Endoplasmic Reticulum Membrane Complex Research Articles

ER membrane complex (EMC): Structure, functions, and roles in diseases.

The endoplasmic reticulum (ER) is the largest membrane system in eukaryotic cells and is the primary site for the biosynthesis of lipids and carbohydrates, as well as for the folding, assembly, modification, and transport of secreted and integrated membrane proteins. The ER membrane complex (EMC) on the ER membrane is an ER multiprotein complex that affects the quality control of membrane proteins, which is abundant and widely preserved. Its disruption has been found to affect a wide range of processes, including protein and lipid synthesis, organelle communication, endoplasmic reticulum stress, and viral maturation, and may lead to neurodevelopmental disorders and cancer. Therefore, EMC has attracted the attention of many scholars and become a hot field. In this paper, we summarized the main contributions of the research of EMC in the past nearly 15 years, and reviewed the structure and function of EMC as well as its related diseases. We hope this review will promote further progress of research on EMC.

Mechanisms Underlying Rare Inherited Pediatric Retinal Vascular Diseases: FEVR, Norrie Disease, Persistent Fetal Vascular Syndrome.

Familial Exudative Vitreoretinopathy (FEVR), Norrie disease, and persistent fetal vascular syndrome (PFVS) are extremely rare retinopathies that are clinically distinct but are unified by abnormal retinal endothelial cell function, and subsequent irregular retinal vascular development and/or aberrant inner blood-retinal-barrier (iBRB) function. The early angiogenesis of the retina and its iBRB is a delicate process that is mediated by the canonical Norrin Wnt-signaling pathway in retinal endothelial cells. Pathogenic variants in genes that play key roles within this pathway, such as NDP, FZD4, TSPAN12, and LRP5, have been associated with the incidence of these retinal diseases. Recent efforts to further elucidate the etiology of these conditions have not only highlighted their multigenic nature but have also resulted in the discovery of pathological variants in additional genes such as CTNNB1, KIF11, and ZNF408, some of which operate outside of the Norrin Wnt-signaling pathway. Recent discoveries of FEVR-linked variants in two other Catenin genes (CTNND1, CTNNA1) and the Endoplasmic Reticulum Membrane Complex Subunit-1 gene (EMC1) suggest that we will continue to find additional genes that impact the neural retinal vasculature, especially in multi-syndromic conditions. The goal of this review is to briefly highlight the current understanding of the roles of their encoded proteins in retinal endothelial cells to understand the essential functional mechanisms that can be altered to cause these very rare pediatric retinal vascular diseases.

1549-P: scEMC10 Promotes Glucose Intolerance via the Inhibition of PKA-Mediated Insulin Secretion

It has been shown recently that overexpression of the secreted isoform of endoplasmic reticulum membrane complex subunit 10 (scEMC10) impairs, whereas Emc10 knockout (KO) or antibody neutralization of circulating scEMC10 improves glucose tolerance in mice fed with high fat diet. In humans, serum scEMC10 levels correlate positively with fasting plasma glucose and HbA1c. The mechanism underlying the regulation of glucose metabolism by scEMC10 remains largely unknown. In this study, alation of EMC10 significantly improved glucose tolerance in chow diet-fed mice, which was attributed to increased glucose-stimulated insulin secretion (GSIS), but not improved insulin sensitivity evaluated by euglycemic-hyperinsulinemic clamp. Using an adeno-associated virus, either reconstitution of scEMC10 in Emc10 KO mice or overexpression of scEMC10 in wildtype mice significantly decreased glucose tolerance accompanied by blunted GSIS. Recombinant scEMC10 inhibited GSIS in both INS-1 cells and isolated primary islets in vitro. Mechanistically, we presented evidence that extracellular scEMC10 can be transported into beta cells where it binds to the catalytic subunit of PKA and inhibits phosphorylations of PKA substrates including phosphorylated CREB, along with reduced PDX-1 expression. Furthermore, antibody neutralization of circulating scEMC10 alleviated hyperglycemia in both STZ-treated mice and ob/ob mice. This work identifies scEMC10 as a suppressor of insulin secretion via the inhibition of PKA signaling in islet beta cells and suggests scEMC10 neutralizing antibody as a promising pharmaceutical agent to treat diabetes. Disclosure J.Dai: None. S.Liu: None. K.Chen: None. S.Jin: None. Y.Wang: None. L.Chen: None. X.Wang: None. Funding National Natural Science Foundation of China (81873645); Science and Technology Commission of Shanghai Municipality (22140902700)

Serum scEMC10 Levels are Negatively Associated With Resting Metabolic Rate and age in Humans.

We have recently shown that the secreted isoform of endoplasmic reticulum membrane complex subunit 10 (scEMC10) is upregulated in human obesity and that overexpression of scEMC10 promotes, whereas antibody neutralization of circulating scEMC10 prevents diet-induced obesity in mice. To explore associations of serum scEMC10 with body mass index (BMI), resting metabolism rate (RMR), and age in humans. A cross-sectional study. A total of 833 participants from a Chinese physical examination cohort and 191 participants from the Leipzig Obesity Biobank cohort. Serum scEMC10 concentrations are measured using chemiluminescent immunoassay. RMR is calculated based on measurements from indirect calorimetry with an open-circuit ventilated-hood system. In the Chinese physical examination cohort, a J-shaped nonlinear correlation between BMI and serum scEMC10 was identified in participants where underweight, overweight, and obese people all presented higher serum scEMC10 levels than normal weight people. Participants younger than age 30 years old exhibited significantly higher serum scEMC10 levels than those older than 50 years of age. In addition, participants aged 30 to 40 years also had significantly higher serum scEMC10 levels than those aged 50 to 60 years. In the Leipzig Obesity Biobank cohort, we observed a significantly negative correlation between serum scEMC10 and resting energy expenditure after adjusting for BMI. Participants in the highest quartile of serum scEMC10 levels had significantly lower RMR than those in the first quartile. RMR had an independently inverse association with serum scEMC10. Serum scEMC10 levels are negatively associated with age and RMR in humans.

Magnaporthe oryzae endoplasmic reticulum membrane complex regulates the biogenesis of membrane proteins for pathogenicity.

In eukaryotes, the majority of newly synthesized integral membrane proteins are inserted into the endoplasmic reticulum (ER) membrane before transferred to their functional sites. The conserved ER membrane complex (EMC) takes part in the insertion process for tail-anchored membrane proteins. However, the function of EMC in phytopathogenic fungi has not been characterized. Here, we report the identification and functional characterization of two EMC subunits MoEmc5 and MoEmc2 in Magnaporthe oryzae. The knockout mutants ΔMoemc5 and ΔMoemc2 exhibit substantial defect in autophagy, pathogenicity, cell wall integrity, and magnesium ion sensitivity. We demonstrate that the autophagy process was severely impaired in the ΔMoemc5 and ΔMoemc2 mutants because of the low-protein steady-state level of Atg9, the sole membrane-associated autophagy protein. Furthermore, the protein level of membrane proteins Chs4, Fks1, and MoMnr2 is also significantly reduced in the ΔMoemc5 and ΔMoemc2 strains, leading to their supersensitivity to Calcofluor white, Congo red, and magnesium. In addition, MoEmc5, but not MoEmc2, acts as a magnesium transporter independent of its EMC function. Magnaporthe oryzae EMC regulates the biogenesis of membrane proteins for autophagy and virulence; therefore, EMC subunits could be potential targets for fungicide design in the future.

Secreted EMC10 is upregulated in human obesity and its neutralizing antibody prevents diet-induced obesity in mice

Secreted isoform of endoplasmic reticulum membrane complex subunit 10 (scEMC10) is a poorly characterized secreted protein of largely unknown physiological function. Here we demonstrate that scEMC10 is upregulated in people with obesity and is positively associated with insulin resistance. Consistent with a causal role for scEMC10 in obesity, Emc10-/- mice are resistant to diet-induced obesity due to an increase in energy expenditure, while scEMC10 overexpression decreases energy expenditure, thus promoting obesity in mouse. Furthermore, neutralization of circulating scEMC10 using a monoclonal antibody reduces body weight and enhances insulin sensitivity in obese mice. Mechanistically, we provide evidence that scEMC10 can be transported into cells where it binds to the catalytic subunit of PKA and inhibits its stimulatory action on CREB while ablation of EMC10 promotes thermogenesis in adipocytes via activation of the PKA signalling pathway and its downstream targets. Taken together, our data identify scEMC10 as a circulating inhibitor of thermogenesis and a potential therapeutic target for obesity and its cardiometabolic complications.

A specific EMC subunit supports Dengue virus infection by promoting virus membrane fusion essential for cytosolic genome delivery.

Dengue virus (DENV) represents the most common human arboviral infection, yet its cellular entry mechanism remains unclear. The multi-subunit endoplasmic reticulum membrane complex (EMC) supports DENV infection, in part, by assisting the biosynthesis of viral proteins critical for downstream replication steps. Intriguingly, the EMC has also been shown to act at an earlier step prior to viral protein biogenesis, although this event is not well-defined. Here we demonstrate that the EMC subunit EMC4 promotes fusion of the DENV and endosomal membranes during entry, enabling delivery of the viral genome into the cytosol which is then targeted to the ER for viral protein biosynthesis. We also found that EMC4 mediates ER-to-endosome transfer of phosphatidylserine, a phospholipid whose presence in the endosome facilitates DENV-endosomal membrane fusion. These findings clarify the EMC-dependent DENV early entry step, suggesting a mechanism by which an ER-localized host factor can regulate viral fusion at the endosome.

The endoplasmic reticulum membrane complex promotes proteostasis of GABAA receptors

SummaryThe endoplasmic reticulum membrane complex (EMC) plays a critical role in the biogenesis of tail-anchored proteins and a subset of multi-pass membrane proteins in the endoplasmic reticulum (ER). However, because of nearly exclusive expression of neurotransmitter-gated ion channels in the central nervous system (CNS), the role of the EMC in their biogenesis is not well understood. In this study, we demonstrated that the EMC positively regulates the surface trafficking and thus function of endogenous γ-aminobutyric acid type A (GABAA) receptors, the primary inhibitory ion channels in the mammalian brain. Moreover, among ten EMC subunits, EMC3 and EMC6 have the most prominent effect, and overexpression of EMC3 or EMC6 is sufficient to restore the function of epilepsy-associated GABAA receptor variants. In addition, EMC3 and EMC6 demonstrate endogenous interactions with major neuroreceptors, which depends on their transmembrane domains, suggesting a general role of the EMC in the biogenesis of neuroreceptors.

The endoplasmic reticulum membrane protein complex localizes to the mitochondrial - endoplasmic reticulum interface and its subunits modulate phospholipid biosynthesis in Trypanosoma brucei.

The endoplasmic reticulum membrane complex (EMC) is a versatile complex that plays a key role in membrane protein biogenesis in the ER. Deletion of the complex has wide-ranging consequences including ER stress, disturbance in lipid transport and organelle tethering, among others. Here we report the function and organization of the evolutionarily conserved EMC (TbEMC) in the highly diverged eukaryote, Trypanosoma brucei. Using (co-) immunoprecipitation experiments in combination with mass spectrometry and whole cell proteomic analyses of parasites after depletion of select TbEMC subunits, we demonstrate that the TbEMC is composed of 9 subunits that are present in a high molecular mass complex localizing to the mitochondrial-endoplasmic reticulum interface. Knocking out or knocking down of single TbEMC subunits led to growth defects of T. brucei procyclic forms in culture. Interestingly, we found that depletion of individual TbEMC subunits lead to disruption of de novo synthesis of phosphatidylcholine (PC) or phosphatidylethanolamine (PE), the two most abundant phospholipid classes in T. brucei. Downregulation of TbEMC1 or TbEMC3 inhibited formation of PC while depletion of TbEMC8 inhibited PE synthesis, pointing to a role of the TbEMC in phospholipid synthesis. In addition, we found that in TbEMC7 knock-out parasites, TbEMC3 is released from the complex, implying that TbEMC7 is essential for the formation or the maintenance of the TbEMC.

The ER membrane complex (EMC) can functionally replace the Oxa1 insertase in mitochondria.

Two multisubunit protein complexes for membrane protein insertion were recently identified in the endoplasmic reticulum (ER): the guided entry of tail anchor proteins (GET) complex and ER membrane complex (EMC). The structures of both of their hydrophobic core subunits, which are required for the insertion reaction, revealed an overall similarity to the YidC/Oxa1/Alb3 family members found in bacteria, mitochondria, and chloroplasts. This suggests that these membrane insertion machineries all share a common ancestry. To test whether these ER proteins can functionally replace Oxa1 in yeast mitochondria, we generated strains that express mitochondria-targeted Get2-Get1 and Emc6-Emc3 fusion proteins in Oxa1 deletion mutants. Interestingly, the Emc6-Emc3 fusion was able to complement an Δoxa1 mutant and restored its respiratory competence. The Emc6-Emc3 fusion promoted the insertion of the mitochondrially encoded protein Cox2, as well as of nuclear encoded inner membrane proteins, although was not able to facilitate the assembly of the Atp9 ring. Our observations indicate that protein insertion into the ER is functionally conserved to the insertion mechanism in bacteria and mitochondria and adheres to similar topological principles.

The phenotype of homozygous EMC10 variant: A new syndrome with intellectual disability and language impairment.

To explore the cognitive and behavioral phenotype associated with a recently reported variant in endoplasmic reticulum membrane complex EMC10 c.287delG (Gly96Alafs∗9), suggested to cause a novel syndromic neurodevelopmental disorder. Homozygous EMC10 variant identified by a combination of autozygosity mapping and exome sequencing was found in five children (aged 7-18) from a large extended family. Their functioning was compared to normative data as well as to that of age-matched relatives (siblings/cousins), sharing similar familial and demographic characteristics. Neuropsychological, behavioral, and daily functioning were assessed. Performance of all participants with EMC10 variant on both cognitive functioning and adaptive skills was lower than the normal range fulfilling diagnostic criteria for intellectual disability. Their functioning was also lower than that of their matched relatives on most areas of functioning, except visual memory that was found higher, in the low average range. Language difficulty was apparent in all participants with EMC10, and a discrepancy within participants' phenotype was found, with lower verbal abilities compared to visuospatial ability. More behavioral problems were found, although not in all participants with EMC10. Homozygous EMC10 variant was found associated with a phenotype of intellectual disability and language deficits.

629: Endoplasmic reticulum membrane complex is required for CFTR biogenesis and activation in vivo

629: Endoplasmic reticulum membrane complex is required for CFTR biogenesis and activation in vivo

An alternative pathway for membrane protein biogenesis at the endoplasmic reticulum

The heterotrimeric Sec61 complex is a major site for the biogenesis of transmembrane proteins (TMPs), accepting nascent TMP precursors that are targeted to the endoplasmic reticulum (ER) by the signal recognition particle (SRP). Unlike most single-spanning membrane proteins, the integration of type III TMPs is completely resistant to small molecule inhibitors of the Sec61 translocon. Using siRNA-mediated depletion of specific ER components, in combination with the potent Sec61 inhibitor ipomoeassin F (Ipom-F), we show that type III TMPs utilise a distinct pathway for membrane integration at the ER. Hence, following SRP-mediated delivery to the ER, type III TMPs can uniquely access the membrane insertase activity of the ER membrane complex (EMC) via a mechanism that is facilitated by the Sec61 translocon. This alternative EMC-mediated insertion pathway allows type III TMPs to bypass the Ipom-F-mediated blockade of membrane integration that is seen with obligate Sec61 clients.

Membrane protein biogenesis at the ER: the highways and byways.

The Sec61 complex is the major protein translocation channel of the endoplasmic reticulum (ER), where it plays a central role in the biogenesis of membrane and secretory proteins. Whilst Sec61-mediated protein translocation is typically coupled to polypeptide synthesis, suggestive of significant complexity, an obvious characteristic of this core translocation machinery is its surprising simplicity. Over thirty years after its initial discovery, we now understand that the Sec61 complex is in fact the central piece of an elaborate jigsaw puzzle, which can be partly solved using new research findings. We propose that the Sec61 complex acts as a dynamic hub for co-translational protein translocation at the ER, proactively recruiting a range of accessory complexes that enhance and regulate its function in response to different protein clients. It is now clear that the Sec61 complex does not have a monopoly on co-translational insertion, with some transmembrane proteins preferentially utilising the ER membrane complex instead. We also have a better understanding of post-insertion events, where at least one membrane-embedded chaperone complex can capture the newly inserted transmembrane domains of multi-span proteins and co-ordinate their assembly into a native structure. Having discovered this array of Sec61-associated components and competitors, our next challenge is to understand how they act together in order to expand the range and complexity of the membrane proteins that can be synthesised at the ER. Furthermore, this diversity of components and pathways may open up new opportunities for targeted therapeutic interventions designed to selectively modulate protein biogenesis at the ER.

Cryo-EM structures of the endoplasmic reticulum membrane complex.

The transmembrane α-helices of membrane proteins are in general highly hydrophobic, and they enter the lipid bilayer through a lateral gate in the Sec61 translocon. However, some transmembrane α-helices are less hydrophobic and form membrane channels or substrate-binding pockets. Insertion of these amphipathic transmembrane α-helices into the membrane requires the specific membrane-embedded insertase called the endoplasmic reticulum membrane complex (EMC), which is a multi-subunit chaperone distinct from the GET insertase complex. Four recent cryo-electron microscopy studies on the eukaryotic EMC have revealed their remarkable architectural conservation from yeast to humans; a general consensus on the substrate transmembrane helix-binding pocket; and the evolutionary link to the prokaryotic insertases of the tail-anchored membrane proteins. These structures provide a solid framework for future mechanistic investigation.

Quality Control of ER Membrane Proteins by the RNF185/Membralin Ubiquitin Ligase Complex

SummaryMisfolded proteins in the endoplasmic reticulum (ER) are degraded by ER-associated degradation (ERAD). Although ERAD components involved in degradation of luminal substrates are well characterized, much less is known about quality control of membrane proteins. Here, we analyzed the degradation pathways of two short-lived ER membrane model proteins in mammalian cells. Using a CRISPR-Cas9 genome-wide library screen, we identified an ERAD branch required for quality control of a subset of membrane proteins. Using biochemical and mass spectrometry approaches, we showed that this ERAD branch is defined by an ER membrane complex consisting of the ubiquitin ligase RNF185, the ubiquitin-like domain containing proteins TMUB1/2 and TMEM259/Membralin, a poorly characterized protein. This complex cooperates with cytosolic ubiquitin ligase UBE3C and p97 ATPase in degrading their membrane substrates. Our data reveal that ERAD branches have remarkable specificity for their membrane substrates, suggesting that multiple, perhaps combinatorial, determinants are involved in substrate selection.

208-OR: Emc10emc10, a Novel Circulating Factor for Systemic Energy and Metabolic Homeostasis

Proteins secreted from metabolic organs play critical roles in fine-tuning and maintenance of systemic metabolic and energy homeostasis. In this study, we identified a novel obesity-regulated circulating protein, scEmc10 (secreted-isoform of endoplasmic reticulum membrane complex subunit 10), that regulates systemic energy homeostasis. We initially observed that scEmc10 is modulated in adipose tissues in diet-induced obesity (DIO) models and subsequently in obese patients. Using an in-house-developed chemiluminescent immunoassay, we found that circulating Emc10 is significantly upregulated in overweight and obese patients and can be normalized by bariatric surgery-induced weight loss. Subsequently, we observed that ablation of Emc10 protects mice from DIO and associated metabolic dysregulation. Conversely, enhanced circulating Emc10 by expression of secretable AAV-hscEmc10 in the liver promotes obesity and associated metabolic dysfunctions in mice fed with either chow or high fat diet. Subsequent metabolic analysis revealed that Emc10 KO mice on HFD have elevated whole body energy expenditure compared to WT controls via an increase in oxygen consumption in both the brown and inguinal subcutaneous fat. Our mechanistic analysis demonstrated that ablation of scEmc10 promotes adaptive thermogenesis in adipocytes via activation of the PKA signaling pathway and its downstream targets, particularly CREB and p38MAPK. Finally, we also showed that neutralization of scEmc10 in the circulation with a monoclonal antibody reduced body weight and improved diet-induced metabolic dysfunction in obese mice. Our findings implicate Emc10 as a novel therapeutic target for counteracting the development of obesity and obesity-induced metabolic dysfunction. Disclosure Y. Li: None. X. Wang: None. G. Qiang: None. M.A. McCann: None. V. Gil: None. M. Blüher: Advisory Panel; Self; AstraZeneca, Boehringer Ingelheim Pharmaceuticals, Inc., Lilly Diabetes, Novo Nordisk A/S, Sanofi. Speaker’s Bureau; Self; Amgen, Novartis AG. C. Liew: None. Funding National Institutes of Health (R01DK090210, R01DK109015); National Natural Science Foundation of China (81070647, 81370936, 81873645, 81873853)

Pre-emptive Quality Control of a Misfolded Membrane Protein by Ribosome-Driven Effects.

SummaryCells possess multiple mechanisms that protect against the accumulation of toxic aggregation-prone proteins. Here, we identify a pre-emptive pathway that reduces synthesis of membrane proteins that have failed to properly assemble in the endoplasmic reticulum (ER). We show that loss of the ER membrane complex (EMC) or mutation of the Sec61 translocon causes reduced synthesis of misfolded forms of the yeast ABC transporter Yor1. Synthesis defects are rescued by various ribosomal mutations, as well as by reducing cellular ribosome abundance. Genetic and biochemical evidence point to a ribosome-associated quality-control pathway triggered by ribosome collisions when membrane domain insertion and/or folding fails. In support of this model, translation initiation also contributes to synthesis defects, likely by modulating ribosome abundance on the message. Examination of translation efficiency across the yeast membrane proteome revealed that polytopic membrane proteins have relatively low ribosome abundance, providing evidence for translational tuning to balance protein synthesis and folding. We propose that by modulating translation rates of poorly folded proteins, cells can pre-emptively protect themselves from potentially toxic aberrant transmembrane proteins.

Assessment of tick-borne flavivirus host factors through genome-scale screens

Powassan virus (POWV) is the only tick-borne flavivirus (TBFV) known to circulate in North America. Although there are relatively few documented cases of POWV disease, investigations into POWV are justified due to the increasing incidence of infection and significant case fatality rate associated with this virus. To better describe the molecular biology of the POWV replication cycle in mammalian cells, we performed genome-scale screens to uncover host factors required for viral replication. Putative proviral host factors were identified by infecting pools of cells containing knockout mutations in non-essential genes with POWV, followed by analysis of cells resistant to virus-induced cell death. Many endoplasmic reticulum membrane complex proteins were revealed in these screens, suggesting that TBFVs share some common host cell hijacking strategies with mosquito-borne flaviviruses. Candidate proteins that function in cell-matrix adhesion, glycosylation, or RNA binding in uninfected cells were the focus of validation studies. We used single-gene knockout cell lines to investigate possible proviral roles for specific proteins in the replication cycle of either POWV or Langat virus (LGTV), a non-pathogenic and model TBFV. Proteins identified in the POWV screens were not necessarily critical for LGTV replication. These results suggest that our screens were able to identify both pan-flaviviral, as well as POWV-specific, host gene products exploited during virus replication. Ongoing work is focused on characterizing distinct host-cell requirements of diverse flaviviruses. This genetic assessment of POWV replication factors, in combination with ensuing mechanistic studies, will provide possible avenues for the development of host-targeting countermeasures.

Important role of MAMs in bifurcation and coherence resonance of calcium oscillations

MAM (mitochondria-associated endoplasmic reticulum membrane complex) is kind of complexes formed between the endoplasmic reticulum (ER) and mitochondria, and it plays an important role in calcium signal transduction of cell. Coherence resonance is cooperative effect of noise and nonlinear system. In the field of life science, coherence resonance reflects the rhythm of life, so studying it by using the theory and method of nonlinear stochastic dynamics has very important biological significance. On the basis of the calcium oscillation model considering the role of MAMs proposed by Szopa et al., and taking two important indexes - the maximal permeability of the Ca2+ channels in the ER membrane (kch) and the maximal permeability of MCU (mitochondrial calcium uniporter) at MAMs (kMAM) as the analysis parameters, this paper further studies the bifurcation characteristic of the system, including two-parameter bifurcation and one-parameter bifurcation, and theoretically expounds the reasons for the generation and disappearance of calcium oscillations. Then, on the basis of bifurcation analysis, we focus on the coherence resonance characteristic of calcium signals and its physiological significance when analysis parameters are affected by environmental noise.