Post-Expression Isotope Labeling: NMR Insights into Glycoprotein Structure

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Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for characterizing the structure of proteins and gaining insight into their function. However, structures solved by NMR have been heavily biased toward soluble proteins that can be easily expressed in bacterial hosts with uniform isotope enrichment. Restriction to bacterial expression leaves many glycosylated proteins inaccessible; these represent as much as 50% of coded human proteins. The objective of the proposed research is to provide isotope-labeling and assignment methodologies for NMR, which are not limited to uniform, isotope labeling. These assignment strategies will become the underpinnings for structural data and functional assays on a specific class of glycosylated proteins. The assignment approach we plan to investigate is to isotopically label proteins post-expression via chemical and enzymatic methods. Mass spectrometry (MS) will be used to identify the peptides labeled and assist in the assignment of NMR resonances. I hypothesize that postexpression, isotope labeling strategies, coupled with MS-assisted NMR assignment, will expand the scope of NMR spectroscopy to include new classes of proteins, and allow characterization of protein-protein and carbohydrate mediated interactions that have been previously inaccessible. The specific aims during the mentored phase of this Pathway to Independence Award are to improve the resolution of the MS-assisted NMR assignment strategy for reductively 13C-methylated lysines and to structurally characterize proteins and ligand binding sites using reductively 13C-methylated model proteins. Specific aims to be pursued as an independent investigator are to apply enzymatic isotope labeling methods for glutamine and glycan labeling, develop NMR experiments for assignment, and apply isotopic labeling and assignment strategies to domains from Notchl and other biologically relevant targets. Understanding the structure and function of a protein is important to disease diagnosis, treatment, and prevention. With the ability to characterize proteins limited to a subset of soluble, non-glycosylated proteins we would exclude from study a class of proteins that accounts for as much as 50% of all human proteins, and many proteins specifically linked to disease. The proposed research will introduce new tools to characterize the structure of proteins with particular emphasis on glycoproteins.