Identification and characterization of spodoptera frugiperda furin: a thermostable subtilisin-like endopeptidase

The Possible Role of Microbial Proteases in Facilitating SARS-CoV-2 Brain Invasion

SARS-CoV-2 has been shown to display proclivity towards organs bearing angiotensin-converting enzyme (ACE2) expression cells. Of interest herein is the ability of the virus to exhibit neurotropism. However, there is limited information on how this virus invades the brain. With this contribution, we explore how, in the context of a microbial co-infection using a cryptococcal co-infection as a model, SARS-CoV-2 could reach the brain. We theorise that the secretion of proteases by disseminated fungal cells might also activate the S2 domain of the viral spike glycoprotein for membrane fusion with brain endothelial cells leading to endocytosis. Understanding this potential invasion mechanism could lead to better SARS-CoV-2 intervention measures, which may also be applicable in instances of co-infection, especially with protease-secreting pathogens.

Improving the baculovirus expression vector system with vankyrin-enhanced technology

The baculovirus expression vector system (BEVS) is a widely used platform for the production of recombinant eukaryotic proteins. However, the BEVS has limitations in comparison to other higher eukaryotic expression systems. First, the insect cell lines used in the BEVS cannot produce glycoproteins with complex‐type N‐glycosylation patterns. Second, protein production is limited as cells die and lyse in response to baculovirus infection. To delay cell death and lysis, we transformed several insect cell lines with an expression plasmid harboring a vankyrin gene (P‐vank‐1), which encodes an anti‐apoptotic protein. Specifically, we transformed Sf9 cells, Trichoplusia ni High Five^TM cells, and SfSWT‐4 cells, which can produce glycoproteins with complex‐type N‐glycosylation patterns. The latter was included with the aim to increase production of glycoproteins with complex N‐glycans, thereby overcoming the two aforementioned limitations of the BEVS. To further increase vankyrin expression levels and further delay cell death, we also modified baculovirus vectors with the P‐vank‐1 gene. We found that cell lysis was delayed and recombinant glycoprotein yield increased when SfSWT‐4 cells were infected with a vankyrin‐encoding baculovirus. A synergistic effect in elevated levels of recombinant protein production was observed when vankyrin‐expressing cells were combined with a vankyrin‐encoding baculovirus. These effects were observed with various model proteins including medically relevant therapeutic proteins. In summary, we found that cell lysis could be delayed and recombinant protein yields could be increased by using cell lines constitutively expressing vankyrin or vankyrin‐encoding baculovirus vectors. © 2017 The Authors Biotechnology Progress published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers Biotechnol. Prog., 33:1496–1507, 2017

Crystal Structure of the Human Cytomegalovirus Glycoprotein B

Human cytomegalovirus (HCMV), a dsDNA, enveloped virus, is a ubiquitous pathogen that establishes lifelong latent infections and caused disease in persons with compromised immune systems, e.g., organ transplant recipients or AIDS patients. HCMV is also a leading cause of congenital viral infections in newborns. Entry of HCMV into cells requires the conserved glycoprotein B (gB), thought to function as a fusogen and reported to bind signaling receptors. gB also elicits a strong immune response in humans and induces the production of neutralizing antibodies although most anti-gB Abs are non-neutralizing. Here, we report the crystal structure of the HCMV gB ectodomain determined to 3.6-Å resolution, which is the first atomic-level structure of any betaherpesvirus glycoprotein. The structure of HCMV gB resembles the postfusion structures of HSV-1 and EBV homologs, establishing it as a new member of the class III viral fusogens. Despite structural similarities, each gB has a unique domain arrangement, demonstrating structural plasticity of gB that may accommodate virus-specific functional requirements. The structure illustrates how extensive glycosylation of the gB ectodomain influences antibody recognition. Antigenic sites that elicit neutralizing antibodies are more heavily glycosylated than those that elicit non-neutralizing antibodies, which suggest that HCMV gB uses glycans to shield neutralizing epitopes while exposing non-neutralizing epitopes. This glycosylation pattern may have evolved to direct the immune response towards generation of non-neutralizing antibodies thus helping HCMV to avoid clearance. HCMV gB structure provides a starting point for elucidation of its antigenic and immunogenic properties and aid in the design of recombinant vaccines and monoclonal antibody therapies.

Human cytomegalovirus (HCMV) establishes lifelong infection in a majority of the world’s population and causes disease in neonates and the immunocompromised patients such as organ transplant recipients or persons with AIDS. There is no vaccine against HCMV, and current HCMV antivirals are toxic and an increasing prevalence of resistance. Glycoprotein B (gB), displayed on the viral surface is a major viral immunogen and is necessary for viral penetration into cells. The crystal structure of gB reported here provides a detailed 3D map of gB. A thick glycan layer covers a large surface area, which may explain why anti-gB neutralizing antibodies are relatively rare. The structure is expected to aid in the development of a HCMV vaccine and monoclonal antibody therapies.

HCMV gB shares structural and functional properties with gB proteins from other herpesviruses

Glycoprotein B (gB) facilitates HCMV entry into cells by binding receptors and mediating membrane fusion. The crystal structures of gB ectodomains from HSV-1 and EBV are available, but little is known about the HCMV gB structure. Using multiangle light scattering and electron microscopy, we show here that HCMV gB ectodomain is a trimer with the overall shape similar to HSV-1 and EBV gB ectodomains. HCMV gB ectodomain forms rosettes similar to rosettes formed by EBV gB and the postfusion forms of other viral fusogens. Substitution of several bulky hydrophobic residues within the putative fusion loops with more hydrophilic residues reduced rosette formation and abolished cell fusion. We propose that like gB proteins from HSV-1 and EBV, HCMV gB has two internal hydrophobic fusion loops that likely interact with target membranes. Our work establishes structural and functional similarities between gB proteins from three subfamilies of herpesviruses.

Neutralizing epitopes of influenza virus hemagglutinin: target for the development of a universal vaccine against H5N1 lineages

The nature of influenza virus to randomly mutate and evolve into new types with diverse antigenic determinants is an important challenge in the control of influenza infection. Particularly, variations within the amino acid sequences of major neutralizing epitopes of influenza virus hemagglutinin (HA) hindered the development of universal vaccines against H5N1 lineages. Based on distribution analyses of the identified major neutralizing epitopes of hemagglutinin, we selected three vaccine strains that cover the entire variants in the neutralizing epitopes among the H5N1 lineages. HA proteins of selected vaccine strains were expressed on the baculovirus surface (BacHA), and the preclinical efficacy of the vaccine formulations was evaluated in a mouse model. The combination of three selected vaccine strains could effectively neutralize viruses from clades 1, 2.1, 2.2, 4, 7, and 8 of influenza H5N1 viruses. In contrast, a vaccine formulation containing only adjuvanted monovalent BacHA (mono-BacHA) or a single strain of inactivated whole viral vaccine was able to neutralize only clade 1 (homologous), clade 2.1, and clade 8.0 viruses. Also, the trivalent BacHA vaccine was able to protect 100% of the mice against challenge with three different clades (clade 1.0, clade 2.1, and clade 7.0) of H5N1 strains compared to mono-BacHA or inactivated whole viral vaccine. The present findings provide a rationale for the development of a universal vaccine against H5N1 lineages. Furthermore, baculoviruses displaying HA will serve as an ideal choice for a vaccine in prepandemic or pandemic situations and expedite vaccine technology without the requirement of high-level-biocontainment facilities or tedious protein purification processes.

An arthropod enzyme, Dfurin1, and a vertebrate furin homolog display distinct cleavage site sequence preferences for a shared viral proprotein substrate

Alphaviruses replicate in vertebrate and arthropod cells and utilize a cellular enzyme called furin to process the PE2 glycoprotein precursor during virus replication in both cell types. Furin cleaves PE2 at a site immediately following a highly conserved four residue cleavage signal. Prior studies demonstrated that the amino acid immediately adjacent to the cleavage site influenced PE2 cleavage differently in vertebrate and mosquito cells (HW Heidner et al. 1996 . Journal of Virology 70: 2069-2073.). This finding was tentatively attributed to potential differences in the substrate specificities of the vertebrate and arthropod furin enzymes or to differences in the carbohydrate processing phenotypes of arthropod and vertebrate cells. To further address this issue, we evaluated Sindbis virus replication and PE2 cleavage in the Chinese hamster, Cricetulus griseus Milne-Edwards (Rodentia: Cricetidae) ovary cells (CHO-K1) and in a CHO-K1-derived furin-negative cell line (RPE.40) engineered to stably express the Dfurin1 enzyme of Drosophila melanogaster Meigen (Diptera: Drosophilidae). Expression of Dfurin1 enhanced Sindbis virus titers in RPE.40 cells by a factor of 10(2)-10(3), and this increase correlated with efficient cleavage of PE2. The PE2-cleavage phenotypes of viruses containing different amino acid substitutions adjacent to the furin cleavage site were compared in mosquito (C6/36), CHO-K1, and Dfurin1-expressing RPE.40 cells. This analysis confirmed that the substrate specificities of Dfurin1 and the putative mosquito furin homolog present in C6/36 cells are similar and suggested that the alternative PE2 cleavage phenotypes observed in vertebrate and arthropod cells were due to differences in substrate specificity between the arthropod and vertebrate furin enzymes and not to differences in host cell glycoprotein processing pathways.

Isolation and functional characterization of a new shrimp ovarian peritrophin with antimicrobial activity from Fenneropenaeus merguiensis

Shrimp ovarian peritrophin (SOP), a major protein in jelly layer (JL) and cortical rods (CRs), is proposed to play a role in the protection of spawned eggs. The full sequence of SOP cDNA from Fenneropenaeus merguiensis (Fm-SOP) shares approximately 50% identity with other SOP sequences and contains several putative chitin-binding or peritrophin-A domains. Interestingly, Fm-SOP contains a putative 61-amino acid propeptide located at the N-terminal end, downstream of a 19-amino acid signal peptide, which is unique among penaeid SOP sequences described so far. This 61-amino-acid sequence constitutes a putative chitin-binding domain with six conserved cysteines, and is cleaved at a dibasic recognition site for a furin (subtilisin-like endoprotease). Expression analyses indicated that Fm-SOP mRNA is abundant in early vitellogenic ovaries and scarce in late-vitellogenic ovaries. Conversely, Fm-SOP protein is the most abundant at the end of vitellogenesis. To investigate its biological function, a recombinant Fm-SOP was expressed to generate a glycosylated protein in Spodoptera frugiperda Sf9 cells (rSOP-Sf9) and a nonglycosylated protein (rSOP-Ec) in Escherichia coli. rSOP-Sf9 and rSOP-Ec were found to bind to chitin, similarly to the native protein extracted from F. merguiensis ovaries. Most interestingly, rSOP-Ec displayed a chitinase activity and efficiently inhibited the growth of Vibrio harveyi and Staphylococcus aureus, with minimum inhibitory concentrations of 2.4 and 15.7 microM, respectively. This first report shows that a major component of CR and JL is biologically active against known pathogens and predicts a significant role of JL in the protection of the spawned eggs against pathogens.

The C-terminal region of the proprotein convertase 1/3 (PC1/3) exerts a bimodal regulation of the enzyme activity in vitro

The proprotein convertase PC1/3 preferentially cleaves its substrates in the dense core secretory granules of endocrine and neuroendocrine cells. Similar to most proteinases synthesized first as zymogens, PC1/3 is synthesized as a larger precursor that undergoes proteolytic processing of its signal peptide and propeptide. The N-terminally located propeptide has been shown to be essential for folding and self-inhibition. Furthermore, PC1/3 also possesses a C-terminal region (CT-peptide) which, for maximal enzymatic activity, must also be cleaved. To date, its role has been documented through transfection studies in terms of sorting and targeting of PC1/3 and chimeric proteins into secretory granules. In this study, we examined the properties of a 135-residue purified bacterially produced CT-peptide on the in vitro enzymatic activity of PC1/3. Depending on the amount of CT-peptide used, it is shown that the CT-peptide increases PC1/3 activity at low concentrations (nm) and decreases it at high concentrations (microm), a feature typical of an activator. Furthermore, we show that, contrary to the propeptide, the CT-peptide is not further cleaved by PC1/3 although it is sensitive to human furin activity. Based on these results, it is proposed that PC1/3, through its various domains, is capable of controlling its enzymatic activity in all regions of the cell that it encounters. This mode of self-control is unique among members of all proteinases families.

The Spn4 gene from Drosophila melanogaster is a multipurpose defence tool directed against proteases from three different peptidase families

By alternative use of four RSL (reactive site loop) coding exon cassettes, the serpin (serine protease inhibitor) gene Spn4 from Drosophila melanogaster was proposed to enable the synthesis of multiple protease inhibitor isoforms, one of which has been shown to be a potent inhibitor of human furin. Here, we have investigated the inhibitory spectrum of all Spn4 RSL variants. The analyses indicate that the Spn4 gene encodes inhibitors that may inhibit serine proteases of the subtilase family (S8), the chymotrypsin family (S1), and the papain-like cysteine protease family (C1), most of them at high rates. Thus a cohort of different protease inhibitors is generated simply by grafting enzyme-adapted RSL sequences on to a single serpin scaffold, even though the target proteases contain different types and/or a varying order of catalytic residues and are descendents of different phylogenetic lineages. Since all of the Spn4 RSL isoforms are produced as intracellular residents and additionally as variants destined for export or associated with the secretory pathway, the Spn4 gene represents a versatile defence tool kit that may provide multiple antiproteolytic functions.

Biosynthesis and secretion of insect lipoprotein: involvement of furin in cleavage of the apoB homolog, apolipophorin-II/I

The biosynthesis of neutral fat-transporting lipoproteins involves the lipidation of their nonexchangeable apolipoprotein. In contrast to its mammalian homolog apolipoprotein B, however, insect apolipophorin-II/I (apoLp-II/I) is cleaved posttranslationally at a consensus substrate sequence for furin, resulting in the appearance of two apolipoproteins in insect lipoprotein. To characterize the cleavage process, a truncated cDNA encoding the N-terminal 38% of Locusta migratoria apoLp-II/I, including the cleavage site, was expressed in insect Sf9 cells. This resulted in the secretion of correctly processed apoLp-II and truncated apoLp-I. The cleavage could be impaired by the furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (decRVKRcmk) as well as by mutagenesis of the consensus substrate sequence for furin, as indicated by the secretion of uncleaved apoLp-II/I-38. Treatment of L. migratoria fat body, the physiological site of lipoprotein biosynthesis, with decRVKRcmk similarly resulted in the secretion of uncleaved apoLp-II/I, which was integrated in lipoprotein particles of buoyant density identical to wild-type high density lipophorin (HDLp). These results show that apoLp-II/I is posttranslationally cleaved by an insect furin and that biosynthesis and secretion of HDLp can occur independent of this processing step. Structure modeling indicates that the cleavage of apoLp-II/I represents a molecular adaptation in homologous apolipoprotein structures. We propose that cleavage enables specific features of insect lipoproteins, such as low density lipoprotein formation, endocytic recycling, and involvement in coagulation.

Improved PC1/3 production through recombinant expression in insect cells and larvae

Protein convertase 1/3 is a serine endoproteinase present in the regulated secretory pathway of endocrine and neuroendocrine cells. It is responsible for the processing of numerous prohormones and proneuropeptides into their biologically active moieties, often following cleavage at pairs of basic residues. The determination of its three-dimensional structure, as well as the understanding of its enzymatic properties, would greatly benefit from the production and availability of large amounts of recombinant enzyme. We report herein improvements in the production of PC1/3 by expressing recombinant mutated forms in both insect cells (Spodoptera frugiperda, Sf9 cells) and larvae (Trichoplusia ni commonly referred to as cabbage looper). On one hand, we deleted the last 135 COOH-terminal residues of mPC1/3 and, on the other hand, we replaced the signal peptide of mPC1/3 by the viral glycoprotein gp67 signal peptide. These modifications were shown to improve markedly (up to 125%) the secretion into the Sf9 cells medium and the amount of enzymatic activity recovered when compared to the original vector. Moreover, intracoelemic injection of the vectors into insect larvae led to the production and purification of enzymatically active enzyme at a level of 30 microg/larva in the case of mPC1/3 and to the production of a high amount of another enzymatically active convertase, PC7. The optimal viral titer for infection of larvae was determined to be 10(6)pfu/ml. Taking into account the purification protocol combined with the ease and efficiency of using larvae, it should now be possible to meet the needs for biochemical and structural studies.

A chimeric protein of simian immunodeficiency virus envelope glycoprotein gp140 and Escherichia coli aspartate transcarbamoylase

The envelope glycoproteins of the human immunodeficiency virus and the related simian immunodeficiency virus (SIV) mediate viral entry into host cells by fusing viral and target cell membranes. We have reported expression, purification, and characterization of gp140 (also called gp160e), the soluble, trimeric ectodomain of the SIV envelope glycoprotein, gp160 (B. Chen et al., J. Biol. Chem. 275:34946-34953, 2000). We have now expressed and purified chimeric proteins of SIV gp140 and its variants with the catalytic subunit (C) of Escherichia coli aspartate transcarbamoylase (ATCase). The fusion proteins (SIV gp140-ATC) bind viral receptor CD4 and a number of monoclonal antibodies specific for SIV gp140. The chimeric molecule also has ATCase activity, which requires trimerization of the ATCase C chains. Thus, the fusion protein is trimeric. When ATCase regulatory subunit dimers (R(2)) are added, the fusion protein assembles into dimers of trimers as expected from the structure of C(6)R(6) ATCase. Negative-stain electron microscopy reveals spikey features of both SIV gp140 and SIV gp140-ATC. The production of the fusion proteins may enhance the possibilities for structure determination of the envelope glycoprotein either by electron cryomicroscopy or X-ray crystallography.

Functional analysis of a conserved region of the baculovirus envelope fusion protein, LD130

The envelope fusion protein from a baculovirus pathogenic for Lymantria dispar was characterized. N-terminal sequence analysis determined that it was cleaved downstream of predicted signal peptide and furin cleavage motifs. Mutation of the furin motif resulted in a protein that was not cleaved and did not mediate fusion. Mutagenesis of three charged amino acids in a conserved sequence with the features of a fusion peptide resulted in significant reduction of the ability of the constructs to mediate fusion. None of the mutations inhibited transport of the proteins to the cell surface. In addition, the mutations of the predicted fusion peptide region yielded no inhibition of cleavage. No difference in cleavage was detected between constructs expressed in Spodoptera frugiperda or L. dispar cells.

Production, purification, and characterization of recombinant prohormone convertase 5 from baculovirus-infected insect cells

The discovery of the prohormone convertase (PC) family of enzymes has provided several good candidates (PC1, PC2, and PC5) for the enzymes responsible for the endoproteolytic cleavage of procholecystokinin (pro-CCK). Determination of the role of individual pro-hormone convertases in the processing of pro-CCK is complicated because several of these enzymes are found in endocrine tumor cells expressing CCK mRNA and in identified neurons in the brain. Production of active recombinant PC5 permits the determination of its ability to cleave substrates related to pro-CCK. Active PC5, secreted from baculovirus-infected Sf9 cells, was partially purified by ion-exchange chromatography. Western blot analysis confirmed the presence of the active form of the enzyme in infected cell media and its absence from uninfected cell media. The enzyme is most active at acidic pH 6.5 and is maximally activated by 5 mM calcium. PC5 was able to cleave both monobasic and dibasic substrates without a requirement for a basic residue at P-4 and it displayed a K(m) in the micromolar range. The enzyme was inhibited by EDTA, 1,10-phenanthroline, and p-CMS, as well as by two specific PC inhibitors. This is the first reported preparation of active recombinant PC5. Like the other members of its family, it has the correct catalytic characteristics in vitro to play a role in the processing of neuropeptide precursor proteins into their final bioactive forms.

A short sequence in the N-terminal region is required for the trimerization of type XIII collagen and is conserved in other collagenous transmembrane proteins

The recombinant transmembrane protein type XIII collagen is shown to reside on the plasma membrane of insect cells in a 'type II' orientation. Expressions of deletion constructs showed that sequences important for the association of three alpha1(XIII) chains reside in their N- rather than C-terminal portion. In particular, a deletion of residues 63-83 immediately adjacent to the transmembrane domain abolished the formation of disulfide-bonded trimers. The results imply that nucleation of the type XIII collagen triple helix occurs at the N-terminal region and that triple helix formation proceeds from the N- to the C-terminus, in opposite orientation to that of the fibrillar collagens. Interestingly, a sequence homologous to the deleted residues was found at the same plasma membrane-adjacent location in other collagenous transmembrane proteins, suggesting that it may be a conserved association domain. The type XIII collagen was secreted into insect cell medium in low amounts, but this secretion was markedly enhanced when the cytosolic portion was lacking. The cleavage occurred in the non-collagenous NC1 domain after four arginines and was inhibited by a furin protease inhibitor.

A novel baculovirus envelope fusion protein with a proprotein convertase cleavage site

The entry mechanism of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV), a group II NPV, in cultured cells was examined. SeMNPV budded virus (BV) enters by endocytosis as do the BVs of the group I NPVs, Autographa californica (Ac) MNPV and Orgyia pseudotsugata (Op) MNPV. In group I NPVs, upon infection acidification of the endosome triggers fusion of the viral and endosomal membrane, which is mediated by the BV envelope glycoprotein GP64. However, the SeMNPV genome lacks a homolog of GP64 envelope fusion protein (EFP). A functional homolog of the OpMNPV GP64 EFP was identified in SeMNPV ORF8 (Se8; 76 kDa) and appeared to be the major BV envelope protein. Surprisingly, a 60-kDa cleavage product of this protein is present in the BV envelope. A furin-like proprotein convertase cleavage site (R-X-K/R-R) was identified immediately upstream of the N-terminus of the mature Se8 protein and this site was also conserved in the Lymantria dispar (Ld) MNPV homolog (Ld130) of Se8. Syncytium formation assays showed that Se8 and Ld130 alone were sufficient to mediate membrane fusion upon acidification of the medium. Furthermore, C-terminal GFP-fusion proteins of Se8 and Ld130 were primarily localized in the plasma membrane of insect cells. This is consistent with their fusogenic activity and supports the conclusion that the Se8 gene product is a functional homolog of the GP64 EFP.