Molecular Analysis of Carbohydrate Recognition by Collectin-10 and Collectin-11

Abstract

Collectins are pathogen-recognition receptors that recognize pathogens via C-terminal carbohydrate-recognition domains (CRD), linked to N-terminal collagen-like domains via an α-helical coiled coil neck. Mannan-binding lectin, collectin liver-1 (CL-L1; aka collectin-10) and collectin kidney-1 (CL-K1; aka collectin-11) initiate the lectin pathway of complement activation via associated MASPs. CL-10 and CL-11 form heterooligomers in serum, while CL-11 can also form homooligomers. As well as targeting pathogens, both CL-11 and CL-10 have an as yet unknown role in development, with mutations associated with the developmental disorder, 3MC syndrome (Michels, Maleic, Mingarelli and Carnevale). They are also known to initiate kidney damage following ischaemia when local collectin-mediated complement activation follows binding to fucosylated structures that are upregulated on host tissues. CL-11 interacts with high-mannose oligosaccharides and fucosylated glycans, while the targets of CL-10 are less well characterised. This study uses structural biology techniques (X-ray crystallography), glycan array analysis, and recombinant expression systems to elucidate the molecular basis of CL-11 and CL-10 oligomerization and ligand recognition. I have shown that cysteine residues present in the neck of CL-11 form interchain disulphide bonds arranged in a symmetrical pattern. These together with unique CRD-CRD contacts probably stabilise the assembly of trimeric subunits during biosynthesis. My findings reveal that CL-11 bind mannose- and fucose-containing ligands via an extended binding site in the CRD that recognises two carbohydrate residues. This binding site accommodates two mannose residues linked by α1-2 glycosidic bonds, but not α1-3 or α1-6 linkages, explaining how CL-11 targets high mannose oligosaccharides through terminal disaccharides. Furthermore, this research reveals the structure of the CRD of CL-10 for the first time and shows that it binds to mannose-capped lipoarabinomannan from the cell wall of Mycobacterium tuberculosis offering novel insights into its functional specificity.