Structures and binding specificity of galactose- and mannose-binding lectins from champedak: differences from jackfruit lectins

Abstract

Galactose-binding and mannose-binding lectins from the champedak fruit, which is native to South-east Asia, exhibit useful potential clinical applications. The specificity of the two lectins for their respective ligands allows the detection of potential cancer biomarkers and monitoring of the glycosylated state of proteins in human serum and/or urine. To fully understand and expand the use of these natural proteins, their complete sequences and crystal structures are presented here, together with details of sugar binding.

Keywords: champedak; galactose-binding lectin; mannose-binding lectin.

Figure 1

Top, amino-acid sequence alignment of champedak galactose-binding lectin (CGB) deduced as part of this study aligned with its close homologue jacalin from jackfruit. The cleaved fragment of the propeptide is highlighted in grey. Bottom, amino-acid sequence alignment of champedak mannose-binding lecting (CMB), identified as part of this study, aligned with its close homologue artocarpin from jackfruit. The residues that are non-identical have been highlighted with a black background.

Figure 2

Cartoon representation of (a) the αβ subunit of CGB lectin and (b) CMB lectin. The protein structures are coloured to highlight the three sheets making up the prisms [marine for sheet (i), green/cyan for sheet (ii) and warm pink for sheet (iii)]. The cleaved β-chain is highlighted in purple in sheet (iii) for CGB, whereas in CMB this β-strand is part of the single oligopetptide chain. An asterisk (*) highlights the sugar-binding sites of the two proteins. (c) Shows a topology representation of the β-prism that is formed by both proteins. Mature CGB has been post-translationally processed, leaving β1 as a separate chain. The processed loop is coloured in grey. The numbering assumes the β-chain to be the first strand of the protein, to make comparison between the two lectins easier.

Figure 3

(a) Cartoon representation of the CGB lectin tetramer, with the main parts of the four subunits coloured green, dark purple, marine and warm pink, and the separate β-chains in lighter shades. (b) Close-up of the interface between subunits I (green/cyan) and II (dark purple). (c) Close-up of the interface between subunits I and III (marine). The residues are named by subunit (in roman numerals) and by α-chain or β-chain (no prime or prime, respectively).

Figure 4

(a) Cartoon representation of the CMB lectin tetramer, with the four subunits coloured green, dark purple, marine and warm pink. (b) Close-up of the interface between subunit I (green/cyan) and subunit II (dark purple). For clarity, only one of the two symmetrical parts of the interface is shown. (c) Close-up of the interface between subunits I and III (marine). The residues are named by subunit (in roman numerals).

Figure 5

Sugar-binding sites of the lectins. (a) CGB–Gal structure with galactose observed in the binding site. The upper part shows the output from PDBsum (Laskowski, 2009 ▶), with hydrogen bonds shown as dashed lines and hydrophobic interactions shown as red arches. The lower part shows the same residues in the X-ray structure. (b) CGB–GalNac structure, with Gal observed in the binding site. (c) The CMB structure with superposed Me-α-Man from artocarpin. Superposition is based on protein backbone. For the sake of clarity artocarpin is not shown.