycosylated NKCC2, further eliminating the possibility that mutant co-transporters were processed by the Golgi apparatus. Serial truncation of the NKCC2 COOH terminus, followed by site-directed mutagenesis, identified hydrophobic residues ¹⁰⁸¹LLV¹⁰⁸³ as an ER exit signal necessary for maturation of NKCC2. Mutation of ¹⁰⁸¹LLV¹⁰⁸³ to AAA within the context of the full-length protein prevented NKCC2 ER exit independently of the expression system. This trihydrophobic motif is highly conserved in the COOH-terminal tails of all members of the cation-chloride co-transporter family, and thus may function as a common motif mediating their transport from the ER to the cell surface. Taken together, these data are consistent with a model whereby naturally occurring premature terminations that interfere with the LLV motif compromise co-transporter surface delivery through defective trafficking.The Na-K-2Cl co-transporter, NKCC2, provides the major route for sodium/chloride transport across the apical plasma membrane of the thick ascending limb (TAL)³ of the kidney (1). This co-transporter is critical for salt reabsorption, acid-base regulation, and divalent mineral cation metabolism (2). The prominent importance of NKCC2 in renal functions is evidenced by the effect of loop diuretics, which as pharmacologic inhibitors of NKCC2, are extensively used in the treatment of edematous states (2). Even more impressive, inactivating mutations of the NKCC2 gene in humans causes Bartter syndrome type 1 (BS1), a life-threatening renal tubular disorder for which the diagnosis is usually made in the antenatal-neonatal period, due to the presence of polyhydramnios, premature delivery, salt loss, hypokalemia, metabolic alkalosis, hypercalciuria, and nephrocalcinosis (3). Without appropriate treatment, patients with BS1 will not survive the early neonatal period (4). In congruence with the severity of the symptoms and the uniformity of the clinical picture, functional analysis of diverse NKCC2 mutants consistently revealed a loss of function effect of the tested mutations (5, 6). However, regulatory characterizations of mutants NKCC2 were limited to Xenopus laevis oocytes. Indeed, studies aimed at understanding the post-translational regulation of NKCC2 have been hampered by the difficulty of expressing the co-transporter protein in mammalian cells (7, 8). As a consequence, our knowledge of the molecular mechanisms underlying membrane trafficking of mutated NKCC2 proteins in mammalian cells is nil. Increasing our understanding of the molecular determinants underlying NKCC2 expression in renal cells is essential for elucidating the pathophysiology of BS1 and for improving the available treatments (9, 10). Undeniably, only analysis of the expression such NKCC2 of mutants in renal cells would definitively establish their cellular fate.NKCC2 belongs to the superfamily of electroneutral cation-coupled chloride (CCC) co-transporters (SLC12A) (1). The cation-chloride co-transporters (CCCs) family comprises two principal branches of homologous membrane proteins. One branch includes the Na⁺-dependent chloride co-transporters composed of the Na⁺-K⁺-2Cl⁻ co-transporters (NKCC1 and NKCC2) and the Na⁺-Cl⁻ co-transporter (NCC). The second branch includes the Na⁺-independent K⁺-Cl⁻ co-transporters composed of at least four different isoforms: KCC1 KCC2, KCC3, and KCC4 (11). Within the families, the CCCs share 25–75% amino acid identity. All of these co-transporters exhibit similar hydropathy profiles with 12 transmembrane-spanning domains, an amino terminus of variable length, and a long cytoplasmic carboxyl terminus. Because the COOH-terminal domain of NKCC2 is the predominant cytoplasmic region, it is likely to be a major factor in the trafficking of the NKCC2 protein. Moreover, there have been several reports demonstrating that COOH-terminal residues are important for correct protein targeting (12–14). Occasionally, COOH-terminal mutations are known to cause genetic disorders (15–17). Although studies of other ion transporters support the importance of the COOH-terminal signals in protein stability, maturation, surface delivery, and ER export (18–22), little is known about the role of COOH-terminal signals in the biogenesis of NKCC2.We were recently able to express NKCC2 protein in mammalian cells (23), providing therefore a powerful tool to study and understand the molecular mechanisms underlying the co-transporter expression and regulation in renal cells. This allowed us, in this study, to take the advantage of the existence of natural mutants altering the COOH-terminal tail of the co-transporter to investigate the role of the COOH terminus in the biogenesis of NKCC2 and to explore possible mechanisms implicated in BS1. The results demonstrate the importance of the COOH terminus in normal maturation of the NKCC2 protein. Indeed, we identified a motif of three hydrophobic residues, ¹⁰⁸¹LLV¹⁰⁸³, highly conserved in the COOH-terminal tails of all members of the CCC family, that controls the rate of ER export and thus of surface expression of NKCC2. Loss of the motif disrupts glycosylation and plasma membrane localization of NKCC2. Therefore, we propose abnormal trafficking as a common BS1 mechanism associated with mutations depriving NKCC2 of its COOH terminus.     

Identification of an Htm1 (EDEM)-dependent,Mns1-independent Endoplasmic Reticulum-associated Degradation (ERAD) Pathway in Saccharomyces cerevisiae: APPLICATION OF A NOVEL ASSAY FOR GLYCOPROTEIN ERAD*

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a quality control system for newly synthesized proteins in the ER; nonfunctional proteins, which fail to form their correct folding state, are then degraded. The cytoplasmic peptide:N-glycanase is a deglycosylating enzyme that is involved in the ERAD and releases N-glycans from misfolded glycoproteins/glycopeptides. We have previously identified a mutant plant toxin protein, RTA (ricin A-chain nontoxic mutant), as the first in vivo Png1 (the cytoplasmic peptide:N-glycanase in Saccharomyces cerevisiae)-dependent ERAD substrate. Here, we report a new genetic device to assay the Png1-dependent ERAD pathway using the new model protein designated RTL (RTA-transmembrane-Leu2). Our extensive studies using different yeast mutants identified various factors involved in RTL degradation. The degradation of RTA/RTL was independent of functional Sec61 but was dependent on Der1. Interestingly, ER-mannosidase Mns1 was not involved in RTA degradation, but it was dependent on Htm1 (ERAD-related α-mannosidase in yeast) and Yos9 (a putative degradation lectin), indicating that mannose trimming by Mns1 is not essential for efficient ERAD of RTA/RTL. The newly established RTL assay will allow us to gain further insight into the mechanisms involved in the Png1-dependent ERAD-L pathway.     

The endoplasmic reticulum‐associated protein,OS‐9, behaves as a lectin in targeting the immature calcium‐sensing receptor

The mechanisms responsible for the processing and quality control of the calcium‐sensing receptor (CaSR) in the endoplasmic reticulum (ER) are largely unknown. In a yeast two‐hybrid screen of the CaSR C‐terminal tail (residues 865–1078), we identified osteosarcoma‐9 (OS‐9) protein as a binding partner. OS‐9 is an ER‐resident lectin that targets misfolded glycoproteins to the ER‐associated degradation (ERAD) pathway through recognition of specific N‐glycans by its mannose‐6‐phosphate receptor homology (MRH) domain. We show by confocal microscopy that the CaSR and OS‐9 co‐localize in the ER in COS‐1 cells. In immunoprecipitation studies with co‐expressed OS‐9 and CaSR, OS‐9 specifically bound the immature form of wild‐type CaSR in the ER. OS‐9 also bound the immature forms of a CaSR C‐terminal deletion mutant and a C677A mutant that remains trapped in the ER, although binding to neither mutant was favored over wild‐type receptor. OS‐9 binding to immature CaSR required the MRH domain of OS‐9 indicating that OS‐9 acts as a lectin most likely to target misfolded CaSR to ERAD. Our results also identify two distinct binding interactions between OS‐9 and the CaSR, one involving both C‐terminal domains of the two proteins and the other involving both N‐terminal domains. This suggests the possibility of more than one functional interaction between OS‐9 and the CaSR. When we investigated the functional consequences of altered OS‐9 expression, neither knockdown nor overexpression of OS‐9 was found to have a significant effect on CaSR cell surface expression or CaSR‐mediated ERK1/2 phosphorylation.     

Vesicle-associated Membrane Protein 2 (VAMP2) but Not VAMP3 Mediates cAMP-stimulated Trafficking of the Renal Na+-K+-2Cl? Co-transporter NKCC2 in Thick Ascending Limbs

In the kidney, epithelial cells of the thick ascending limb (TAL) reabsorb NaCl via the apical Na⁺/K⁺/2Cl⁻ co-transporter NKCC2. Steady-state surface NKCC2 levels in the apical membrane are maintained by a balance between exocytic delivery, endocytosis, and recycling. cAMP is the second messenger of hormones that enhance NaCl absorption. cAMP stimulates NKCC2 exocytic delivery via protein kinase A (PKA), increasing steady-state surface NKCC2. However, the molecular mechanism involved has not been studied. We found that several members of the SNARE family of membrane fusion proteins are expressed in TALs. Here we report that NKCC2 co-immunoprecipitates with VAMP2 in rat TALs, and they co-localize in discrete domains at the apical surface. cAMP stimulation enhanced VAMP2 exocytic delivery to the plasma membrane of renal cells, and stimulation of PKA enhanced VAMP2-NKCC2 co-immunoprecipitation in TALs. In vivo silencing of VAMP2 but not VAMP3 in TALs blunted cAMP-stimulated steady-state surface NKCC2 expression and completely blocked cAMP-stimulated NKCC2 exocytic delivery. VAMP2 was not involved in constitutive NKCC2 delivery. We concluded that VAMP2 but not VAMP3 selectively mediates cAMP-stimulated NKCC2 exocytic delivery and surface expression in TALs. We also demonstrated that cAMP stimulation enhances VAMP2 exocytosis and promotes VAMP2 interaction with NKCC2.     

Na(+) -K(+) -2Cl(-) cotransporter type 2 trafficking and activity: The role of interacting proteins

The central role of Na(+) -K(+) -2Cl(-) cotransporter type 2 (NKCC2) in vectorial transepithelial salt reabsorption in thick ascending limb cells from Henle's loop in the kidney is evidenced by the effects of loop diuretics, the pharmacological inhibitors of NKCC2, that are amongst the most powerful antihypertensive drugs available to date. Moreover, genetic mutations of the NKCC2 encoding gene resulting in impaired apical targeting and function of NKCC2 transporter give rise to a pathological phenotype known as type I Bartter syndrome, characterised by a severe volume depletion, hypokalaemia and metabolic alkalosis with high prenatal mortality. On the contrary, excessive NKCC2 activity has been linked with inherited hypertension in humans and in rodent models. Interestingly, in animal models of hypertension, NKCC2 upregulation is achieved by post-translational mechanisms underlining the need to analyse the molecular mechanisms involved in the regulation of NKCC2 trafficking and activity to gain insights in the pathogenesis of hypertension.     

Misfolded Proteins Induce Aggregation of the Lectin Yos9

A substantial fraction of nascent proteins delivered into the endoplasmic reticulum (ER) never reach their native conformations. Eukaryotes use a series of complementary pathways to efficiently recognize and dispose of these terminally misfolded proteins. In this process, collectively termed ER-associated degradation (ERAD), misfolded proteins are retrotranslocated to the cytosol, polyubiquitinated, and degraded by the proteasome. Although there has been great progress in identifying ERAD components, how these factors accurately identify substrates remains poorly understood. The targeting of misfolded glycoproteins in the ER lumen for ERAD requires the lectin Yos9, which recognizes the glycan species found on terminally misfolded proteins. In a role that remains poorly characterized, Yos9 also binds the protein component of ERAD substrates. Here, we identified a 45-kDa domain of Yos9, consisting of residues 22–421, that is proteolytically stable, highly structured, and able to fully support ERAD in vivo. In vitro binding studies show that Yos9(22–421) exhibits sequence-specific recognition of linear peptides from the ERAD substrate, carboxypeptidase Y G255R (CPY*), and binds a model unfolded peptide ΔEspP and protein Δ131Δ in solution. Binding of Yos9 to these substrates results in their cooperative aggregation. Although the physiological consequences of this substrate-induced aggregation remain to be seen, it has the potential to play a role in the regulation of ERAD.     

Ectopic RING activity at the ER membrane differentially impacts ERAD protein quality control pathways

Endoplasmic reticulum-associated degradation (ERAD) is a protein quality control pathway that ensures misfolded proteins are removed from the ER and destroyed. In ERAD, membrane and luminal substrates are ubiquitylated by ER-resident RING-type E3 ubiquitin ligases, retrotranslocated into the cytosol, and degraded by the proteasome. Overexpression of ERAD factors is frequently used in yeast and mammalian cells to study this process. Here, we analyze the impact of ERAD E3 overexpression on substrate turnover in yeast, where there are three ERAD E3 complexes (Doa10, Hrd1, and Asi1-3). Elevated Doa10 or Hrd1 (but not Asi1) RING activity at the ER membrane resulting from protein overexpression inhibits the degradation of specific Doa10 substrates. The ERAD E2 ubiquitin-conjugating enzyme Ubc6 becomes limiting under these conditions, and UBC6 overexpression restores Ubc6-mediated ERAD. Using a subset of the dominant-negative mutants, which contain the Doa10 RING domain but lack the E2-binding region, we show that they induce degradation of membrane tail-anchored Ubc6 independently of endogenous Doa10 and the other ERAD E3 complexes. This remains true even if the cells lack the Dfm1 rhomboid pseudoprotease, which is also a proposed retrotranslocon. Hence, rogue RING activity at the ER membrane elicits a highly specific off-pathway defect in the Doa10 pathway, and the data point to an additional ERAD E3-independent retrotranslocation mechanism.     

Unconventional roles of nonlipidated LC3 in ERAD tuning and coronavirus infection

Secretory and membrane proteins attain their native structure in the endoplasmic reticulum (ER). Folding-defective polypeptides are selected for degradation by processes collectively defined as ER-associated degradation (ERAD). Enhanced ERAD activity may interfere with protein biogenesis by inappropriately targeting not-yet-native protein folding intermediates for disposal. The regulation of ERAD is therefore crucial to maintain cellular proteostasis. At steady-state, select ERAD regulators are constitutively removed from the ER in a series of processes collectively defined as ERAD tuning. This sets the ERAD activity at levels that do not interfere with completion of ongoing folding programs. Our latest work highlights a crucial, autophagy-independent role of nonlipidated LC3 (LC3-I) as part of a membrane-bound receptor that insures the vesicle-mediated clearance of at least two ERAD regulators from the ER, EDEM1 and OS9. This pathway is hijacked by coronaviruses (CoV), and silencing of LC3 substantially inhibits viral replication.     

Bypass of glycan-dependent glycoprotein delivery to ERAD by up-regulated EDEM1

Trimming of mannose residues from the N-linked oligosaccharide precursor is a stringent requirement for glycoprotein endoplasmic reticulum (ER)-associated degradation (ERAD). In this paper, we show that, surprisingly, overexpression of ER degradation-enhancing α-mannosidase-like protein 1 (EDEM1) or its up-regulation by IRE1, as occurs in the unfolded protein response, overrides this requirement and renders unnecessary the expression of ER mannosidase I. An EDEM1 deletion mutant lacking most of the carbohydrate-recognition domain also accelerated ERAD, delivering the substrate to XTP3-B and OS9. EDEM1 overexpression also accelerated the degradation of a mutant nonglycosylated substrate. Upon proteasomal inhibition, EDEM1 concentrated together with the ERAD substrate in the pericentriolar ER-derived quality control compartment (ERQC), where ER mannosidase I and ERAD machinery components are localized, including, as we show here, OS9. We suggest that a nascent glycoprotein can normally dissociate from EDEM1 and be rescued from ERAD by reentering calnexin-refolding cycles, a condition terminated by mannose trimming. At high EDEM1 levels, glycoprotein release is prevented and glycan interactions are no longer required, canceling the otherwise mandatory ERAD timing by mannose trimming and accelerating the targeting to degradation.     

The Lhs1/GRP170 Chaperones Facilitate the Endoplasmic Reticulum-associated Degradation of the Epithelial Sodium Channel

The epithelial sodium channel, ENaC, plays a critical role in maintaining salt and water homeostasis, and not surprisingly defects in ENaC function are associated with disease. Like many other membrane-spanning proteins, this trimeric protein complex folds and assembles inefficiently in the endoplasmic reticulum (ER), which results in a substantial percentage of the channel being targeted for ER-associated degradation (ERAD). Because the spectrum of factors that facilitates the degradation of ENaC is incomplete, we developed yeast expression systems for each ENaC subunit. We discovered that a conserved Hsp70-like chaperone, Lhs1, is required for maximal turnover of the ENaC α subunit. By expressing Lhs1 ATP binding mutants, we also found that the nucleotide exchange properties of this chaperone are dispensable for ENaC degradation. Consistent with the precipitation of an Lhs1-αENaC complex, Lhs1 holdase activity was instead most likely required to support the ERAD of αENaC. Moreover, a complex containing the mammalian Lhs1 homolog GRP170 and αENaC co-precipitated, and GRP170 also facilitated ENaC degradation in human, HEK293 cells, and in a Xenopus oocyte expression system. In both yeast and higher cell types, the effect of Lhs1 on the ERAD of αENaC was selective for the unglycosylated form of the protein. These data establish the first evidence that Lhs1/Grp170 chaperones can act as mediators of ERAD substrate selection.     

Importin beta interacts with the endoplasmic reticulum-associated degradation machinery and promotes ubiquitination and degradation of mutant alpha1-antitrypsin

The mechanism by which misfolded proteins in the endoplasmic reticulum (ER) are retrotranslocated to the cytosol for proteasomal degradation is still poorly understood. Here, we show that importin β, a well established nucleocytoplasmic transport protein, interacts with components of the retrotranslocation complex and promotes ER-associated degradation (ERAD). Knockdown of importin β specifically inhibited the degradation of misfolded ERAD substrates but did not affect turnover of non-ERAD proteasome substrates. Genetic studies and in vitro reconstitution assays demonstrate that importin β is critically required for ubiquitination of mutant α1-antitrypsin, a luminal ERAD substrate. Furthermore, we show that importin β cooperates with Ran GTPase to promote ubiquitination and proteasomal degradation of mutant α1-antitrypsin. These results establish an unanticipated role for importin β in ER protein quality control.     

PDI reductase acts on Akita mutant proinsulin to initiate retrotranslocation along the Hrd1/Sel1L-p97 axis

In mutant INS gene–induced diabetes of youth (MIDY), characterized by insulin deficiency, MIDY proinsulin mutants misfold and fail to exit the endoplasmic reticulum (ER). Moreover, these mutants bind and block ER exit of wild-type (WT) proinsulin, inhibiting insulin production. The ultimate fate of ER-entrapped MIDY mutants is unclear, but previous studies implicated ER-associated degradation (ERAD), a pathway that retrotranslocates misfolded ER proteins to the cytosol for proteasomal degradation. Here we establish key ERAD machinery components used to triage the Akita proinsulin mutant, including the Hrd1-Sel1L membrane complex, which conducts Akita proinsulin from the ER lumen to the cytosol, and the p97 ATPase, which couples the cytosolic arrival of proinsulin with its proteasomal degradation. Surprisingly, we find that protein disulfide isomerase (PDI), the major protein oxidase of the ER lumen, engages Akita proinsulin in a novel way, reducing proinsulin disulfide bonds and priming the Akita protein for ERAD. Efficient PDI engagement of Akita proinsulin appears linked to the availability of Hrd1, suggesting that retrotranslocation is coordinated on the lumenal side of the ER membrane. We believe that, in principle, this form of diabetes could be alleviated by enhancing the targeting of MIDY mutants for ERAD to restore WT insulin production.     

A genome-wide screen identifies Yos9p as essential for ER-associated degradation of glycoproteins

We undertook a growth-based screen exploiting the degradation of CTL*, a chimeric membrane-bound ERAD substrate derived from soluble lumenal CPY*. We screened the Saccharomyces cerevisiae genomic deletion library containing approximately 5000 viable strains for mutants defective in endoplasmic reticulum (ER) protein quality control and degradation (ERAD). Among the new gene products we identified Yos9p, an ER-localized protein previously involved in the processing of GPI anchored proteins. We show that deficiency in Yos9p affects the degradation only of glycosylated ERAD substrates. Degradation of non-glycosylated substrates is not affected in cells lacking Yos9p. We propose that Yos9p is a lectin or lectin-like protein involved in the quality control of N-glycosylated proteins. It may act sequentially or in concert with the ERAD lectin Htm1p/Mnl1p (EDEM) to prevent secretion of malfolded glycosylated proteins and deliver them to the cytosolic ubiquitin-proteasome machinery for elimination.     

Mammalian OS-9 Is Upregulated in Response to Endoplasmic Reticulum Stress and Facilitates Ubiquitination of Misfolded Glycoproteins

Proteins that fail to fold or assemble with partner subunits are selectively removed from the endoplasmic reticulum (ER) via the ER-associated degradation (ERAD) pathway. Proteins selected for ERAD are polyubiquitinated and retrotranslocated into the cytosol for degradation by the proteasome. Although it is unclear how proteins are initially identified by the ERAD system in mammalian cells, OS-9 was recently proposed to play a key role in this process. Here we show that OS-9 is upregulated in response to ER stress and is associated both with components of the ERAD machinery and with ERAD substrates. Using RNA interference, we show that OS-9 is required for efficient ubquitination of glycosylated ERAD substrates, suggesting that it helps transfer misfolded proteins to the ubiquitination machinery. We also find that OS-9 binds to a misfolded nonglycosylated protein destined for ERAD, but not to the properly folded wild-type protein. Surprisingly, however, OS-9 is not required for ubiquitination or degradation of this nonglycosylated ERAD substrate. We propose a model in which OS-9 recognises terminally misfolded proteins via polypeptide-based rather than glycan-based signals, but is only required for transferring those bearing N-glycans to the ubiquitination machinery.     

A vacuolar carboxypeptidase mutant of Arabidopsis thaliana is degraded by the ERAD pathway independently of its N-glycan

Misfolded proteins produced in the endoplasmic reticulum (ER) are degraded by a mechanism, the ER-associated degradation (ERAD). Here we report establishment of the experimental system to analyze the ERAD in plant cells. Carboxypeptidase Y (CPY) is a vacuolar enzyme and its mutant CPY∗ is degraded by the ERAD in yeast. Since Arabidopsis thaliana has AtCPY, an ortholog of yeast CPY, we constructed and expressed fusion proteins consisting of AtCPY and GFP and of AtCPY∗, which carries a mutation homologous to yeast CPY∗, and GFP in A. thaliana cells. While AtCPY-GFP was efficiently transported to the vacuole, AtCPY∗-GFP was retained in the ER to be degraded in proteasome- and Cdc48-dependent manners. We also found that AtCPY∗-GFP was degraded by the ERAD in yeast cells, but that its single N-glycan did not function as a degradation signal in yeast or plant cells. Therefore, AtCPY∗-GFP can be used as a marker protein to analyze the ERAD pathway, likely for nonglycosylated substrates, in plant cells.     

A Cyclooxygenase-2-dependent Prostaglandin E2 Biosynthetic System in the Golgi Apparatus

Cyclooxygenases (COXs) catalyze the committed step in prostaglandin (PG) biosynthesis. COX-1 is constitutively expressed and stable, whereas COX-2 is inducible and short lived. COX-2 is degraded via endoplasmic reticulum (ER)-associated degradation (ERAD) following post-translational glycosylation of Asn-594. COX-1 and COX-2 are found in abundance on the luminal surfaces of the ER and inner membrane of the nuclear envelope. Using confocal immunocytofluorescence, we detected both COX-2 and microsomal PGE synthase-1 (mPGES-1) but not COX-1 in the Golgi apparatus. Inhibition of trafficking between the ER and Golgi retarded COX-2 ERAD. COX-2 has a C-terminal STEL sequence, which is an inefficient ER retention signal. Substituting this sequence with KDEL, a robust ER retention signal, concentrated COX-2 in the ER where it was stable and slowly glycosylated on Asn-594. Native COX-2 and a recombinant COX-2 having a Golgi targeting signal but not native COX-1 exhibited efficient catalytic coupling to mPGES-1. We conclude that N-glycosylation of Asn-594 of COX-2 occurs in the ER, leading to anterograde movement of COX-2 to the Golgi where the Asn-594-linked glycan is trimmed prior to retrograde COX-2 transport to the ER for ERAD. Having an inefficient ER retention signal leads to sluggish Golgi to ER transit of COX-2. This permits significant Golgi residence time during which COX-2 can function catalytically. Cytosolic phospholipase A_2α, which mobilizes arachidonic acid for PG synthesis, preferentially translocates to the Golgi in response to physiologic Ca²⁺ mobilization. We propose that cytosolic phospholipase A_2α, COX-2, and mPGES-1 in the Golgi comprise a dedicated system for COX-2-dependent PGE₂ biosynthesis.