Mass Spectrometry-based Strategies for Protein Characterization: Amyloid Formation, Protein-Ligand Interactions and Structures of Membrane Proteins in Live Cells

Mass spectrometry (MS)-based protein footprinting characterizes protein structure and protein-ligand interactions by interrogating protein solvent-accessible surfaces by using chemical reagents as probes. The method is highly applicable to protein or protein-ligand complexes that are difficult to study by conventional means such as X-ray crystallography and nuclear magnetic resonance. In this dissertation, we describe the development and application of MS-based protein footprinting from three perspectives, including I) protein aggregation and amyloid formation (Chapter 2-3), II) protein-ligand interactions (Chapter 4-5), and III) in-cellulo structures and dynamic motion of membrane proteins (Chapter 6). Fast Photochemical Oxidation of Proteins (FPOP) is the main methodology implemented in the work presented in this dissertation. Chapter 1 provides an overview of FPOP and discusses its fundamentals as well as its important applications in both academic research and biotechnology drug development. In Part I, Chapter 2 covers the early method development of FPOP for monitoring amyloid beta (Aβ) aggregation. In this work, we demonstrated the high sensitivity and spatial resolution of the method in probing the solvent accessibility of Aβ at global, sub-regional, and some amino-acid residue levels as a function of its aggregation, and revealed Aβ species at various oligomeric states identified by their characteristic modification levels. In Chapter 3, we extended the application of the platform to assess the effect of a putative polyphenol inhibitor on amyloid formation and to provide insights into the mechanism of action of the inhibitor in remodeling Aβ aggregation pathways. In Part II, we evaluated different protein footprinting techniques, including FPOP, hydrogen-deuterium exchange (HDX), and carboxyl group footprinting, for probing protein-ligand (drug candidates) interaction in the context of a therapeutic development. Chapter 4 focused on protein-protein interaction by investigating the epitope of IL-6 receptor for two adnectins that have similar apparent biophysical properties. In Chapter 5, we probed the hydrophobic binding cavity of bromodomain protein for a small molecule inhibitor. This study serves as an example of interrogating protein-small molecule interactions. The two studies in Part II demonstrate the unique capabilities and limitations of protein footprinting methods in protein structural characterization. In Part III, we pushed the boundary of MS-based protein footprinting by applying the method to footprint live cells and investigate the dynamic structures/motion of membrane-transport proteins in their native cellular environment. We employed protein engineering, suspension cell expression and isotopic-encoded carboxyl group footprinting to identify salt bridges in two proteins, GLUT1 and GLUT5, that control their alternating access motions for substrate translocation. With functional analysis and mutagenesis, live-cell footprinting provides new insights into the transport mechanism of proteins in the major facilitator superfamily. The five studies in the dissertation demonstrate the powerful capability of MS-based protein footprinting in protein structural biology and biophysics research. The method also holds great potential in studying more complicated biological systems and solving demanding problems related to protein structure and properties