Overview of the Structure⁻Function Relationships of Mannose-Specific Lectins from Plants, Algae and Fungi

Abstract

To date, a number of mannose-binding lectins have been isolated and characterized from plants and fungi. These proteins are composed of different structural scaffold structures which harbor a single or multiple carbohydrate-binding sites involved in the specific recognition of mannose-containing glycans. Generally, the mannose-binding site consists of a small, central, carbohydrate-binding pocket responsible for the "broad sugar-binding specificity" toward a single mannose molecule, surrounded by a more extended binding area responsible for the specific recognition of larger mannose-containing N-glycan chains. Accordingly, the mannose-binding specificity of the so-called mannose-binding lectins towards complex mannose-containing N-glycans depends largely on the topography of their mannose-binding site(s). This structure⁻function relationship introduces a high degree of specificity in the apparently homogeneous group of mannose-binding lectins, with respect to the specific recognition of high-mannose and complex N-glycans. Because of the high specificity towards mannose these lectins are valuable tools for deciphering and characterizing the complex mannose-containing glycans that decorate both normal and transformed cells, e.g., the altered high-mannose N-glycans that often occur at the surface of various cancer cells.

Keywords: function; fungi; lectin; mannose-binding specificity; plant; structure; use as tools.

Figure 1

Structural diversity of the mannose-binding lectins. (A). Two-chain lectin protomer of Lathyrus ochrus (PDB code 1LOE [48]). Light chain and heavy chains are colored green and red, respectively. (B). Homodimeric organization of the L. ochrus isolectin-I (1LOE). The light and heavy chains of the dimer are colored differently. (C). Homotetrameric organization of Con A (PDB code 3CNA). The four single-chain protomers are shown in different colors. (D). The β-prism organization of the artocarpin protomer from Artocarpus integrifolia (PDB code 1J4S). The three bundles of β-strands forming the β-prism are colored green, red and orange, respectively. (E). Homotetrameric organization of artocarpin from A. integrifolia (1J4U). The β-prism protomers are colored differently. (F). Homooctameric organization of Heltuba from Helianthus tuberosus (1C3M) [81]. The β-prism protomers are colored differently. (G). The β-prism II organization of the protomer of GNA from Galanthus nivalis (PDB code 1MSA). (H). Organization of the β-prism II protomers in the GNA tetramer (PDB code 1MSA). (I). Hexameric structure of the tarin lectin from Colocasia esculenta (PDB code 5T20). The six β-prism-folded protomers are colored differently.

Figure 2

Three-dimensional models for the EUL domain of EUL-domains of rice lectin Orysata, showing the β-trefoill organization made of three bundles of antiparallel β-sheets (I, II, III).

Figure 3

Three-dimensional model of griffithsin (PDB code 2GTY), showing the β-prism organization made of three four-stranded β-sheets in each monomer. The four stranded β-sheets are colored red, pink and magenta in monomer (A), and blue, light blue and purple in monomer (B), respectively. The stars indicate the localization of the carbohydrate-binding sites in each monomer.

Figure 4

(A). Beta-propeller organization of tectonin 2 from the mushroom Laccaria bicolor in complex with allyl-α4-methyl-mannoside. The lectin consists of 6 antiparallel strands of β-sheet (colored differently) organized in 6 blades around the axis of the β-propeller. The allyl-mannoside residues (M) anchored to the carbohydrate-binding sites of the lectin are colored purple (PDB code 5FSC) (B). Sixth mannose-binding site of tectonin 2 in complex with allyl-α4-methyl-mannoside. Hydrogen bonds connecting the monosaccharides to the amino acid residues Ser200, Asn216 and Tyr222, forming the monosaccharide-binding site are represented by black dashed lines. Aromatic residues Trp3 and Tyr222, paticipating in stacking interactions with the sugar ring are colored orange. The molecular surface of the lectins is colored dark grey and their extended oligosaccharide-binding areas are delineated by white dashed lines. (C). The shallow depression corresponding to the monosaccharide-binding site that accommodates the allyl-mannoside residue (colored purple) at the molecular surface (colored according to the oulombic charges) of tectonin 2, is delineated by a green dashed line.

Figure 5

(A,C). Beta-sandwich organization of Flo5 from the yeast Saccharomyces cerevisiae in complex with mannose (A) (PDB code 2XJP) and α1,2-mannobiose (C) (PDB code 2XJS). The mannose-binding N-terminal domain of Flo5 consists of two strands of β-sheet forming a β-sandwich structure. (B). Network of hydrogen bonds anchoring mannose (colored purple) to the amino acid residues forming the carbohydrate-binding site located at the top of the β-sandwich. Two stacking interactions of the pyranose ring of mannose with aromatic residues Tyr54 and Trp228 (colored orange), complete the interaction. (D). Network of hydrogen bonds anchoring α1,2-mannobiose (colored purple) Flo5, showing additional hydrogen bonds anchoring α1,2-mannobiose to Gln117 and Ser 227 residues. Residues Asp160, Asp161, Val226 and Trp228, also serve as ligands for a Ca²⁺ ion (colored red in A and C) located at the bottom of the mannose-binding pocket.

Figure 6

(A). Beta-sandwich organization of NcCVNH from Neurospora crassa (PDB code 2JZL), showing the two-swapped domains A (colored purple) and B (colored pink). Strands of β-sheet are numbered 1–10. N and C indicate the N-terminal and C-terminal extremities of the polypeptide chain, respectively. The mannose-binding site has been identified at the top of domain B (red star ★). (B). Ribbon diagram showing the structural organization of the two-domain (A and B) cyanobacterial microvirin from Microcystis aeruginosa (PDB code 2YHH) The β-strands, β-hairpins and turns, are colored purple, red and green, respectively.

Figure 7

(A,B). ConA from Canavalia ensiformis in complex with α-methylmannoside (PDB code 5CNA). (C,D). Isolectin LoLI from Lathyrus ochrus in complex with Man (PDB code 1LOB). (E,F). Artocarpin from Artocarpus integrifolia in complex with α-mthylmannoside (PDB code 1J4U). (G,H). Heltuba from Helianthus tuberosus in complex with Manα1,3Man (PDB code 1C3M). (I,J). Third Man-binding site of GNA from Galanthus nivalis in complex with α-methylmannoside (PDB code 1MSA). Hydrogen bonds connecting the monosaccharides to the amino acid residues forming the monosaccharide-binding site are represented by black dashed lines. Aromatic residues participating in stacking interactions with the sugar rings are colored orange. The molecular surface of the lectins is colored dark grey and their extended oligosaccharide-binding areas are delineated by white dashed lines. The shallow depression corresponding to the monosaccharide-binding site that accommodates simple sugars is delineated by a green dashed line. The green and violet spheres correspond to the Ca²⁺ and Mn²⁺ ions, that have a stabilizing effect on the carbohydrate-binding site.

Figure 8

Docking of αMeMan to the monosaccharide-binding site of the active sub-domain III of OsEULS3. Hydrogen bonds connecting Man to the amino acid residues forming the monosaccharide-binding site are shown by black dashed lines and distances are indicated in Å. The aromatic Trp136 residue participating in stacking interactions with the sugar ring is colored orange.

Figure 9

(A,B). ConA from Canavalia ensiformis in complex with β-D-GlcNAc-(1,2)-α-D-Man-(1,6)-[β-D-GlcNAc-(1,2)-α-D-Man-(1,6]-αD-Man (PDB code 1TEI) [196]. (C,D). Isolectin LoLII from Lathyrus ochrus in complex with a biantennary octasaccharide of the N-acetyllactosamine type from lactotransferrin (PDB code 1LOF). (E,F). GNA from Galanthus nivalis in complex with three mannosyl residues from a mannopentaose (PDB code 1JPC). (G,H). PAL from Pterocarpus angolensis in complex with a mannotetraose (PDB code 2PHF). Hydrogen bonds connecting the oligosaccharides to the amino acid residues forming the extended carbohydrate-binding site are represented by black dashed lines. Aromatic residues participating in stacking interactions with the sugar rings are colored orange. The electrostatic potentials were calculated and mapped on the molecular surface of the lectins, using YASARA. The extended oligosaccharide-binding areas are delineated by white dashed lines. The shallow depression corresponding to the monosaccharide-binding site that accommodates simple sugars, is delineated by a green dashed line.

Figure 10

Structural diversity of the mannose-binding lectins. (A–C). Ribbon diagrams (A lateral view, B upper view) and surface electrostatic potentials (C) of griffthsin in complex with a high-mannose branched glycan (colored cyan) (PDB code 3LL2), showing the β-prism organization of the lectin. Note the electronegatively charged character (colored red) of the Man-binding pockets at the upper face of the β-prism. (D–F). Ribbon diagrams (D lateral view, E upper view) and surface electrostatic potentials (F) of actinohivin in complex with a high-mannose branched glycan (colored cyan) (PDB code 3S5X), showing the β-trefoil (β-prism II) organization of the lectin. Note the electronegatively (colored red) and electropositively (colored blue) charged character of the Man-binding pockets at the edges of the β-trefoil. (G–I). Ribbon diagrams (G lateral view, H upper view) and surface electrostatic potentials (I) of actinohivin in complex with α-1,2-mannotriose (colored cyan) (PDB code 4P6A), showing the organization of the lectin. Note the electronegatively charged character (colored red) of the Man-binding pockets.

Figure 11

Three-dimensional structure of the highly-mannosylated gp120 molecule associated to the O-glycosylated gp41 molecule (PDB code 5FYK). Surface-exposed Man residues of high-mannose N-glycoproteins decorating gp120 are colored green. O-glycans of gp41 are colored blue.

Figure 12

Three-dimensional structure of the gp120-gp41 tandem complexed to a CD4 molecule (PDB code 47VP). Gp120, gp41 and CD4 are colored pink, purple, and orange/yellow, respectively. The high-mannose N-glycan chains decorating gp120 are represented in cyan colored sticks. The carbohydrate binding agents (red arrow) specifically recognize some of the high-mannose N-glycans exposed at the surface of gp120, thus preventing the recognition of gp120 by the CD4 molecule of the CD4+ T lymphocytes. In fact, the association of three gp120-gp41 tandems forms the HIV-1-envelope spike, which facilitates the HIV-1 entry. The Env spike consists of a transmembrane trimer of gp41 associated to an extracellular trimer of gp120 offering exposed high-mannose glycans to the CD4 recognition process.