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.

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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-N's 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.

The HIV-inactivating protein Cyanovirin-N (CV-N) is a cyanobacterial lectin that exhibits potent antiviral activity at nanomolar concentrations by interacting with high-mannose carbohydrates on viral glycoproteins. To date there is no molecular explanation for this potent virucidal activity, given the experimentally measured micromolar affinities for small sugars and the problems encountered with aggregation and precipitation of high-mannose/CV-N complexes. Here, we present results for two CV-N variants, CV-N(mutDA) and CV-N(mutDB), compare their binding properties with monomeric [P51G]CV-N (a stabilized version of wtCV-N) and test their in vitro activities. The mutations in CV-N(mutDA) and CV-N(mutDB) comprise changes in amino acids that alter the trimannose specificity of domain A(M) and abolish the sugar binding site on domain B(M), respectively. We demonstrate that carbohydrate binding via domain B(M) is essential for antiviral activity, whereas alterations in sugar binding specificity on domain A(M) have little effect on envelope glycoprotein recognition and antiviral activity. Changes in A(M), however, affect the cross-linking activity of CV-N. Our findings augment and clarify the existing models of CV-N binding to N-linked glycans on viral glycoproteins, and demonstrate that the nanomolar antiviral potency of CV-N is related to the constricted and spatially crowded arrangement of the mannoses in the glycan clusters on viral glycoproteins and not due to CV-N induced virus particle agglutination, making CV-N a true viral entry inhibitor.

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-N's 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.

Cyanovirin-N can exist in solution in monomeric and domain-swapped dimeric forms, with HIV-antiviral activity being reported for both. Here we present results for CV-N variants that form stable solution dimers: the obligate dimer [DeltaQ50]CV-N and the preferential dimer [S52P]CV-N. These variants exhibit comparable DeltaG values (10.6 +/- 0.5 and 9.4 +/- 0.5 kcal.mol(-1), respectively), similar to that of stabilized, monomeric [P51G]CV-N (9.8 +/- 0.5 kcal.mol(-1)), but significantly higher than wild-type CV-N (4.1 +/- 0.2 kcal.mol(-1)). During folding/unfolding, no stably folded monomer was observed under any condition for the obligate dimer [DeltaQ50]CV-N, whereas two monomeric, metastable species were detected for [S52P]CV-N at low concentrations. This is in contrast to our previous results for [P51G]CV-N and wild-type CV-N, for which the dimeric forms were found to be the metastable species. The dimeric mutants exhibit comparable antiviral activity against HIV and Ebola, similar to that of wild-type CV-N and the stabilized [P51G]CV-N variant.

Phytases are the primary enzymes responsible for the hydrolysis of phytic acid, myo-inositol-1,2,3,4,5,6-hexakisphosphate (I-1,2,3,4,5,6-P,). A number of phytases with varying specificities, properties, and localizations hydrolyze phytic acid present in cells. l h e specificity of hydrolysis of phytic acid by alkaline phytase from lily (Lilium longiflorum 1.) pollen is described. Structures of the intermediate inositol phosphates and the final product were established by a variety of nuclear magnetic resonance techniques ('H-, 31P-, and 3'P-'H-detected multiple quantum coherence spectroscopy, and total correlation spectroscopy). O n the basis of the structures identified we have proposed a scheme of hydrolysis of phytic acid. Initial hydrolysis of the phosphate ester occurs at the D-5 position of phytic acid to yield the symmetrical I-1,2,3,4,6-P5. The two subsequent dephosphorylations occur adjacent to the D-5 hydroxyl group to yield I-1,2,3-P3 as the final product. Alkaline phytase differs from other phytases in the specificity of hydrolysis of phosphate esters on the inositol ring, its high substrate specificity for phytic acid, and biochemical properties such as susceptibility to activation by calcium and inhibition by fluoride. l h e physiological significance of alkaline phytase and the biological role of I-1,2,3-P3 remain to be identified.Phytic acid, myo-inositol hexakisphosphate, is a major constituent of seeds and pollen grains (1-5% of dry weight) (Loewus, 1990;Raboy, 1990). In mature lily (Lilium longiflorum L.) pollen and seeds, phytic acid is localized in membrane-bound phytic-rich granules. The presence of phytic acid in plant cells has been known for some time (Loewus, 1990;Raboy, 1990). However, it was only recently recognized that phytic acid is present in virtually all mammalian cells in concentrations higher than most other inositol phosphates (Menniti et al., 1993) and may function as a neurotransmitter (Vallejo et al., 1988). The discovery that I-1,4,5-P3 plays a crucial role in calcium cellular signaling (Bemdge et al., 1989) has greatly increased interest in the structure, metabolism, and biological role of inositol phosphates, including phytic acid.The primary enzymes responsible for the degradation of phytic acid are phytases. Phytases are a special class of phosphatases that catalyze the sequential hydrolysis of phytic acid to inositol phosphates and, in some cases, to inositol. Phytases occur in a variety of organisms including plants, fungi, and animals (reviewed by Cosgrove, 1980a). A variety of phytases differing in pH optima, substrate specificity, and * Corresponding author; fax 1-906-487-2061. specificity of hydrolysis have been identified in plants and fungi (Cosgrove, 1980a(Cosgrove, , 1980b. Acid phytases from wheat bran and Aspergilli have been extensively studied and the stereospecificity of hydrolysis has been well established (Cosgrove, 1980b). Based on the specificity of initial hydrolysis, two classes of acid phytases are recognized by the Intemational Union of Pure a...

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