The Characteristic Structure of Anti‐HIV Actinohivin in Complex with Three HMTG D1 Chains of HIV‐gp120

Abstract

The anti-HIV lectin actinohivin (AH) specifically interacts with HMTG (high-mannose-type glycan), which is attached to the glycoprotein gp120 of HIV-1 in a process in which the three branched mannotriose chains (D1, D2, and D3) of HMTG exhibit different binding affinities, it being estimated that that of D1 is the strongest, that of D3 is weaker, and that of D2 is undetectable. These properties have been ascribed to the stereochemical differences in linkages between the second and the third mannose residues of the three chains. In order to clarify the interaction geometry between AH and the major target D1, an X-ray determination of the crystal structure of AH in complex with D1-which is α(1,2)mannotriose composed of three mannose (Man) residues linked together only by α(1,2) bonding-has been performed. In each of the three D1-binding pockets of AH, two Man residues of D1 are accommodated at zones 1 and 2 in the pocket, in the same way as those found in the α(1,2)mannobiose-bound AH crystals. However, an OMIT map shows poor densities at both ends of the two residues. This suggests the existence of positional disorder of D1 in the pocket: the two zones are each occupied by two Man residues in two different modes, with mode A involving the Man1 and Man2 residues and mode B the Man2 and Man3 residues. In each mode, D1 is stabilized by adopting a double-bracket-shaped conformation through CH⋅⋅⋅O interactions. In mode B, however, the Man1 residue, which is the most sensitive residue to AH binding, protrudes wholly into the solvent region without contacts with AH. In mode A, in contrast, the Man3 residue interacts with the essential hydrophobic amino acid residues (Tyr and Leu conserved between the three pockets) of AH. Therefore, mode A is likely to be the one that occurs when whole HMTG is bound. In this mode, the two hydroxy groups (O3 and O4) of the Man2 residue are anchored in zone 2 by four hydrogen bonds with Asp, Asn, and Tyr residues of AH. In addition, it has been found that an isolated water molecule buried in the hydrophobic long loop bridges between Asp of AH and the hydroxy group of Man2 through hydrogen bonds. The most interesting feature is found in the interaction of the Man1 and Man3 residues with AH. All eight hydroxy groups of the two residues are completely exposed in the solvent region, whereas their hydrophobic parts make contacts with a Leu residue and two Tyr residues so that the shape of D1 and the surface of AH fit well over a wide area. These structural characteristics are potentially useful for development of AH to produce more effective antiretroviral drugs to suppress the infectious expansion of HIV/AIDS and to help expedite an end to the HIV/AIDS pandemic in the near future.

Engineering of a Lectibody Targeting High-Mannose-Type Glycans of the HIV Envelope

“…AH is composed of three near-homologous tandem repeats of a 38amino-acid domain harboring a binding pocket for an HMG. 13,14 We hypothesized that the high aggregation propensity of AH could be improved by neutralizing its surface charge variation. Thus, we designed multiple AH variants by changing amino acids in one or two domains to corresponding residues in the other domain(s) so that all three domains are alike and have similar overall surface charge properties.…”

“…Actinohivin (AH) is an actinomycete-derived lectin recognizing the terminal a1,2-linked mannose residues on HMGs. 13,14 AH was shown to neutralize HIV without inducing cytotoxicity or mitogenicity in human blood cells, 15 suggesting that this lectin may provide a useful tool for developing a new HMG-targeting agent. A major drawback, however, is that AH is highly hydrophobic, prone to aggregate, and, thus, recalcitrant to efficient recombinant production, hampering its development as a drug and/or biological tool.…”

Antiviral lectins: Selective inhibitors of viral entry

“…The lectin is active against HIV-1 and HIV-2 with IC 50 values of 2e110 nM in the MAGI assay and inhibited cell-cell fusion with EC 50 values ranging from 60 to 700 nM (Chiba et al, 2004). The x-ray crystal structures of AH reveal that the protein is organized into a Ricin B-chain-like b-trefoil with a1,2-mannotriose bound in each CRD located between the second and third b strands of the carbohydrate binding site (Zhang et al, 2014). Specifically, the side chains of D15, Y23, and N28 hydrogen bond with O3 and O4 of M2 of the trimannose ligand.…”

Mannose-Specific Lectins from Marine Algae: Diverse Structural Scaffolds Associated to Common Virucidal and Anti-Cancer Properties

“…The cyanovirin-N lectin domain consists of a β-barrel (PDB code 2EZM) [47], whereas the actinovirin lectin domain exhibits a β-trefoil architecture (PDB code 3A07) [48]. The accommodation of high-mannose glycans by both lectins, closely resembles that found in griffithsin and other man-specific seaweed lectins, with a prominent role for the oligosaccharide-binding pocket in the CBSs [49,50], (Figure 5).…”

“…Finally, the accommodation of high-mannose glycan chains by the man-specific seaweed lectins consists of a complex interaction process in which the monosaccharide-binding pocket plays a prominent role. Similarly, the mannose-binding pocket of the closely-related bacterial lectin cyanovirin-N [49] and fungal lectin actinohivin [50], plays a key role in the accommodation of high-mannose glycans (see Figure 5).…”