Furin is important but not essential for the proteolytic maturation of gp160 of HIV-1

S Protein, ACE2 and Host Cell Proteases in SARS-CoV-2 Cell Entry and Infectivity; Is Soluble ACE2 a Two Blade Sword? A Narrative Review

Since the spread of the deadly virus SARS-CoV-2 in late 2019, researchers have restlessly sought to unravel how the virus enters the host cells. Some proteins on each side of the interaction between the virus and the host cells are involved as the major contributors to this process: (1) the nano-machine spike protein on behalf of the virus, (2) angiotensin converting enzyme II, the mono-carboxypeptidase and the key component of renin angiotensin system on behalf of the host cell, (3) some host proteases and proteins exploited by SARS-CoV-2. In this review, the complex process of SARS-CoV-2 entrance into the host cells with the contribution of the involved host proteins as well as the sequential conformational changes in the spike protein tending to increase the probability of complexification of the latter with angiotensin converting enzyme II, the receptor of the virus on the host cells, are discussed. Moreover, the release of the catalytic ectodomain of angiotensin converting enzyme II as its soluble form in the extracellular space and its positive or negative impact on the infectivity of the virus are considered.

How Do Enveloped Viruses Exploit the Secretory Proprotein Convertases to Regulate Infectivity and Spread?

Inhibition of the binding of enveloped viruses surface glycoproteins to host cell receptor(s) is a major target of vaccines and constitutes an efficient strategy to block viral entry and infection of various host cells and tissues. Cellular entry usually requires the fusion of the viral envelope with host plasma membranes. Such entry mechanism is often preceded by “priming” and/or “activation” steps requiring limited proteolysis of the viral surface glycoprotein to expose a fusogenic domain for efficient membrane juxtapositions. The 9-membered family of Proprotein Convertases related to Subtilisin/Kexin (PCSK) serine proteases (PC1, PC2, Furin, PC4, PC5, PACE4, PC7, SKI-1/S1P, and PCSK9) participate in post-translational cleavages and/or regulation of multiple secretory proteins. The type-I membrane-bound Furin and SKI-1/S1P are the major convertases responsible for the processing of surface glycoproteins of enveloped viruses. Stefan Kunz has considerably contributed to define the role of SKI-1/S1P in the activation of arenaviruses causing hemorrhagic fever. Furin was recently implicated in the activation of the spike S-protein of SARS-CoV-2 and Furin-inhibitors are being tested as antivirals in COVID-19. Other members of the PCSK-family are also implicated in some viral infections, such as PCSK9 in Dengue. Herein, we summarize the various functions of the PCSKs and present arguments whereby their inhibition could represent a powerful arsenal to limit viral infections causing the present and future pandemics.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection: let the virus be its own demise

There has been a collaborative global effort to construct novel therapeutic and prophylactic approaches to SARS-CoV-2 management. Although vaccine development is crucial, acute management of newly infected patients, especially those with severe acute respiratory distress syndrome, is a priority. Herein we describe the rationale and potential of repurposing a dual plasmid, Vigil (pbi-shRNA^furin-GM-CSF), now in Phase III cancer trials, for the treatment of and, in certain circumstances, enhancement of the immune response to SARS-CoV-2.

Same same but different - Antiviral factors interfering with the infectivity of HIV particles

Human immunodeficiency virus (HIV) is the causative agent of acquired immunodeficiency syndrome (AIDS). Novel strategies to combat this pandemic include the discovery of cellular proteins targeting distinct steps of the HIV replication cycle. Here, we summarize our current knowledge on antiviral proteins interfering with the infectivity of released HIV particles.

Production of complex viral glycoproteins in plants as vaccine immunogens

Plant molecular farming offers a cost-effective and scalable approach to the expression of recombinant proteins which has been proposed as an alternative to conventional production platforms for developing countries. In recent years, numerous proofs of concept have established that plants can produce biologically active recombinant proteins and immunologically relevant vaccine antigens that are comparable to those made in conventional expression systems. Driving many of these advances is the remarkable plasticity of the plant proteome which enables extensive engineering of the host cell, as well as the development of improved expression vectors facilitating higher levels of protein production. To date, the only plant-derived viral glycoprotein to be tested in humans is the influenza haemagglutinin which expresses at ~50 mg/kg. However, many other viral glycoproteins that have potential as vaccine immunogens only accumulate at low levels in planta. A critical consideration for the production of many of these proteins in heterologous expression systems is the complexity of post-translational modifications, such as control of folding, glycosylation and disulphide bridging, which is required to reproduce the native glycoprotein structure. In this review, we will address potential shortcomings of plant expression systems and discuss strategies to optimally exploit the technology for the production of immunologically relevant and structurally authentic glycoproteins for use as vaccine immunogens.

Selection of Single-Stranded DNA Molecular Recognition Elements against Exotoxin A Using a Novel Decoy-SELEX Method and Sensitive Detection of Exotoxin A in Human Serum

Exotoxin A is one of the virulence factors of Pseudomonas aeruginosa, a bacterium that can cause infections resulting in adverse health outcomes and increased burden to health care systems. Current methods of diagnosing P. aeruginosa infections are time consuming and can require significant preparation of patient samples. This study utilized a novel variation of the Systematic Evolution of Ligand by Exponential Enrichment, Decoy-SELEX, to identify an Exotoxin A specific single-stranded DNA (ssDNA) molecular recognition element (MRE). Its emphasis is on increasing stringency in directing binding toward free target of interest and at the same time decreasing binding toward negative targets. A ssDNA MRE with specificity and affinity was identified after fourteen rounds of Decoy-SELEX. Utilizing surface plasmon resonance measurements, the determined equilibrium dissociation constant (K_d) of the MRE is between 4.2 µM and 4.5 µM, and is highly selective for Exotoxin A over negative targets. A ssDNA MRE modified sandwich enzyme-linked immunosorbent assay (ELISA) has been developed and achieved sensitive detection of Exotoxin A at nanomolar concentrations in human serum. This study has demonstrated the proof-of-principle of using a ssDNA MRE as a clinical diagnostic tool.

Assembly and characterization of gp160-nanodiscs: A new platform for biochemical characterization of HIV envelope spikes

The human immunodeficiency virus (HIV) is the causative agent of acquired immune deficiency syndrome (AIDS) and is thus responsible for significant morbidity and mortality worldwide. Despite considerable effort, preparation of an effective vaccine for AIDS has been elusive and it has become clear that a fundamental understanding of the relevant antigenic targets on HIV is essential. The Env trimer spike is the only viral antigen present on the surface of the viral particle and it is the target of all broadly neutralizing antibodies isolated to date. Thus, a soluble, homogeneous, and well-defined preparation of Env trimers is an important first step toward biochemical and structural characterization of the antigenic spike. Phospholipid bilayer nanodiscs represent a relatively new technology that can serve as a platform for the assembly of membrane proteins into a native membrane-like environment. Here we describe the preparation and characterization of unprocessed full-length, natively glycoslyated gp160 Env proteins incorporated into nanodiscs (gp160-ND). The particles are soluble and well defined in the absence of detergent, and possess a morphology anticipated of Env incorporated into a lipid ND. Importantly, the gp160-NDs retain CD4 and Env antibody binding characteristics expected of a functional trimer spike and their incorporation into a lipid membrane allows interrogation of epitopes associated with the membrane-proximal ectodomain region of gp41. These studies provide the groundwork for the use of gp160-ND in more detailed biochemical and structural studies that may set the stage for their use in vaccine development.

Targeted coating with antigenic peptide renders tumor cells susceptible to CD8(+) T cell-mediated killing

The potency of immunotherapies targeting endogenous tumor antigens is hindered by immune tolerance. We created a therapeutic agent comprised of a tumor-homing module fused to a functional domain capable of selectively rendering tumor cells sensitive to foreign antigen-specific CD8(+) T cell-mediated immune attack, and thereby, circumventing concerns for immune tolerance. The tumor-homing module is comprised of a single-chain variable fragment (scFv) that specifically binds to mesothelin (Meso), which is commonly overexpressed in human cancers, including ovarian tumors. The functional domain is comprised of the Fc portion of IgG2a protein and foreign immunogenic CD8(+) T cell epitope flanked by furin cleavage sites (R), which can be recognized and cleaved by furin that is highly expressed in the tumor microenvironment. We show that our therapeutic protein specifically loaded antigenic epitope onto the surface of mesothelin-expressing tumor cells, rendering tumors susceptible to antigen-specific cytotoxic CD8(+) T lymphocytes (CTL)-mediated killing in vitro and in vivo. Our findings have important implications for bypassing immune tolerance to enhance cancer immunotherapy.

Exon level transcriptomic profiling of HIV-1-infected CD4(+) T cells reveals virus-induced genes and host environment favorable for viral replication

HIV-1 is extremely specialized since, even amongst CD4⁺ T lymphocytes (its major natural reservoir in peripheral blood), the virus productively infects only a small proportion of cells under an activated state. As the percentage of HIV-1-infected cells is very low, most studies have so far failed to capture the precise transcriptomic profile at the whole-genome scale of cells highly susceptible to virus infection. Using Affymetrix Exon array technology and a reporter virus allowing the magnetic isolation of HIV-1-infected cells, we describe the host cell factors most favorable for virus establishment and replication along with an overview of virus-induced changes in host gene expression occurring exclusively in target cells productively infected with HIV-1. We also establish that within a population of activated CD4⁺ T cells, HIV-1 has no detectable effect on the transcriptome of uninfected bystander cells at early time points following infection. The data gathered in this study provides unique insights into the biology of HIV-1-infected CD4⁺ T cells and identifies genes thought to play a determinant role in the interplay between the virus and its host. Furthermore, it provides the first catalogue of alternative splicing events found in primary human CD4⁺ T cells productively infected with HIV-1.

Some previous studies have monitored HIV-1-induced gene expression in various host cell targets and tissues but the discrimination between productively infected cells and uninfected bystander cells represents a technical challenge yet to be solved. Consequently, data interpretation has always been biased towards the transcriptional response of a majority of uninfected bystander cells that were exposed to soluble factors released by virus-infected cells. Following the design of a unique and innovative molecular tool to identify cells productively infected with HIV-1 and the description of an efficient magnetic beads-based technique to separate them from uninfected bystander cells, we undertake this challenge and perform the first comparative whole-genome transcriptomic and large-scale proteomic profiling of both HIV-1-infected and uninfected bystander CD4⁺ T cells. We demonstrate herein that HIV-1- infected and uninfected bystander cells display distinctive transcriptomic signatures which might permit to identify new susceptibility and resistance factors.

Highly potent inhibitors of proprotein convertase furin as potential drugs for treatment of infectious diseases

Optimization of our previously described peptidomimetic furin inhibitors was performed and yielded several analogs with a significantly improved activity. The most potent compounds containing an N-terminal 4- or 3-(guanidinomethyl)phenylacetyl residue inhibit furin with K(i) values of 16 and 8 pM, respectively. These analogs inhibit other proprotein convertases, such as PC1/3, PC4, PACE4, and PC5/6, with similar potency, whereas PC2, PC7, and trypsin-like serine proteases are poorly affected. Incubation of selected compounds with Madin-Darby canine kidney cells over a period of 96 h revealed that they exhibit great stability, making them suitable candidates for further studies in cell culture. Two of the most potent derivatives were used to inhibit the hemagglutinin cleavage and viral propagation of a highly pathogenic avian H7N1 influenza virus strain. The treatment with inhibitor 24 (4-(guanidinomethyl)phenylacetyl-Arg-Val-Arg-4-amidinobenzylamide) resulted in significantly delayed virus propagation compared with an inhibitor-free control. The same analog was also effective in inhibiting Shiga toxin activation in HEp-2 cells. This antiviral effect, as well as the protective effect against a bacterial toxin, suggests that inhibitors of furin or furin-like proprotein convertases could represent promising lead structures for future drug development, in particular for the treatment of infectious diseases.

Conserved arginine residue in the membrane-spanning domain of HIV-1 gp41 is required for efficient membrane fusion

Despite the high mutation rate of HIV-1, the amino acid sequences of the membrane-spanning domain (MSD) of HIV-1 gp41 are well conserved. Arginine residues are rarely found in single membrane-spanning domains, yet an arginine residue, R(696) (the numbering is based on that of HXB2), is highly conserved in HIV-1 gp41. To examine the role of R(696), it was mutated to K, A, I, L, D, E, N, and Q. Most of these substitutions did not affect the expression, processing or surface distribution of the envelope protein (Env). However, a syncytia formation assay showed that the substitution of R(696) with amino acid residues other than K, a naturally observed mutation in the gp41 MSD, decreased fusion activity. Substitution with hydrophobic amino acid residues (A, I, and L) resulted in a modest decrease, while substitution with D or E, potentially negatively-charged residues, almost abolished the syncytia formation. All the fusion-defective mutants showed slower kinetics with the cell-based dual split protein (DSP) assay that scores the degree of membrane fusion based on pore formation between fusing cells. Interestingly, the D and E substitutions did show some fusion activity in the DSP assays, suggesting that proteins containing D or E substitutions retained some fusion pore-forming capability. However, nascent pores failed to develop, due probably to impaired activity in the pore enlargement process. Our data show the importance of this conserved arginine residue for efficient membrane fusion.

The membrane-spanning domain of gp41 plays a critical role in intracellular trafficking of the HIV envelope protein

Background

The sequences of membrane-spanning domains (MSDs) on the gp41 subunit are highly conserved among many isolates of HIV-1. The GXXXG motif, a potential helix-helix interaction motif, and an arginine residue (rare in hydrophobic MSDs) are especially well conserved. These two conserved elements are expected to locate on the opposite sides of the MSD, if the MSD takes a α-helical secondary structure. A scanning alanine-insertion mutagenesis was performed to elucidate the structure-function relationship of gp41 MSD.

Results

A circular dichroism analysis of a synthetic gp41 MSD peptide determined that the secondary structure of the gp41 MSD was α-helical. We then performed a scanning alanine-insertion mutagenesis of the entire gp41 MSD, progressively shifting the relative positions of MSD segments around the helix axis. Altering the position of Gly694, the last residue of the GXXXG motif, relative to Arg696 (the number indicates the position of the amino acid residues in HXB2 Env) around the axis resulted in defective fusion. These mutants showed impaired processing of the gp160 precursor into gp120 and gp41. Furthermore, these Env mutants manifested inefficient intracellular transport in the endoplasmic reticulum and Golgi regions. Indeed, a transplantation of the gp41 MSD portion into the transmembrane domain of another membrane protein, Tac, altered its intracellular distribution. Our data suggest that the intact MSD α-helix is critical in the intracellular trafficking of HIV-1 Env.

Conclusions

The relative position between the highly conserved GXXXG motif and an arginine residue around the gp41 MSD α-helix is critical for intracellular trafficking of HIV-1 Env. The gp41 MSD region not only modulates membrane fusion but also controls biosynthesis of HIV-1 Env.

A study of human furin specificity using synthetic peptides derived from natural substrates, and effects of potassium ions

We explored furin substrate requirements in addition to the motif R-X-K/R-R using synthetic fluorescent resonance energy transfer (FRET) decapeptides. These decapeptides were derived from furin cleavage sites in viral coat glycoproteins and human and bacterial protein precursors. The hydrolysis by furin of most substrate was activated by K(+) ion, whereas kosmotropic anions of the Hofmeister series were inhibitors. The analysis of furin hydrolytic activity showed that its efficiency is highly dependent on the particular combinations of amino acids at different substrate positions. There is a clear interdependence of furin subsites that must be taken in account in determining its specificity and also for the design of inhibitors. However, clear preferences were detected for substrates with S at P(1)', and V at P(2)', at P(3)' the amino acids D, S, L and A are almost equally frequent. In the non-prime subsites the best substrates presented S and H at P(6); basic amino acids at P(5); and no clear tendency at P(3). Interestingly, two amino acid substitutions on the prime side of the peptide derived from H5N1 influenza hemagglutinin furin processing site highly improved its hydrolysis. These modifications are possible by single point mutations, suggesting a potential yield of a more infectious virus.

LEKTI fragments specifically inhibit KLK5, KLK7, and KLK14 and control desquamation through a pH-dependent interaction

LEKTI is a 15-domain serine proteinase inhibitor whose defective expression underlies the severe autosomal recessive ichthyosiform skin disease, Netherton syndrome. Here, we show that LEKTI is produced as a precursor rapidly cleaved by furin, generating a variety of single or multidomain LEKTI fragments secreted in cultured keratinocytes and in the epidermis. The identity of these biological fragments (D1, D5, D6, D8-D11, and D9-D15) was inferred from biochemical analysis, using a panel of LEKTI antibodies. The functional inhibitory capacity of each fragment was tested on a panel of serine proteases. All LEKTI fragments, except D1, showed specific and differential inhibition of human kallikreins 5, 7, and 14. The strongest inhibition was observed with D8-D11, toward KLK5. Kinetics analysis revealed that this interaction is rapid and irreversible, reflecting an extremely tight binding complex. We demonstrated that pH variations govern this interaction, leading to the release of active KLK5 from the complex at acidic pH. These results identify KLK5, a key actor of the desquamation process, as the major target of LEKTI. They disclose a new mechanism of skin homeostasis by which the epidermal pH gradient allows precisely regulated KLK5 activity and corneodesmosomal cleavage in the most superficial layers of the stratum corneum.

Insulin-like peptide 6: characterization of secretory status and posttranslational modifications

Insulin-like peptide 6 (Insl6) is a member of the insulin/relaxin superfamily with unknown biological function(s). In the current report, we establish that meiotic and postmeiotic germ cells of the testis are the principal sites of expression of Insl6. Analysis of stably or transiently transfected cells revealed that Insl6 is a secreted protein localized to the endoplasmic reticulum and Golgi. Secretion could be detected in both CHO and GC2 germ cells and was sensitive to brefeldin A treatment. In cell lysates, the predominant Insl6 band was approximately 28 kDa in size. In contrast, the predominant Insl6 species in the supernatant was 8 kDa in size, suggesting posttranslational processing of the precursor protein. Ectopically expressed Insl6 is processed and secreted in furin-deficient LoVo cells and in CHO cells treated with a furin inhibitor, although the size profile of the secreted protein is altered suggesting that Insl6 is a substrate for furin action. Furthermore, mutation of a putative furin cleavage site in the Insl6 peptide resulted in aberrant processing of the Insl6 peptide. Additional investigations of the structure of Insl6 protein provided evidence for posttranslational modifications of Insl6, including the presence of disulfide bonds, glycosylation, and ubiquitination. On the basis of the demonstrated secretory status of Insl6, we speculate that the physical proximity of the germ cell to the Sertoli cell renders the Sertoli cell a likely candidate for Insl6 action.

Synthesis, assembly, and processing of the Env ERVWE1/syncytin human endogenous retroviral envelope

Syncytin is a fusogenic protein involved in the formation of the placental syncytiotrophoblast layer. This protein is encoded by the envelope gene of the ERVWE1 proviral locus belonging to the human endogenous retrovirus W (HERV-W) family. The HERV-W infectious ancestor entered the primate lineage 25 to 40 million years ago. Although the syncytin fusion property has been clearly demonstrated, little is known about this cellular protein maturation process with respect to classical infectious retrovirus envelope proteins. Here we show that the cellular syncytin protein is synthesized as a glycosylated gPr73 precursor cleaved into two mature proteins, a gp50 surface subunit (SU) and a gp24 transmembrane subunit (TM). These SU and TM subunits are found associated as homotrimers. The intracytoplasmic tail is critical to the fusogenic phenotype, although its cleavage requirements seem to have diverged from those of classical retroviral maturation.

Beta-amyloid protein converting enzyme 1 and brain-specific type II membrane protein BRI3: binding partners processed by furin

Using a yeast two-hybrid system, we screened a human brain cDNA library for possible interacting proteins with the C-terminal cytosolic tail of the beta-secretase beta-amyloid protein converting enzyme (BACE)1. This identified seven potential candidates, including the brain-specific type II membrane protein BRI3. Co-localization and co-immunoprecipitation experiments confirmed that BACE1 and BRI3 co-localize and interact with each other via the cytosolic tail of BACE1. Furthermore, pulse and pulse-chase analyses revealed that the pro-protein convertases furin, and to a lesser extent PC7 and PC5A, process BRI3 into a C-terminal secreted approximately 4-kDa product. Thus, furin efficiently processes both pro-BACE1 and its novel interacting protein pro-BRI3.

Polyarginine inhibits gp160 processing by furin and suppresses productive human immunodeficiency virus type 1 infection

Correct endoproteolytic maturation of gp160 is essential for the infectivity of human immunodeficiency virus type 1. This processing of human immunodeficiency virus-1 envelope protein, gp160, into gp120 and gp41 has been attributed to the activity of the cellular subtilisin-like proprotein convertase furin. The prototypic furin recognition cleavage site is Arg-X-Arg/Lys-Arg. Arg-Arg-Arg-Arg-Arg-Arg or longer iterations of polyarginine have been shown to be competitive inhibitors of substrate cleavage by furin. Here, we tested polyarginine for inhibition of productive human immunodeficiency virus-1-infection in T-cell lines, primary peripheral blood mononuclear cells, and macrophages. We found that polyarginine inhibited significantly human immunodeficiency virus-1 replication at concentrations that were benign to cell cultures ex vivo and mice in vivo. Using a fluorogenic assay, we demonstrated that polyarginine potently inhibited substrate-specific proteolytic cleavage by furin. Moreover, we verified that authentic processing of human immunodeficiency virus-1 gp160 synthesized in human cells from an infectious human immunodeficiency virus-1 (HIV-1) molecular clone was effectively blocked by polyarginine. Taken together, our data support that inhibitors of proteolytic processing of gp160 may be useful for combating human immunodeficiency virus-1 and that polyarginine represents a lead example of such inhibitors.

Peptide inhibitors of virus-cell fusion: enfuvirtide as a case study in clinical discovery and development

The peptidic antiretroviral enfuvirtide (Fuzeon) is the first clinically approved antiviral fusion inhibitor and the first antiretroviral that must routinely be administered parenterally. Its extracellular activity results both in activity against current drug-resistant strains of HIV-1 and a low potential for systemic toxicities. As a peptide, enfuvirtide also exhibits few interactions with other antiretrovirals and concomitant medications used in HIV disease. Enfuvirtide shows potent antiretroviral activity and significantly improves medical outcomes in highly treatment-experienced patients with HIV-1 infection, but like other antiretrovirals must be given as part of a carefully selected combination regimen to minimise the risk of emergent drug resistance. Despite its subcutaneous route of administration, clinical data indicate that most patients can accept long-term enfuvirtide treatment with little difficulty or impact on daily activities. The only common adverse event associated with enfuvirtide use is injection-site reactions of generally mild-to-moderate severity, which are seldom treatment-limiting.

Furin processing and proteolytic activation of Semliki Forest virus

The alphavirus Semliki Forest virus (SFV) infects cells via a low-pH-dependent membrane fusion reaction mediated by the E1 envelope protein. Fusion is regulated by the interaction of E1 with the receptor-binding protein E2. E2 is synthesized as a precursor termed "p62," which forms a stable heterodimer with E1 and is processed late in the secretory pathway by a cellular furin-like protease. Once processing to E2 occurs, the E1/E2 heterodimer is destabilized so that it is more readily dissociated by exposure to low pH, allowing fusion and infection. We have used FD11 cells, a furin-deficient CHO cell line, to characterize the processing of p62 and its role in the control of virus fusion and infection. p62 was not cleaved in FD11 cells and cleavage was restored in FD11 cell transfectants expressing human furin. Studies of unprocessed virus produced in FD11 cells (wt/p62) demonstrated that the p62 protein was efficiently cleaved by purified furin in vitro, without requiring prior exposure to low pH. wt/p62 virus particles were also processed during their endocytic uptake in furin-containing cells, resulting in more efficient virus infection. wt/p62 virus was compared with mutant L, in which p62 cleavage was blocked by mutation of the furin-recognition motif. wt/p62 and mutant L had similar fusion properties, requiring a much lower pH than control virus to trigger fusion and fusogenic E1 conformational changes. However, the in vivo infectivity of mutant L was more strongly inhibited than that of wt/p62, due to additional effects of the mutation on virus-cell binding.

Structure-function analysis of the prosegment of the proprotein convertase PC5A

To investigate if some residues within the prosegment of PC5A are important for its optimal proteolytic function, various PC5A mutants were cellularly expressed, and their processing activities were compared using pro-vascular endothelial growth factor C (pro-VEGF-C) as a substrate. Although wild type PC5A almost completely processes pro-VEGF-C, a prosegment deletion as well as both P1 mutants of the primary (R116A) and secondary (R84A) autocatalytic cleavage sites are inactive. The in vitro inhibitory potency of various decapeptides mimicking the C-terminal sequence of PC5 prosegment (pPC5) revealed that the native (107)QQVVKKRTKR(116) peptide is a nanomolar inhibitor, whereas its P6 mutant K111H is more selective toward PC5A than Furin. In vitro activity assays using the bacterially expressed pPC5 and its mutants revealed them to be very potent nanomolar inhibitors (IC(50)) and only approximately 6-fold more selective inhibitors of PC5A versus Furin. Expression of the preprosegment of PC5 (ppPC5) and its mutants in Chinese hamster ovary FD11 cells overexpressing pro-VEGF-C with either PC5A or Furin showed them to be as good inhibitors of PC5A as the serpin alpha1-antitrypsin Portland (alpha1-PDX), ppFurin, or ppPACE4 but less potent toward overexpressed Furin. In conclusion, cleavages of the prosegment of PC5A at both Arg(116) and Arg(84) are required for PC5A cellular activity, and ppPC5 is a very potent but modestly selective cellular inhibitor of PC5A.

Regulation of membrane type-1 matrix metalloproteinase activation by proprotein convertases

Membrane type-1 matrix metalloproteinase (MT1-MMP) is the prototypical member of a subgroup of membrane-anchored proteinases that belong to the matrix metalloproteinase family. Although synthesized as a zymogen, MT1-MMP plays an essential role in extracellular matrix remodeling after an undefined process that unmasks its catalytic domain. We now report the existence of a proprotein convertase-MT1-MMP axis that regulates the processing and functional activity of the metalloproteinase. Two sets of basic motifs in the propeptide region of MT1-MMP are identified that potentially can be recognized by the proprotein convertase family of subtilisin-like proteases. Processing of proMT1-MMP as well as the expression of its proteolytic activity were blocked by mutating these recognition motifs or by inhibiting the proprotein convertases furin and PC6 with the serpin-based inhibitor alpha(1) antitrypsin Portland. Furthermore, both furin-dependent and furin-independent MT1-MMP processing pathways are identified that require tethering of the metalloproteinase to the cell surface. These findings demonstrate the existence of a proprotein convertase-MT1-MMP axis that can regulate extracellular matrix remodeling.

Endoproteolytic processing of the ebola virus envelope glycoprotein: cleavage is not required for function

Proteolytic processing is required for the activation of numerous viral glycoproteins. Here we show that the envelope glycoprotein from the Zaire strain of Ebola virus (Ebo-GP) is proteolytically processed into two subunits, GP1 and GP2, that are likely covalently associated through a disulfide linkage. Murine leukemia virions pseudotyped with Ebo-GP contain almost exclusively processed glycoprotein, indicating that this is the mature form of Ebo-GP. Mutational analysis identified a dibasic motif, reminiscent of furin-like protease processing sites, as the Ebo-GP cleavage site. However, analysis of Ebo-GP processing in LoVo cells that lack the proprotein convertase furin demonstrated that furin is not required for processing of Ebo-GP. In sharp contrast to other viral systems, we found that an uncleaved mutant of Ebo-GP was able to mediate infection of various cell lines as efficiently as the wild-type, proteolytically cleaved glycoprotein, indicating that cleavage is not required for the activation of Ebo-GP despite the conservation of a dibasic cleavage site in all filoviral envelope glycoproteins.

An anti-HIV peptide construct derived from the cleavage region of the Env precursor acts on Env fusogenicity through the presence of a functional cleavage sequence

A 22-amino-acid-long multibranched peptide construct (CLV) derived from the cleavage region (KIEPLGVAPTKAKRR*VVQREKR*) of the human immunodeficiency virus (HIV) type-1 envelope precursor inhibits HIV infection (Virology, 1996, 223, 406-408). We attempted to characterize its activity for Env expressed via a recombinant vaccinia virus (rVV): gp 160 cleavage was delayed, but not impaired, in the presence of CLV (10 microM), whereas neither Env production nor Env membrane expression was significantly altered. Through the synthesis of analogs, we concluded that the presence of a cleavage sequence was required for inhibition of syncytium formation by CLV in rVV-infected CD(4+) cell cultures: indeed, a single amino acid residue substitution (R* > S) in the cleavage sites presented by CLV abolished its activity. Other analogs allowed us to further determine the region of CLV which mediates its activity. The ability of a radiolabeled CLV analog to enter cells was also shown. Although, these data strongly suggest that CLV acts on Env fusogenicity at least partially through interference with gp160 processing.

The life-cycle of human immunodeficiency virus type 1

The life-cycle of human immunodeficiency virus type 1 (HIV-1) has been studied using several techniques including immunoelectron microscopy and cryomicroscopy. The HIV-1 particle consists of an envelope, a core and the region between the core and the envelope (matrix). Virus particles in the extracellular space are observed as having various profiles: a central or an eccentric round electron-dense core, a bar-shaped electron-dense core, and immature doughnut-shaped particle. HIV-1 particles in the hydrated state were observed by high-resolution electron cryomicroscopy to be spherical and the lipid membrane was clearly resolved as a bilayer. Projections around the circumference were seen to be knob-like. The shapes and sizes of the projections, especially the head parts, were found to vary with each projection. HIV-1 cores were isolated with a mixture of Nonidet P40 and glutaraldehyde, and were confirmed to consist of HIV-1 Gag p24 protein by immunogold labelling. On infection, the HIV-1 virus was found to enter the cell in two ways: membrane fusion and endocytosis. After viral entry, no structures resembling virus particles could be seen in the cytoplasm. In the infected cells, positive reactions by immunolabelling suggest that HIV-1 Gag is produced in membrane-bound structures and transported to the cell surface by the cytoskeletons. A crescent electron-dense layer is then formed underneath the cell membrane. Finally, the virus particle is released from the cell surface and found extracellularly to be a complete virus particle with an electron-dense core. However, several cell clones producing defective mature, doughnut-shaped (immature) or teardrop-shaped particles were found to be produced in the extracellular space. In the doughnut-shaped particles, Gag p17 and p24 proteins exist facing each other against an inner electron-dense ring, suggesting that the inner ring consists of a precursor Gag protein showing a defect at the viral proteinase.

Inhibition of intracellular proteolytic processing of soluble proproteins by an engineered alpha 2-macroglobulin containing a furin recognition sequence in the bait region

The bait region of the general protease inhibitor alpha 2-macroglobulin (alpha 2M) was mutated by introducing a recognition sequence of furin. This did not interfere with folding, S-ester formation or tetramerization of the mutant recombinant alpha 2M (r alpha 2M). Mutant r alpha 2M inhibited furin in vitro, by a similar mechanism to that used by plasma alpha 2M to inhibit high-molecular-mass proteases. The mutant alpha 2M was intracellularly active in COS-1 cells in inhibiting the endogenous processing of the soluble substrates for furin (von Willebrand factor, transforming growth factor beta1 and a soluble form of the envelope glycoprotein gp160 from HIV-1) but not the membrane-bound form of gp160. The intracellular activity of mutant alpha 2M strongly indicated that alpha 2M attains its native conformation, and thus that the unusual internal S-ester is formed, before alpha 2M passes through the cleavage compartment(s). Our results show for the first time that modulation of the bait region of alpha 2M allows the creation of an inhibitor against membrane-bound proteases. It can be expected that the use of alpha 2M-bait mutants will become important as a technique for the study of various proteolytic processes and for the identification of the proteases involved.

The role of eukaryotic subtilisin-like endoproteases for the activation of human immunodeficiency virus glycoproteins in natural host cells

Proteolytic activation of the precursor envelope glycoproteins gp160 of human immunodeficiency virus type 1 (HIV-1) and gp140 of HIV-2, a prerequisite for viral infection, results in the formation of gp120/gp41 and gp125/gp36, respectively. Cleavage is mediated by cellular proteases. Furin, a member of the eukaryotic subtilisin family, has been shown to be an activating protease for HIV. Here, we compared the presence of furin and other mammalian subtilisins in lymphatic cells and tissues. Northern blot analyses revealed that furin and the recently discovered protease LPC/PC7 were the only subtilisin-like enzymes transcribed in such cells. Furin was identified as an enzymatically active endoprotease present in different lymphocytic, as well as monocytic, cell lines. When expressed from vaccinia virus vectors, the proprotein convertases were correctly processed, transported, and secreted into the media and enzymatically active. Coexpression of different subtilisins with the HIV envelope precursors revealed that furin and LPC/PC7 are able to cleave HIV-1 gp160. Moreover, both enzymes proteolytically processed the envelope precursor of HIV-2. gp140 was also cleaved to some extent by PC1, which is not, however, present in lymphatic cells. Furin- and LPC/PC7-catalyzed cleavage of HIV-1 gp160 resulted in biologically active envelope protein. In conclusion, among the known members of the subtilisin family, only furin and LPC/PC7 fulfill the requirements of a protease responsible for in vivo activation of HIV envelope glycoproteins.

PACE4: a subtilisin-like endoprotease with unique properties

PACE4 is one of the neuroendocrine-specific mammalian subtilisin-related endoproteases believed to function in the secretory pathway. The biosynthesis and secretion of PACE4 have been studied using transfected neuroendocrine and fibroblast cell lines. as well as primary pituitary cultures. ProPACE4 (approx. 106 kDa) is cleaved intracellularly before secretion of PACE4 (approx. 97 kDa); the N-terminal propeptide cleavage is accelerated in a truncated form of PACE4 lacking the Cys-rich C-terminal region (PACE4s). Neither PACE4 nor PACE4s is stored in regulated neuroendocrine secretory granules, whereas pro-opiomelanocortin-derived peptides and prohormone convertase I enter the regulated secretory pathway efficiently. The relatively slow cleavage of the proregion of proPACE4 in primary anterior pituitary cells, followed by rapid secretion of PACE4, is similar to the results for proPACE4 in transfected cell lines. The enzyme activity of PACE4 is distinct from furin and prohormone convertases, both in the marked sensitivity of PACE4 to inhibition by leupeptin and the relative insensitivity of PACE4 to inhibition by Ca2+ chelators and dithiothreitol; PACE4 is not inhibited by the alpha1-antitrypsin Portland variant that is very potent at inhibiting furin. The unique biosynthetic and enzymic patterns seen for PACE4 suggest a role for this neuroendocrine-specific subtilisin-like endoprotease outside the pathway for peptide biosynthesis.

Endoprotease activities other than furin and PACE4 with a role in processing of HIV-I gp160 glycoproteins in CHO-K1 cells

We addressed the question of whether furin is the endoprotease primarily responsible for processing the human immunodeficiency virus type I (HIV-I) envelope protein gp160 in mammalian cells. The furin-deficient Chinese hamster ovary (CHO)-K1 strain RPE.40 processed gp160 as efficiently as wild-type CHO-K1 cells in vivo. Although furin can process gp160 in vitro, this processing is probably not physiologically relevent, because it occurs with very low efficiency. PACE4, a furin homologue, allowed processing of gp160 when both were expressed in RPE.40 cells. Further, PACE4 participated in the activation of a calcium-independent protease activity in RPE.40 cells, which efficiently processed the gp160 precursor in vitro. This calcium-independent protease activity was not found in another furin-deficient cell strain, 7.P15, selected from the monkey kidney cell line COS-7.

Identification of the paired basic convertases implicated in HIV gp160 processing based on in vitro assays and expression in CD4(+) cell lines

The human immunodeficiency virus HIV envelope glycoprotein gp160 is synthesized as an inactive precursor, which is processed into its fusiogenic form gp120/gp41 by host cell proteinases during its intracellular trafficking. Kexin/subtilisin-related endoproteases have been proposed to be enzyme candidates for this maturation process. In the present study, 1) we examined the ability of partially purified precursor convertases and their isoforms to cleave gp160 in vitro. The data demonstrate that all the convertases tested specifically cleave the HIV envelope glycoprotein into gp120 and gp41. 2) We demonstrated that a 19-amino acid model peptide spanning the gp120/gp41 junction is cleaved by all convertases at the same gp160 site as that recognized in HIV-infected cells. 3) In an effort to evaluate specific convertase inhibitors, we showed that the alpha1-antitrypsin variant, alpha1-PDX, inhibits equally well the ability of the tested convertases to cleave gp160 in vitro. 4) Three lymphocyte cell lines were screened by reverse transcription polymerase chain reaction in an effort to identify which are the convertases expressed in the most common HIV target, the CD4(+) lymphocytes. The data demonstrate that furin, PC5/6, and the newly cloned PC7 are the main transcribed convertases, suggesting that these proteinases are the major gp160-converting enzymes in T4 lymphocytes.

Isolation of the human PC6 gene encoding the putative host protease for HIV-1 gp160 processing in CD4+ T lymphocytes

Production of infectious HIV-1 virions is dependent on the processing of envelope glycoprotein gp160 by a host cell protease. The protease in human CD4+ T lymphocytes has not been unequivocally identified, yet members of the family of mammalian subtilisin-like protein convertases (SPCs), which are soluble or membrane-bound proteases of the secretory pathway, best fulfill the criteria. These proteases are required for proprotein maturation and cleave at paired basic amino acid motifs in numerous cellular and viral glycoprotein precursors, both in vivo and in vitro. To identify the gp160 processing protease, we have used reverse transcription-PCR and Northern blot analyses to ascertain the spectrum of SPC proteases in human CD4+ T cells. We have cloned novel members of the SPC family, known as the human PC6 genes. Two isoforms of the hPC6 protease are expressed in human T cells, hPC6A and the larger hPC6B. The patterns of SPC gene expression in human T cells has been compared with the furin-defective LoVo cell line, both of which are competent in the production of infectious HIV virions. This comparison led to the conclusion that the hPC6 gene products are the most likely candidates for the host cell protease responsible for HIV-1 gp160 processing in human CD4+ T cells.

Comparative cellular processing of the human immunodeficiency virus (HIV-1) envelope glycoprotein gp160 by the mammalian subtilisin/kexin-like convertases

We present here the pulse and pulse-chase analysis of the biosynthesis of the envelope glycoprotein gp160 and its intracellular processing by the subtilisin/kexin-like convertases furin, PACE4, PC1, PC5 and its isoform PC5/6-B. We demonstrate that furin and to a much lesser extent PACE4, PC5/6-B and PC1 are candidate enzymes capable of processing gp160 intracellularly. Furthermore we show that furin can also process gp160/gp120 into gp77/gp53 products by cleavage at the sequence RIQR/GPGR just preceding the conserved GPGR structure found at the tip of the hypervariable V3 loop. The results show that processing into gp120 could occur at or before the trans-Golgi network (TGN) where sulphation of the oligosaccharide moieties of gp160 was detected. In contrast, the formation of gp77/gp53 by furin is a late event occurring after exit from the TGN. Our data also revealed that the alpha glucosidase I inhibitor N-butyldeoxynojirimycin, although affecting the oligosaccharide composition of gp160, does not impair the processing of either gp160 or gp120 by either furin or PACE4. Finally, the co-expression of the [Arg355, Arg358]-alpha-1-antitrypsin Portland variant was shown to potently inhibit the processing of both gp160 and gp120 by these convertases.