An essential virulence factor of the pathogenic fungus C. neoformans is the polysaccharide capsular coat, the major constituent of which is glycuronoxylomannan (GXM). Antibodies raised against a GXM-tetanus toxoid conjugate have been examined in depth by our lab. 18B7, a monoclonal anti-GXM antibody, shows catalytic activity against both GXM polysaccharide fragments and the peptide mimetic P1. Previously our lab and colleagues crystalized 2H1, another anti-GXM antibody with high sequence identity with 18B7, with the PA1 GXM mimotype. The PA1 peptide bound by 2H1 structure reveals a type II ß turn within the putative antibody binding site. Peptide alignment between P1 and PA1 GXM mimetics reveals conservation of two areas, the antibody binding motif, and another three amino acid region. Recent studies by our lab show that this other three amino acid region is the cleavage site in the P1 peptide, indicating that the ß turn may be important for both antibody binding and cleavage of GXM peptide mimetics. Preliminary analysis of synthetic GXM oligosaccharides indicates a tight helical secondary structure, along with O-acetylation of the mannose backbone, are essential for 18B7 binding. Currently we are performing structural analysis on the P1 peptide and the GXM oligosaccharide to further define the structural features shared by peptide and oligosaccharide essential for hydrolysis by 18B7. We hope to determine the mechanism and structural features enabling a single antibody is to catalyze the cleavage of both peptide and glycosyl bonds.

Structural analysis of peptide and carbohydrate epitopes cleaved by the Cryptococcus neoformans catalytic monoclonal antibody 18B7

254th National Meeting (2017)

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