Development and application of glycan readers for the detection and analysis of bacterial glycoconjugates

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Project Summary The glycans that decorate cell-surface glycoconjugates, represent a rich molecular language that mediates a myriad of important biological functions. In bacteria, these glycoconjugates are critical determinants in interactions amongst bacterial communities and between both pathogenic and symbiotic bacteria and human host cells. A major challenge in understanding the roles of complex glycans in bacteria is that the pool of monosaccharide building blocks and diversity of glycosidic linkages reflected in the glycoconjugates is greatly expanded relative to eukaryotic organisms. Therefore, the currently-available glycan-binding proteins, which include lectins and monoclonal antibodies, are simply inadequate for detecting a majority bacterial glycan epitopes. In light of the importance of bacterial glycans in human infectious disease, the development of experimental reagents, as “glycan readers”, to selectively characterize and monitor pathogen-specific glycan determinants is of utmost current importance. Selective glycan readers towards bacterial glycan epitopes promise to be valuable reagents for identifying bacterial pathogens and understanding the biological significance of glycoconjugates in infectious disease.  In this exploratory research program, we aim to establish proof-of-principle for engineered protein-based glycan readers for the detection and analysis of pathogen-specific glycans and glycoconjugates. This proposal includes two aims. Aim 1 will involve preparation of chemically-defined C. jejuni glycan epitopes, including pseudaminic acid and N-acetyl bacillosamine, which are prokaryote-specific carbohydrates. These glycan epitopes will be armed, via established linker chemistry, with biotin for directed evolution of novel glycan binding proteins, based on the Sso7d scaffold, using yeast surface display. Aim 2 will develop applications of the evolved modular glycan binders using protein engineering approaches, with a focus on utility for the study of disease-related bacterial glycans. In particular, sortase-mediated ligation will be applied for modifying the C- and N-termini of glycan binders to include fluorophores and biotin. These “glycan readers” will be valuable for applications including glycan array visualization, live and fixed imaging, fluorescence-activated cell sorting (FACS) and affinity chromatography. In addition, azide-modified glycan readers will be amenable to click chemistry-based conjugation reactions with alkynes enabling, for example, multivalent display to exploit avidity effects.