Application of Native Ion MobilityMass Spectrometry in the Study of Protein Structure and Conformational Diseases

Native mass spectrometry (MS) enables the maintenance of intramolecular and intermolecular noncovalent interactions in the gas phase, thus can be used for characterization of proteins and protein complexes under the near “native” conditions. Ion mobility spectrometry (IMS) is a technique that separates ions based on the time they travel through a gas-filled chamber under the influence of an electric field, the combination of IMS and MS can act as a tool to separate complex mixtures, to resolve ions that may be indistinguishable by MS alone, or to determine structural, and it is complementary to more traditional structural biology techniques, such as NMR, X-ray crystallography and cryo-electron microscopy. In this review, the basic principle of IM-MS and the information extracted during IM-MS measurements were firstly introduced. Ion mobility separation adds a third dimension to MS analyses, an IM-MS three-dimensional spectrum containing information regarding the mass to charge ratio, abundance of ions, and the drift time the ions travel through the IM cell. Arrival time distribution reflecting the structural heterogeneity of proteins and complexes, as well as IM-derived collision cross section (CCS) that closely related to the crystal structure of an ion can also be obtained. Another remarkable toolkit often used in IM-MS experiments is collision induced unfolding method, which enables the rapid differentiation of subtly different protein isoforms based on their unfolding patterns and stabilities. In the following sections, the applications of native IM-MS in the study of protein conformation and non-covalent interactions were summarized, including the determination of variation in conformation states or isomers that would result in the same m/z measurement, differentiating proteins with different labile PTMs, and probing protein stability and conformational changes induced by ligand binding. How this technique can be used to understand protein conformational dynamics and analyze highly heterogenous protein mixtures were also discussed. The progress has been made in applying IM-MS to identify transiently populated monomeric and oligomeric species for a number of amyloid systems are highlighted. Although native IM-MS is unable to reveal resolution at the atomic level, the ability to analyze protein heterocomplexes and protein-ligand interactions in their native conformations offers competitive advantages over other structural approaches. More recently, the integration of IM-MS with the native top-down method and multiple dissociation methods has enable native IM-MS to provide multi-dimensional structural information and be applicable to more diverse analytical settings. With the ongoing development and improvement of a new generation of high-resolution, sensitive and convenient ion mobility mass spectrometry technology, as well as the further integration with diversified mass spectrometry technologies, it is believed that native IM-MS will become an invaluable tool for analyzing biological macromolecules structure and interactions.