Abstract
Native mass spectrometry (MS) encompasses methods to keep noncovalent interactions of biomolecular complexes intact in the gas phase throughout the instrument and to measure the mass-to-charge ratios of supramolecular complexes directly in the mass spectrometer. Electrospray ionization (ESI) in nondenaturing conditions is now an established method to characterize noncovalent systems. Matrix-assisted laser desorption/ionization (MALDI), on the other hand, consumes low quantities of samples and largely tolerates contaminants, making it a priori attractive for native MS. However, so-called native MALDI approaches have so far been based on solid deposits, where the rapid transition of the sample through a solid state can engender the loss of native conformations. Here we present a new method for native MS based on liquid deposits and MALDI ionization, unambiguously detecting intact noncovalent protein complexes by direct desorption from a liquid spot for the first time. To control for aggregation, we worked with HUαβ, a heterodimer that does not spontaneously rearrange into homodimers in solution. Screening through numerous matrix solutions to observe first the monomeric protein, then the dimer complex, we settled on a nondenaturing binary matrix solution composed of acidic and basic organic matrices in glycerol, which is stable in vacuo. The role of temporal and spatial laser irradiation patterns was found to be critical. Both a protein-protein and a protein-ligand complex could be observed free of aggregation. To minimize gas-phase dissociation, source parameters were optimized to achieve a conservation of complexes above 50% for both systems.Graphical ᅟ
Highlights
Native mass spectrometry (MS) methods consist in directly analyzing biological molecular assemblies in an instru-M
HU proteins are known as nonsequence-specific DNA-binding proteins and exist predominantly in Escherichia coli cells as an αβ heterodimer
The ubiquitous hydroxy-α-cinnamic acid (HCCA) and dihydroxybenzoic acid (DHB) matrices are often used for this approach
Summary
M. Beaufour et al.: Liquid Native MALDI MS of Protein Complexes. The go-to ionization method for native MS has been electrospray (ESI). Evidence of noncovalent complexes was shown for both ESI-MS [4,5,6] and MALDI (matrix-assisted laser desorption/ionization) MS around 1990 [7, 8], the rapid expansion in number and scope of native MS applications can be credited to ESI-MS methods [1, 2]. Several arguments plead for MALDI development in native MS, as sample consumption is even lower than in ESI, and tolerance towards contaminants and buffers is greater. MALDI’s tolerance to nonionic and zwitterion detergents for example, as demonstrated in MALDI in denaturing conditions, should facilitate the analysis of membrane protein complexes
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