Structural Mass Spectrometry of Protein Assemblies

Native MS and Ion Mobility-MS

Shortly after the introduction of electrospray ionization (ESI) that revolutionized the MS analysis of biomolecules at the end of the 80s, several groups observed the presence of non-covalent complexes in the gas phase. First limited to small complexes, instrumental developments progressively enabled the analysis of increasingly larger complexes such as GroEL, the proteasome or even mega-Da assemblies such as ribosomes or viral capsids. Within the past 10 years, non-covalent or native MS, has emerged and is now an integrative part of structural biology, providing a series of information on the structure and dynamics of protein complexes (Figure).

The toolbox of structural mass spectrometrists has also been further completed by ion mobility MS (IM-MS). This method enables the separation of ions in the gas phase, according to their size and conformation(s). Ionized proteins are introduced in a cell filled with an inert gas with which they collide and submitted to an electric field. The larger the surface of the protein, the higher the number of collisions and the longer it will take to go through the ion mobility tube. By measuring the time taken to cross the cell (drift time), and after calibrating with proteins of known structure, averaged collisional cross section of the protein (CCS, in Å2) can be obtained. By acting like “gas phase capillary electrophoresis”, ion mobility brings an additional dimension of separation, virtually increasing the peak capacity of the instrument allowing not only spectrum cleaning but also the detection of subtle conformational changes or differences in the unfolding pattern upon ligand binding. Moreover, CCSs constitute structural constraints that can be very useful for molecular modeling purposes.

 

Information obtained by native MS and ion mobility
Figure: Information obtained by native MS and ion mobility.