Laura G. Barrientos

Cyanovirin-N binds to the viral surface glycoprotein, GP1,2 and inhibits infectivity of Ebola virus.

Ebola virus (Ebo) causes severe hemorrhagic fever and high mortality in humans. There are currently no effective therapies. Here, we have explored potential anti-Ebo activity of the human immunodeficiency virus (HIV)-inactivating protein cyanovirin-N (CV-N). CV-N is known to potently inhibit the infectivity of a broad spectrum of HIV strains at the level of viral entry. This involves CV-N binding to N-linked high-mannose oligossacharides on the viral glycoprotein gp120. The Ebola envelope contains somewhat similar oligosaccharide constituents, suggesting possible susceptibility to inhibition by CV-N. Our initial results revealed that CV-N had both in vitro and in vivo antiviral activity against the Zaire strain of the Ebola virus (Ebo-Z). Addition of CV-N to the cell culture medium at the time of Ebo-Z infection inhibited the development of viral cytopathic effects (CPEs). CV-N also delayed the death of Ebo-Z-infected mice, both when given as a series of daily subcutaneous injections and when the virus was incubated ex vivo together with CV-N before inoculation into the mice. Furthermore, similar to earlier results with HIV gp120, CV-N bound with considerable affinity to the Ebola surface envelope glycoprotein, GP(1,2). Competition experiments with free oligosaccharides were consistent with the view that carbohydrate-mediated CV-N/GP(1,2) interactions involve oligosaccharides residing on the Ebola viral envelope. Overall, these studies broaden the range of viruses known to be inhibited by CV-N, and further implicate carbohydrate moieties on viral surface proteins as common viral molecular targets for this novel protein.

The Domain-Swapped Dimer of Cyanovirin-N Is in a Metastable Folded State: Reconciliation of X-Ray and NMR Structures

The structure of the potent HIV-inactivating protein cyanovirin-N was previously found by NMR to be a monomer in solution and a domain-swapped dimer by X-ray crystallography. Here we demonstrate that, in solution, CV-N can exist both in monomeric and in domain-swapped dimeric form. The dimer is a metastable, kinetically trapped structure at neutral pH and room temperature. Based on orientational NMR constraints, we show that the domain-swapped solution dimer is similar to structures in two different crystal forms, exhibiting solely a small reorientation around the hinge region. Mutation of the single proline residue in the hinge to glycine significantly stabilizes the protein in both its monomeric and dimeric forms. By contrast, mutation of the neighboring serine to proline results in an exclusively dimeric protein, caused by a drastic destabilization of the monomer.

The Highly Specific Carbohydrate-Binding Protein Cyanovirin-N: Structure, Anti-HIV/Ebola Activity and Possibilities for Therapy

Cyanovirin-N (CV-N), a cyanobacterial lectin, is a potent viral entry inhibitor currently under development as a microbicide against a broad spectrum of enveloped viruses. CV-N was originally identified as a highly active anti-HIV agent and later, as a virucidal agent against other unrelated enveloped viruses such as Ebola, and possibly other viruses. CV-Ns antiviral activity appears to involve unique recognition of N-linked high-mannose oligosaccharides, Man-8 and Man-9, on the viral surface glycoproteins. Due to its distinct mode of action and opportunities for harnessing the associated interaction for therapeutic intervention, a substantial body of research on CV-N has accumulated since its discovery in 1997. In this review we focus in particular on structural studies on CV-N and their relationship to biological activity.

Multisite and Multivalent Binding between Cyanovirin-N and Branched Oligomannosides: Calorimetric and NMR Characterization

Binding of the protein cyanovirin-N to oligomannose-8 and oligomannose-9 of gp120 is crucially involved in its potent virucidal activity against the human immunodeficiency virus (HIV). The interaction between cyanovirin-N and these oligosaccharides has not been thoroughly characterized due to aggregation of the oligosaccharide-protein complexes. Here, cyanovirin-Ns interaction with a nonamannoside, a structural analog of oligomannose-9, has been studied by nuclear magnetic resonance and isothermal titration calorimetry. The nonamannoside interacts with cyanovirin-N in a multivalent fashion, resulting in tight complexes with an average 1:1 stoichiometry. Like the nonamannoside, an alpha1-->2-linked trimannoside substructure interacts with cyanovirin-N at two distinct protein subsites. The chitobiose and internal core trimannoside substructures of oligomannose-9 are not recognized by cyanovirin-N, and binding of the core hexamannoside occurs at only one of the sites on the protein. This is the first detailed analysis of a biologically relevant interaction between cyanovirin-N and high-mannose oligosaccharides of HIV-1 gp120.

Characterization of surfactant liquid crystal phases suitable for molecular alignment and measurement of dipolar couplings

Media employed for imparting partial alignment onto solute molecules have recently attracted considerable attention, since they permit the measurement of NMR parameters for solute biomolecules commonly associated with solid state NMR. Here we characterize a medium which is based on a quasi-ternary surfactant system comprising cetylpyridinium bromide/hexanol/sodium bromide. We demonstrate that dilute solutions of this system can exist in liquid crystalline phases which orient in the magnetic field and allow the measurement of residual dipolar couplings under a variety of conditions. The present system is extremely versatile and robust, tolerating different buffer conditions, temperature ranges and concentrations.

Functional homologs of cyanovirin-N amenable to mass production in prokaryotic and eukaryotic hosts.

Cyanovirin-N (CV-N) is under development as a topical (vaginal or rectal) microbicide to prevent sexual transmission of human immunodeficiency virus (HIV), and an economically feasible means for very large-scale production of the protein is an urgent priority. We observed that N-glycosylation of CV-N in yeast eliminated the anti-HIV activity, and that dimeric forms and aggregates of CV-N occurred under certain conditions, potentially complicating the efficient, large-scale manufacture of pure monomeric CV-N. We therefore expressed and tested CV-N homologs in which the glycosylation-susceptible Asn residue at position 30 was replaced with Ala, Gln, or Val, and/or the Pro at position 51 was replaced by Gly to eliminate potential conformational heterogeneity. All homologs exhibited anti-HIV activity comparable to wild-type CV-N, and the Pro51Gly homologs were significantly more stable proteins. These glycosylation-resistant, functional cyanovirins should be amenable to large-scale production either in bacteria or in eukaryotic hosts.

The domain-swapped dimer of cyanovirin-N contains two sets of oligosaccharide binding sites in solution

The binding of high-mannose oligosaccharides to the domain-swapped dimeric form of the potent HIV-inactivating protein cyanovirin-N (CV-N) was investigated in solution by NMR, complementing recent structural studies by X-ray crystallography on similar complexes [J. Biol. Chem. 277 (2002) 34336]. The crystal structures of CV-N dimer complexed with Man-9 and hexamannoside revealed two carbohydrate binding sites on opposite ends of the molecule. No binding was observed at site 1, previously identified on the solution monomer of CV-N [Structure 9 (2001) 931; Shenoy et al., Chem. Biol. 9 (2002) 1109]. Here, we report the presence of four sugar binding sites on the CV-N dimer in solution, identified by chemical shift mapping with hexamannoside and nonamannoside, synthetic substructures of Man-9. Our results demonstrate that in solution the domain-swapped CV-N dimer, like the CV-N monomer, contains two types of sites that are available for carbohydrate binding, suggesting that the occlusion of the primary sites in the crystal is due to specific features of the solid state.

In Vitro Evaluation of Cyanovirin-N Antiviral Activity, by Use of Lentiviral Vectors Pseudotyped with Filovirus Envelope Glycoproteins

Cyanovirin-N (CV-N) has been shown to inhibit Ebola Zaire virus (EboZV) infection, both in vitro and in vivo, through its ability to bind to oligomannoses-8/9 on the EboZV surface glycoprotein (GP). Here, we report the in vitro potency of CV-N to inhibit EboZV GP- and Marburg virus GP-pseudotyped viruses (EC50 approximately 40-60 nmol/L and approximately 6-25 nmol/L, respectively) from mediating gene transduction into HeLa cells. In addition, we provide evidence that CV-N can effectively inhibit DC-SIGN-mediated EboZV infection. Our data emphasize both the utility of GP-pseudotyped vectors in the assessment of compounds that affect cell entry by filovirus and the use of CV-N as a reagent for the probing of carbohydrate-dependent interactions at viral entry.

Solution Structure of a Circular-Permuted Variant of the Potent HIV-inactivating Protein Cyanovirin-N: Structural Basis for Protein Stability and Oligosaccharide Interaction

The high-resolution solution structure of a monomeric circular permuted (cp) variant of the potent HIV-inactivating protein cyanovirin-N (CV-N) was determined by NMR. Comparison with the wild-type (wt) structure revealed that the observed loss in stability of cpCV-N compared to the wt protein is due to less favorable packing of several residues at the pseudo twofold axis that are responsible for holding the two halves of the molecule together. In particular, the N and C-terminal amino acid residues exhibit conformational flexibility, resulting in fewer and less favorable contacts between them. The important hydrophobic and hydrogen-bonding network between residues W49, D89, H90, Y100 and E101 that was observed in wt CV-N is no longer present. For instance, Y100 and E101 are flexible and the tryptophan side-chain is in a different conformation compared to the wt protein. The stability loss amounts to approximately 2kcal/mol and the mobility of the protein is evident by fast amide proton exchange throughout the chain. Mutation of the single proline residue to glycine (P52G) did not substantially affect the stability of the protein, in contrast to the finding for wtCV-N. The binding of high-mannose type oligosaccharides to cpCV-N was also investigated. Similar to wtCV-N, two carbohydrate-binding sites were identified on the protein and the Man alpha1-->2Man linked moieties on the sugar were delineated as binding epitopes. Unlike in wtCV-N, the binding sites on cpCV-N are structurally similar and exhibit comparable binding affinities for the respective sugars. On the basis of the studies presented here and previous results on high-mannose binding to wtCV-N, we discuss a model for the interaction between gp120 and CV-N.

Design and initial characterization of a circular permuted variant of the potent HIV-inactivating protein cyanovirin-N.

A circular permuted variant of the potent human immunodeficiency virus (HIV)‐inactivating protein cyanovirin‐N (CV‐N) was constructed. New N‐ and C‐termini were introduced into an exposed helical loop, and the original termini were linked using residues of the original loop. Since the three‐dimensional structure of wild‐type cyanovirin‐N is a pseudodimer, the mutant essentially exhibits a swap between the two pseudo‐symmetrically related halves. The expressed protein, which accumulates in the insoluble fraction, was purified, and conditions for in vitro refolding were established. During refolding, a transient dimeric species is also formed that converts to a monomer. Similar to the wild‐type CV‐N, the monomeric circular permuted protein exhibits reversible thermal unfolding and urea denaturation. The mutant is moderately less stable than the wild‐type protein, but it displays significantly reduced anti‐HIV activity. Using nuclear magnetic resonance spectroscopy, we demonstrate that this circular permuted monomeric molecule adopts the same fold as the wild‐type protein. Characterization of these two architecturally very similar molecules allows us to embark, for the first time, on a structure guided focused mutational study, aimed at delineating crucial features for the extraordinary difference in the activity of these molecules. Proteins 2002;46:153–160.