SARS-CoV-2 Mpro responds to oxidation by forming disulfide and NOS/SONOS bonds.

The main protease (Mpro) of SARS-CoV-2 is critical for viral function and a key drug target. Mpro is only active when reduced; turnover ceases upon oxidation but is restored by re-reduction. This suggests the system has evolved to survive periods in an oxidative environment, but the mechanism of this protection has not been confirmed. Here, we report a crystal structure of oxidized Mpro showing a disulfide bond between the active site cysteine, C145, and a distal cysteine, C117. Previous work proposed this disulfide provides the mechanism of protection from irreversible oxidation. Mpro forms an obligate homodimer, and the C117-C145 structure shows disruption of interactions bridging the dimer interface, implying a correlation between oxidation and dimerization. We confirm dimer stability is weakened in solution upon oxidation. Finally, we observe the protein's crystallization behavior is linked to its redox state. Oxidized Mpro spontaneously forms a distinct, more loosely packed lattice. Seeding with crystals of this lattice yields a structure with an oxidation pattern incorporating one cysteine-lysine-cysteine (SONOS) and two lysine-cysteine (NOS) bridges. These structures further our understanding of the oxidative regulation of Mpro and the crystallization conditions necessary to study this structurally.

Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges.

G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.

Vitamin K Epoxide Reductase Complex-Protein Disulphide Isomerase Assemblies in the Thiol-Disulphide Exchange Reactions: Portrayal of Precursor-to-Successor Complexes.

The human Vitamin K Epoxide Reductase Complex (hVKORC1), a key enzyme that converts vitamin K into the form necessary for blood clotting, requires for its activation the reducing equivalents supplied by its redox partner through thiol-disulphide exchange reactions. The functionally related molecular complexes assembled during this process have never been described, except for a proposed de novo model of a 'precursor' complex of hVKORC1 associated with protein disulphide isomerase (PDI). Using numerical approaches (in silico modelling and molecular dynamics simulation), we generated alternative 3D models for each molecular complex bonded either covalently or non-covalently. These models differ in the orientation of the PDI relative to hVKORC1 and in the cysteine residue involved in forming protein-protein disulphide bonds. Based on a comparative analysis of these models' shape, folding, and conformational dynamics, the most probable putative complexes, mimicking the 'precursor', 'intermediate', and 'successor' states, were suggested. In addition, we propose using these complexes to develop the 'allo-network drugs' necessary for treating blood diseases.

Linking Disulfide Levels and NAD+ Metabolism with Alzheimer's Disease for Diagnostic Modeling and Target Drug Analysis.

BACKGROUND: Alzheimer's disease (AD) is a condition that affects the nervous system and that requires considerably more in-depth study. Abnormal Nicotinamide Adenine Dinucleotide (NAD+) metabolism and disulfide levels have been demonstrated in AD. This study investigated novel hub genes for disulfide levels and NAD+ metabolism in relation to the diagnosis and therapy of AD. METHODS: Data from the gene expression omnibus (GEO) database were analyzed. Hub genes related to disulfide levels, NAD+ metabolism, and AD were identified from overlapping genes for differentially expressed genes (DEGs), genes in the NAD+ metabolism or disulfide gene sets, and module genes obtained by weighted gene co-expression network analysis (WGCNA). Pathway analysis of these hub genes was performed by Gene Set Enrichment Analysis (GSEA). A diagnostic model for AD was constructed based on the expression level of hub genes in brain samples. CIBERSORT was used to evaluate immune cell infiltration and immune factors correlating with hub gene expression. The DrugBank database was also used to identify drugs that target the hub genes. RESULTS: We identified 3 hub genes related to disulfide levels in AD and 9 related to NAD+ metabolism in AD. Pathway analysis indicated these 12 genes were correlated with AD. Stepwise regression analysis revealed the area under the curve (AUC) for the predictive model based on the expression of these 12 hub genes in brain tissue was 0.935, indicating good diagnostic performance. Additionally, analysis of immune cell infiltration showed the hub genes played an important role in AD immunity. Finally, 33 drugs targeting 10 hub genes were identified using the DrugBank database. Some of these have been clinically approved and may be useful for AD therapy. CONCLUSION: Hub genes related to disulfide levels and NAD+ metabolism are promising biomarkers for the diagnosis of AD. These genes may contribute to a better understanding of the pathogenesis of AD, as well as to improved drug therapy.

[Cloning and functional analysis of AcFMO from onion during alliine biosynthesis].

Flavin-containing monooxygenase (FMO) is the key enzyme in the biosynthesis pathway of CSOs with sulfur oxidation. In order to explore the molecular regulatory mechanism of FMO in the synthesis of onion CSOs, based on transcriptome database and phylogenetic analysis, one AcFMO gene that may be involved in alliin synthesis was obtained, the AcFMO had a cDNA of 1 374 bp and encoded 457 amino acids, which was evolutionarily closest to the AsFMO of garlic. Real-time fluorescence quantitative polymerase chain reaction (qRT-PCR) indicated that AcFMO was the highest in the flowers and the lowest in the leaf sheaths. The results of subcellular localization showed that the AcFMO gene product was widely distributed throughout the cell A yeast expression vector was constructed, and the AcFMO gene was ecotopically overexpressed in yeast to further study the enzyme function in vitro and could catalyze the synthesis of alliin by S-allyl-l-cysteine. In summary, the cloning and functional identification of AcFMO have important reference value for understanding the biosynthesis of CSOs in onions.

The structure of Mycobacterium thermoresistibile MmpS5 reveals a conserved disulfide bond across mycobacteria.

The tuberculosis (TB) emergency has been a pressing health threat for decades. With the emergence of drug-resistant TB and complications from the COVID-19 pandemic, the TB health crisis is more serious than ever. Mycobacterium tuberculosis (Mtb), the causative agent of TB, requires iron for its survival. Thus, Mtb has evolved several mechanisms to acquire iron from the host. Mtb produces two siderophores, mycobactin and carboxymycobactin, which scavenge for host iron. Mtb siderophore-dependent iron acquisition requires the export of apo-siderophores from the cytosol to the host environment and import of iron-bound siderophores. The export of Mtb apo-siderophores across the inner membrane is facilitated by two mycobacterial inner membrane proteins with their cognate periplasmic accessory proteins, designated MmpL4/MmpS4 and MmpL5/MmpS5. Notably, the Mtb MmpL4/MmpS4 and MmpL5/MmpS5 complexes have also been implicated in the efflux of anti-TB drugs. Herein, we solved the crystal structure of M. thermoresistibile MmpS5. The MmpS5 structure reveals a previously uncharacterized, biologically relevant disulfide bond that appears to be conserved across the Mycobacterium MmpS4/S5 homologs, and comparison with structural homologs suggests that MmpS5 may be dimeric.

Protein disulfide isomerase 1 is required for RodA assembling-based conidial hydrophobicity of Aspergillus fumigatus.

The hydrophobic layer of Aspergillus conidia, composed of RodA, plays a crucial role in conidia transfer and immune evasion. It self-assembles into hydrophobic rodlets through intramolecular disulfide bonds. However, the secretory process of RodA and its regulatory elements remain unknown. Since protein disulfide isomerase (PDI) is essential for the secretion of many disulfide-bonded proteins, we investigated whether PDI is also involved in RodA secretion and assembly. By gene knockout and phenotypic analysis, we found that Pdi1, one of the four PDI-related proteins of Aspergillus fumigatus, determines the hydrophobicity and integrity of the rodlet layer of the conidia. Preservation of the thioredoxin-active domain of Pdi1 was sufficient to maintain conidial hydrophobicity, suggesting that Pdi1 mediates RodA assembly through its disulfide isomerase activity. In the absence of Pdi1, the disulfide mismatch of RodA in conidia may prevent its delivery from the inner to the outer layer of the cell wall for rodlet assembly. This was demonstrated using a strain expressing a key cysteine-mutated RodA. The dormant conidia of the Pdi1-deficient strain (Δpdi) elicited an immune response, suggesting that the defective conidia surface in the absence of Pdi1 exposes internal immunogenic sources. In conclusion, Pdi1 ensures the correct folding of RodA in the inner layer of conidia, facilitating its secretion into the outer layer of the cell wall and allowing self-assembly of the hydrophobic layer. This study has identified a regulatory element for conidia rodlet assembly.IMPORTANCEAspergillus fumigatus is the major cause of invasive aspergillosis, which is mainly transmitted by the inhalation of conidia. The spread of conidia is largely dependent on their hydrophobicity, which is primarily attributed to the self-assembly of the hydrophobic protein RodA on the cell wall. However, the mechanisms underlying RodA secretion and transport to the outermost layer of the cell wall are still unclear. Our study identified a critical role for Pdi1, a fungal protein disulfide isomerase found in regulating RodA secretion and assembly. Inhibition of Pdi1 prevents the formation of correct S-S bonds in the inner RodA, creating a barrier to RodA delivery and resulting in a defective hydrophobic layer. Our findings provided insight into the formation of the conidial hydrophobic layer and suggested potential drug targets to inhibit A. fumigatus infections by limiting conidial dispersal and altering their immune inertia.

A genetic screen identifies a role for oprF in Pseudomonas aeruginosa biofilm stimulation by subinhibitory antibiotics.

Biofilms are surface-associated communities of bacteria that grow in a self-produced matrix of polysaccharides, proteins, and extracellular DNA (eDNA). Sub-minimal inhibitory concentrations (sub-MIC) of antibiotics induce biofilm formation, potentially as a defensive response to antibiotic stress. However, the mechanisms behind sub-MIC antibiotic-induced biofilm formation are unclear. We show that treatment of Pseudomonas aeruginosa with multiple classes of sub-MIC antibiotics with distinct targets induces biofilm formation. Further, addition of exogenous eDNA or cell lysate failed to increase biofilm formation to the same extent as antibiotics, suggesting that the release of cellular contents by antibiotic-driven bacteriolysis is insufficient. Using a genetic screen for stimulation-deficient mutants, we identified the outer membrane porin OprF and the ECF sigma factor SigX as important. Similarly, loss of OmpA - the Escherichia coli OprF homolog - prevented sub-MIC antibiotic stimulation of E. coli biofilms. Our screen also identified the periplasmic disulfide bond-forming enzyme DsbA and a predicted cyclic-di-GMP phosphodiesterase encoded by PA2200 as essential for biofilm stimulation. The phosphodiesterase activity of PA2200 is likely controlled by a disulfide bond in its regulatory domain, and folding of OprF is influenced by disulfide bond formation, connecting the mutant phenotypes. Addition of reducing agent dithiothreitol prevented sub-MIC antibiotic biofilm stimulation. Finally, activation of a c-di-GMP-responsive promoter follows treatment with sub-MIC antibiotics in the wild-type but not an oprF mutant. Together, these results show that antibiotic-induced biofilm formation is likely driven by a signaling pathway that translates changes in periplasmic redox state into elevated biofilm formation through increases in c-di-GMP.

Mitochondria-Targeted Prodrug Nanoassemblies for Efficient Ferroptosis-Based Therapy via Devastating Ferroptosis Defense Systems.

Ferroptosis is a form of regulated cell death accompanied by lipid reactive oxygen species (ROS) accumulation in an iron-dependent manner. However, the efficiency of tumorous ferroptosis was seriously restricted by intracellular ferroptosis defense systems, the glutathione peroxidase 4 (GPX4) system, and the ubiquinol (CoQH2) system. Inspired by the crucial role of mitochondria in the ferroptosis process, we reported a prodrug nanoassembly capable of unleashing potent mitochondrial lipid peroxidation and ferroptotic cell death. Dihydroorotate dehydrogenase (DHODH) inhibitor (QA) was combined with triphenylphosphonium moiety through a disulfide-containing linker to engineer well-defined nanoassemblies (QSSP) within a single-molecular framework. After being trapped in cancer cells, the acidic condition provoked the structural disassembly of QSSP to liberate free prodrug molecules. The mitochondrial membrane-potential-driven accumulation of the lipophilic cation prodrug was delivered explicitly into the mitochondria. Afterward, the thiol-disulfide exchange would occur accompanied by downregulation of reduced glutathione levels, thus resulting in mitochondria-localized GPX4 inactivation for ferroptosis. Simultaneously, the released QA from the hydrolysis reaction of the adjacent ester bond could further devastate mitochondrial defense and evoke robust ferroptosis via the DHODH-CoQH2 system. This subcellular targeted nanoassembly provides a reference for designing ferroptosis-based strategy for efficient cancer therapy through interfering antiferroptosis systems.

A phage-displayed disulfide constrained peptide discovery platform yields novel human plasma protein binders.

Disulfide constrained peptides (DCPs) show great potential as templates for drug discovery. They are characterized by conserved cysteine residues that form intramolecular disulfide bonds. Taking advantage of phage display technology, we designed and generated twenty-six DCP phage libraries with enriched molecular diversity to enable the discovery of ligands against disease-causing proteins of interest. The libraries were designed based on five DCP scaffolds, namely Momordica charantia 1 (Mch1), gurmarin, Asteropsin-A, antimicrobial peptide-1 (AMP-1), and potato carboxypeptidase inhibitor (CPI). We also report optimized workflows for screening and producing synthetic and recombinant DCPs. Examples of novel DCP binders identified against various protein targets are presented, including human IgG Fc, serum albumin, vascular endothelial growth factor-A (VEGF-A) and platelet-derived growth factor (PDGF). We identified DCPs against human IgG Fc and serum albumin with sub-micromolar affinity from primary panning campaigns, providing alternative tools for potential half-life extension of peptides and small protein therapeutics. Overall, the molecular diversity of the DCP scaffolds included in the designed libraries, coupled with their distinct biochemical and biophysical properties, enables efficient and robust identification of de novo binders to drug targets of therapeutic relevance.

Deciphering the mechanism of glutaredoxin-catalyzed roGFP2 redox sensing reveals a ternary complex with glutathione for protein disulfide reduction.

Glutaredoxins catalyze the reduction of disulfides and are key players in redox metabolism and regulation. While important insights were gained regarding the reduction of glutathione disulfide substrates, the mechanism of non-glutathione disulfide reduction remains highly debated. Here we determined the rate constants for the individual redox reactions between PfGrx, a model glutaredoxin from Plasmodium falciparum, and redox-sensitive green fluorescent protein 2 (roGFP2), a model substrate and versatile tool for intracellular redox measurements. We show that the PfGrx-catalyzed oxidation of roGFP2 occurs via a monothiol mechanism and is up to three orders of magnitude faster when roGFP2 and PfGrx are fused. The oxidation kinetics of roGFP2-PfGrx fusion constructs reflect at physiological GSSG concentrations the glutathionylation kinetics of the glutaredoxin moiety, thus allowing intracellular structure-function analysis. Reduction of the roGFP2 disulfide occurs via a monothiol mechanism and involves a ternary complex with GSH and PfGrx. Our study provides the mechanistic basis for understanding roGFP2 redox sensing and challenges previous mechanisms for protein disulfide reduction.

Organ/Cell-Selective Intracellular Delivery of Biologics via N-Acetylated Galactosamine-Functionalized Polydisulfide Conjugates.

Biologics, including proteins and antisense oligonucleotides (ASOs), face significant challenges when it comes to achieving intracellular delivery within specific organs or cells through systemic administrations. In this study, we present a novel approach for delivering proteins and ASOs to liver cells, both in vitro and in vivo, using conjugates that tether N-acetylated galactosamine (GalNAc)-functionalized, cell-penetrating polydisulfides (PDSs). The method involves the thiol-bearing cargo-mediated ring-opening polymerization of GalNAc-functionalized lipoamide monomers through the so-called aggregation-induced polymerization, leading to the formation of site-specific protein/ASO-PDS conjugates with narrow dispersity. The hepatocyte-selective intracellular delivery of the conjugates arises from a combination of factors, including first GalNAc binding with ASGPR receptors on liver cells, leading to cell immobilization, and the subsequent thiol-disulfide exchange occurring on the cell surface, promoting internalization. Our findings emphasize the critical role of the close proximity of the PDS backbone to the cell surface, as it governs the success of thiol-disulfide exchange and, consequently, cell penetration. These conjugates hold tremendous potential in overcoming the various biological barriers encountered during systemic and cell-specific delivery of biomacromolecular cargos, opening up new avenues for the diagnosis and treatment of a range of liver-targeting diseases.

Scalable production of recombinant three-finger proteins: from inclusion bodies to high quality molecular probes.

BACKGROUND: The three-finger proteins are a collection of disulfide bond rich proteins of great biomedical interests. Scalable recombinant expression and purification of bioactive three-finger proteins is quite difficult. RESULTS: We introduce a working pipeline for expression, purification and validation of disulfide-bond rich three-finger proteins using E. coli as the expression host. With this pipeline, we have successfully obtained highly purified and bioactive recombinant α-Βungarotoxin, k-Bungarotoxin, Hannalgesin, Mambalgin-1, α-Cobratoxin, MTα, Slurp1, Pate B etc. Milligrams to hundreds of milligrams of recombinant three finger proteins were obtained within weeks in the lab. The recombinant proteins showed specificity in binding assay and six of them were crystallized and structurally validated using X-ray diffraction protein crystallography. CONCLUSIONS: Our pipeline allows refolding and purifying recombinant three finger proteins under optimized conditions and can be scaled up for massive production of three finger proteins. As many three finger proteins have attractive therapeutic or research interests and due to the extremely high quality of the recombinant three finger proteins we obtained, our method provides a competitive alternative to either their native counterparts or chemically synthetic ones and should facilitate related research and applications.

CdbC: a disulfide bond isomerase involved in the refolding of mycoloyltransferases in Corynebacterium glutamicum cells exposed to oxidative conditions.

In Corynebacterium glutamicum cells, cdbC, which encodes a protein containing the CysXXCys motif, is regulated by the global redox-responsive regulator OsnR. In this study, we assessed the role of the periplasmic protein CdbC in disulfide bond formation and its involvement in mycomembrane biosynthesis. Purified CdbC efficiently refolded scrambled RNaseA, exhibiting prominent disulfide bond isomerase activity. The transcription of cdbC was decreased in cells grown in the presence of the reductant dithiothreitol (DTT). Moreover, unlike wild-type and cdbC-deleted cells, cdbC-overexpressing (P180-cdbC) cells grown in the presence of DTT exhibited retarded growth, abnormal cell morphology, increased cell surface hydrophobicity and altered mycolic acid composition. P180-cdbC cells cultured in a reducing environment accumulated trehalose monocorynomycolate, indicating mycomembrane deformation. Similarly, a two-hybrid analysis demonstrated the interaction of CdbC with the mycoloyltransferases MytA and MytB. Collectively, our findings suggest that CdbC, a periplasmic disulfide bond isomerase, refolds misfolded MytA and MytB and thereby assists in mycomembrane biosynthesis in cells exposed to oxidative conditions.

The transport activity of the multidrug ABC transporter BmrA does not require a wide separation of the nucleotide-binding domains.

ATP-binding cassette (ABC) transporters are ubiquitous membrane proteins responsible for the translocation of a wide diversity of substrates across biological membranes. Some of them confer multidrug or antimicrobial resistance to cancer cells and pathogenic microorganisms, respectively. Despite a wealth of structural data gained in the last two decades, the molecular mechanism of these multidrug efflux pumps remains elusive, including the extent of separation between the two nucleotide-binding domains (NBDs) during the transport cycle. Based on recent outward-facing structures of BmrA, a homodimeric multidrug ABC transporter from Bacillus subtilis, we introduced a cysteine mutation near the C-terminal end of the NBDs to analyze the impact of disulfide-bond formation on BmrA function. Interestingly, the presence of the disulfide bond between the NBDs did not prevent the ATPase, nor did it affect the transport of Hoechst 33342 and doxorubicin. Yet, the 7-amino-actinomycin D was less efficiently transported, suggesting that a further opening of the transporter might improve its ability to translocate this larger compound. We solved by cryo-EM the apo structures of the cross-linked mutant and the WT protein. Both structures are highly similar, showing an intermediate opening between their NBDs while their C-terminal extremities remain in close proximity. Distance measurements obtained by electron paramagnetic resonance spectroscopy support the intermediate opening found in these 3D structures. Overall, our data suggest that the NBDs of BmrA function with a tweezers-like mechanism distinct from the related lipid A exporter MsbA.

Thiol-Disulfide Exchange Kinetics and Redox Potential of the Coenzyme M and Coenzyme B Heterodisulfide, an Electron Acceptor Coupled to Energy Conservation in Methanogenic Archaea.

Methanogenic and methanotrophic archaea play important roles in the global carbon cycle by interconverting CO2 and methane. To conserve energy from these metabolic pathways that happen close to the thermodynamic equilibrium, specific electron carriers have evolved to balance the redox potentials between key steps. Reduced ferredoxins required to activate CO2 are provided by energetical coupling to the reduction of the high-potential heterodisulfide (HDS) of coenzyme M (2-mercaptoethanesulfonate) and coenzyme B (7-mercaptoheptanoylthreonine phosphate). While the standard redox potential of this important HDS has been determined previously to be -143 mV (Tietze et al. 2003 DOI: 10.1002/cbic.200390053), we have measured thiol disulfide exchange kinetics and reassessed this value by equilibrating thiol-disulfide mixtures of coenzyme M, coenzyme B, and mercaptoethanol. We determined the redox potential of the HDS of coenzyme M and coenzyme B to be -16.4±1.7 mV relative to the reference thiol mercaptoethanol (E0 '=-264 mV). The resulting E0 ' values are -281 mV for the HDS, -271 mV for the homodisulfide of coenzyme M, and -270 mV for the homodisulfide of coenzyme B. We discuss the importance of these updated values for the physiology of methanogenic and methanotrophic archaea and their implications in terms of energy conservation.

High purity of human secreted phospholipase A2 group IIE in Pichia pastoris using basal salts medium comparison with YPD medium.

Secreted phospholipase A2s (sPLA2s) are a group of enzymes with 6-8 disulfide bonds that participate in numerous physiological processes by catalyzing the hydrolysis of phospholipids at the sn-2 position. Due to their high content of disulfide bonds and hydrolytic activity toward cell membranes, obtaining the protein of sPLA2s in the soluble and active form is challenging, which hampers their functional study. In this study, one member of recombinant human sPLA2s, tag-free group IIE (GIIE), was expressed in Pichia pastoris. The protein GIIE was purified from the crude culture supernatant by a two-step chromatography procedure, a combination of cation exchange and size-exclusion chromatography. In the shake flask fermentation, Protein of GIIE with higher purity was successfully obtained, using basal salts medium (BSM) instead of YPD medium. In the large-scale fermentation, each liter of BSM produced a final yield of 1.2 mg pure protein GIIE. This protocol will facilitate further research of GIIE and provide references for the production of other sPLA2 members.

A Functional Hydrogel Bead-Based High-Throughput Screening System for Mammalian Cells with Enhanced Secretion of Therapeutic Antibodies.

Droplet-based high-throughput screening systems are an emerging technology that provides a quick test to screen millions of cells with distinctive characteristics. Biopharmaceuticals, specifically therapeutic proteins, are produced by culturing cells that secrete heterologous recombinant proteins with different populations and expression levels; therefore, a technology to discriminate cells that produce more target proteins is needed. Here, we present a droplet-based microfluidic strategy for encapsulating, screening, and selecting target cells with redox-responsive hydrogel beads (HBs). As a proof-of-concept study, we demonstrate the enrichment of hybridoma cells with enhanced capability of antibody secretion using horseradish peroxidase (HRP)-catalyzed hydrogelation of tetra-thiolate poly(ethylene glycol); hybridoma cells were encapsulated in disulfide-bonded HBs. Recombinant protein G or protein M with a C-terminal cysteine residue was installed in the HBs via disulfide bonding to capture antibodies secreted from the cells. HBs were fluorescently stained by adding the protein L-HRP conjugate using a tyramide signal amplification system. HBs were then separated by fluorescence-activated droplet sorting and degraded by reducing the disulfide bonds to recover the target cells. Finally, we succeeded in the selection of hybridoma cells with enhanced antibody secretion, indicating the potential of this system in the therapeutic protein production.

Fingerprint of the oxido-reductase ERO1: A protein disulfide bond producer and supporter of cancer.

Endoplasmic reticulum oxidoreductin 1 (ERO1) alpha (ERO1A) is an endoplasmic reticulum (ER)-localized protein disulfide oxidoreductase, involved in the disulfide bond formation of proteins. ERO1's activity in oxidative protein folding is redundant in higher eukaryotes and its loss is well compensated. Although it is dispensable in non-cancer cells, high ERO1 levels are seen with different cancers and predict their malignant phenotype. ERO1 fosters tumor aggressiveness and the response to drug therapy in hypoxic and highly metastatic tumors. It regulates vascular endothelial growth factor (VEGF) levels, oxidative folding and N-glycosylation in hypoxic conditions, boosting tumor fitness and angiogenesis on multiple levels. In addition, ERO1 regulates protein death ligand-1 (PD-L1) on tumors, interfering with the related immune surveillance mechanism, hence acting on the tumors' response to immune check-point inhibitors (ICI). This all points to inhibition of ERO1 as an effective pharmacological tool, selectively targeting tumors while sparing non-cancer cells from cytotoxicity. The critical discussion here closely examines the molecular basis for ERO1's involvement in tumors and ERO1 inhibition strategies for their treatment.

Cytoplasmic production of Fabs in chemically defined media in fed-batch fermentation.

Fragment of antigen-binding region (Fab) of antibodies are important biomolecules, with a broad spectrum of functionality in the biomedical field. While full length antibodies are usually produced in mammalian cells, the smaller size, lack of N-glycosylation and less complex structure of Fabs make production in microbial cell factories feasible. Since Fabs contain disulfide bonds, such production is often done in the periplasm, but there the formation of the inter-molecular disulfide bond between light and heavy chains can be problematic. Here we studied the use of the CyDisCo system (cytoplasmic disulfide bond formation in E. coli) to express two Fabs (Herceptin and Maa48) in the cytoplasm of E. coli in fed-batch fermentation using a generic chemically defined media. We were able to solubly express both Fabs with purified yields of 565 mg/L (Maa48) and 660 mg/L (Herceptin) from low density fermentation. Both proteins exhibited CD spectra consistent with natively folded protein and both were biologically active. To our knowledge this is the first demonstration of high-level production of biological active Fabs in the cytoplasm of E. coli in industrially relevant fermentation conditions.