Protein Glycosylation: Mechanisms and Consequences

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The chemical modification of proteins with a specialized sugar called O-GlcNAc is a ubiquitous process in multicellular organisms, including humans. Over a thousand intracellular proteins are reversibly O-GlcNAcylated, and the presence of this sugar influences a diverse range of biochemical processes spanning from the expression of our genetic code to cellular signaling and various forms of stress response. However, despite its prevalence, the regulation of protein O-GlcNAcylation and the mechanisms by which it impacts cellular functions remain poorly understood at a detailed molecular level. Accordingly, the overall goals of our research program are two-fold.First, we aim to determine how the enzyme called O-GlcNAc transferase (or simply OGT) selects the specific proteins that it modifies with O-GlcNAc. In addition to the catalytic domain where the chemical transfer of O-GlcNAc occurs, OGT has a long tail called a TPR domain. It is hypothesized that the TPR domain is a scaffold that recognizes (binds) certain substrate proteins, and thereby directs their modification by the adjacent catalytic domain. Thus we will use a variety of biochemical screens to search for and identify proteins that bind the TPR domain. Once identified, we will use the powerful technique of NMR spectroscopy as a "molecular microscope" to define the properties of the substrate proteins underpinning their recognition and subsequent O-GlcNAc modification by OGT.Second, we will investigate the consequences of O-GlcNAcylation on protein function. Given the diverse cellular processes associated with this modification, it is likely that the effects of O-GlcNAc are context-dependent. In some cases, the presence of O-GlcNAc appears to prevent unwanted protein aggregation, whereas in other cases, it seems to promote or disrupt interactions with partner cellular proteins or block alternative regulatory chemical modifications such as phosphorylation. Therefore, we will use techniques such as NMR spectroscopy to determine how O-GlcNAc influences the molecular structure, motions and interactions of a representative set of proteins involved in gene expression, signaling and cellular organization. The results of our studies will help explain which proteins are O-GlcNAcylated by OGT and how this modification impacts their function. This fundamental knowledge should provide the scientific community with a much needed molecular-level understanding of the roles played by O-GlcNAcylation in many normal and aberrant cellular processes. Our research will also offer excellent training and career advancement opportunities for graduate students and postdoctoral fellows in areas ranging from biophysics to cellular biology.