Binding Site Geometry and Subdomain Valency Control Effects of Neutralizing Lectins on HIV-1 Viral Particles.

Carbohydrate binding proteins such as griffithsin, cyanovirin-N, and BanLec are potent HIV entry inhibitors and promising microbicides. Each binds to high-mannose glycans on the surface envelope glycoprotein gp120, yet the mechanisms by which they engage viral spikes and exhibit inhibition constants ranging from nanomolar to picomolar are not understood. To determine the structural and mechanistic basis for recognition and potency, we selected a panel of lectins possessing different valencies per subunit, oligomeric states, and relative orientations of carbohydrate binding sites to systematically probe their contributions to inhibiting viral entry. Cryo-electron micrographs and immuno gold staining of lectin-treated viral particles revealed two distinct effects-namely, viral aggregation or clustering of the HIV-1 envelope on the viral membrane-that were dictated by carbohydrate binding site geometry and valency. "Sandwich" surface plasmon resonance experiments revealed that a second binding event occurs only for those lectins that could aggregate viral particles. Furthermore, picomolar Kd values were observed for the second binding event, providing a mechanism by which picomolar IC50 values are achieved. We suggest that these binding and aggregation phenomena translate to neutralization potency.

An intramolecular salt bridge drives the soluble domain of GTP-bound atlastin into the postfusion conformation.

Endoplasmic reticulum (ER) network branching requires homotypic tethering and fusion of tubules mediated by the atlastin (ATL) guanosine triphosphatase (GTPase). Recent structural studies on the ATL soluble domain reveal two dimeric conformers proposed to correspond to a tethered prefusion state and a postfusion state. How the prefusion conformer transitions to the postfusion conformer is unknown. In this paper, we identify an intramolecular salt bridge mediated by two residues outside the GTPase domain near the point of rotation that converts the prefusion dimer to the postfusion state. Charge reversal of either residue blocked ER network branching, whereas a compensatory charge reversal to reestablish electrostatic attraction restored function. In vitro assays using the soluble domain revealed that the salt bridge was dispensable for GTP binding and hydrolysis but was required for forming the postfusion dimer. Unexpectedly, the postfusion conformation of the soluble domain was achieved when bound to the nonhydrolyzable GTP analogue guanosine 5'-[ß,γ-imido]triphosphate, suggesting that nucleotide hydrolysis might not be required for the prefusion to postfusion conformational change.

Nucleotide-dependent self-assembly of Nucleoside Diphosphate Kinase (NDPK) in vitro.

In addition to their role in nucleotide homeostasis, members of the Nucleoside Diphosphate Kinase (NDPK) family have been implicated in tumor metastasis, cell migration and vesicle trafficking. Although its role in most cases depends on nucleotide catalysis, a precise understanding of how the catalytic activity of NDPK supports its function in diverse processes is lacking. Here we report that wild type, but not catalytically inactive (H118C) NDPKB, undergoes dynamic self-assembly into ordered 20-25 nm diameter filaments in vitro. Self-assembly is nucleoside triphosphate dependent, GTP being most effective at promoting polymer formation. In addition, polymerization appears to depend on formation of the phosphoryl-Histidine intermediate of the enzyme, suggesting a previously unappreciated conformational change in NDPK during its catalytic cycle. We hypothesize that the observed nucleotide-dependent self-assembly property of NDPKB may reflect a key feature of NDPK enzymes that enables their function in diverse processes.

Nucleoside diphosphate kinase B (NDKB) scaffolds endoplasmic reticulum membranes in vitro.

The mechanisms that structure the mammalian endoplasmic reticulum (ER) network are not fully understood. Here we show that salt extraction of semi-intact normal rat kidney (NRK) fibroblasts and subsequent incubation of the extracted cells with ATP resulted in dramatic ER network retraction. Under these conditions, addition of a single protein, Nucleoside Diphosphate Kinase B (NDKB), was sufficient to reverse the retraction and to promote ER network extension. The underlying mechanism of membrane extension involved direct lipid binding, as NDKB bound phosphatidylinositol (PtdIns)(4)P, PtdIns(4,5)P(2) and phosphatidic acid (PA); binding to these anionic lipids required clusters of basic residues on the surface of the NDKB hexamer; and amino acid changes in NDKB that blocked lipid binding also blocked ER network extension. Remarkably, purified NDKB transformed a uniform population of synthetic lipid vesicles into extensive membrane networks, and this also required its phospholipid-binding activity. Altogether these results identify a protein sufficient to scaffold extended membrane networks, and suggest a possible role for NDKB-like proteins, as well as phosphoinositides and/or acidic phospholipids, in modulating ER network morphogenesis.