Here we describe a detailed protocol for both data collection and interpretation with respect to ion mobility-mass spectrometry analysis of large protein assemblies. Ion mobility is a technique that can separate gaseous ions based on their size and shape. Specifically, within this protocol, we cover general approaches to data interpretation, methods of predicting whether specific model structures for a given protein assembly can be separated by ion mobility, and generalized strategies for data normalization and modeling. The protocol also covers basic instrument settings and best practices for both observation and detection of large noncovalent protein complexes by ion mobility-mass spectrometry.
INTRODUCTIONLarge-scale interaction maps suggest a complex interplay of proteins within a myriad of functional assemblies 1,2 . A critical step in assigning functions to these assemblies is to determine their structure 3 . This goal is challenging, as many of these assemblies exist in low-copy numbers within cells, are frequently heterogeneous and may interact only transiently. Consequently, structural information for many protein complexes is not readily accessible by using the classical tools of structural biology (e.g., X-ray crystallography, nuclear magnetic resonance spectroscopy). New approaches are being developed that involve integrating data from a number of lower-resolution experimental methods and by combining distance and interaction restraints from these methods with homology modeling, architectural or even atomic models are being generated 4 . These restraints can be derived from a variety of experimental measurements including MS of intact complexes, chemical cross-linking, fluorescence resonance energy transfer, small angle X-ray scattering, and analytical ultracentrifugation 5,6 . One very recent addition to this series of biophysical tools is ion mobility separation coupled to mass spectrometry (IM-MS). IM is an established technique for studying shape and conformation in small molecules and individual proteins in the gas phase 7-10 but has only recently been applied to intact protein complexes 11,12 . When IM is coupled with MS, mass and consequently subunit composition can be determined simultaneously with the overall topology of protein complexes 10,12,13 .IM-MS analysis is performed by first ionizing the protein complex of interest. In our experiments, nano-electrospray ionization is used, typically requiring careful preparation procedures for most protein complexes. These procedures, as well as general practical aspects of sample preparation, are detailed in a protocol by Hernández and Robinson 14 . Although they are not discussed in detail here, knowledge of the materials and protocol steps described in that work are critical to the success of the protocol described below.After ionization, ions are injected into a region containing neutral gas at a controlled pressure (e.g., 0.5 mBar of nitrogen gas). Under the influence of a relatively weak electric field, injected ions undergo IM separation [7]...