Chemical Biology Tools to Study Bacterial Cell Surface Glycans

Abstract

Cell surface glycans are ubiquitous and serve as the first point of contact between a cell and the surrounding environment. Many of the carbohydrate-mediated interactions that occur at this interface regulate key signaling processes such as cell-cell recognition, communication, and adhesion. Bacterial glycans in particular play critical roles in maintaining cellular structure and are implicated in infections and pathogenesis. Understanding molecular determinants of these important biological functions is critical for both fundamental and translational research. Despite the need to better understand these important biological structures, methods for probing glycan structure and function remain limited. Glycans are incompatible with common strategies for studying other biomacromolecules, which often exploit chemoselective reactions for covalent modification, capture, or imaging. Unlike the amino acid residues that constitute proteins, glycan building blocks are composed primarily of polyol isomers and lack distinguishing reactivity required for selective labeling. Moreover, unlike protein synthesis, glycan biosynthesis is not templated, making perturbation through genetic manipulation is often convoluted. Finally, the molecular complexity of glycan composition presents an added challenge. Unlike the 20 canonical amino acids used in proteins, bacteria use more than 600 distinct monosaccharide building blocks. To address this open challenge, we developed novel chemical biology tools to study bacterial cell surface glycans. We have established a new, generalizable strategy for chemoselective glycan modification to enable the study of specific bacterial cell wall glycans. Our method relies on the direct incorporation of reactive glycan building block surrogates by cell surface glycosyltransferases, a technique termed “biosynthetic incorporation”. We first validated this approach by labeling the arabinan (Chapter 2), which enabled several important downstream applications, including assay development and controlled cell surface perturbation (Chapter 3). We then demonstrated the generalizability of this approach by developing probes for mannose-containing glycans using this strategy (Chapter 4). In this work, we have also targeted modification of the cell wall assembly enzymes themselves (Chapter 5), in addition to the structures they produce. Ultimately, we envision that the chemical biology tools developed in this work will be useful for both answering fundamental biological questions and towards efforts to develop new antibiotics.