Collision induced unfolding of isolated proteins in the gas phase: past, present, and future

Collision-Induced Unfolding Reveals Unique Fingerprints for Remote Protein Interaction Sites in the KIX Regulation Domain

The kinase-inducible domain (KIX) of the transcriptional coactivator CBP binds multiple transcriptional regulators through two allosterically connected sites. Establishing a method for observing activator-specific KIX conformations would facilitate the discovery of drug-like molecules that capture specific conformations and further elucidate how distinct activator-KIX complexes produce differential transcriptional effects. However, the transient and low to moderate affinity interactions between activators and KIX are difficult to capture using traditional biophysical assays. Here, we describe a collision-induced unfolding-based approach that produces unique fingerprints for peptides bound to each of the two available sites within KIX, as well as a third fingerprint for ternary KIX complexes. Furthermore, we evaluate the analytical utility of unfolding fingerprints for KIX complexes using CIUSuite, and conclude by speculating as to the structural origins of the conformational families created from KIX:peptide complexes following collisional activation. Graphical Abstract ᅟ.

Quantitative collision-induced unfolding differentiates model antibody-drug conjugates

Antibody-drug conjugates (ADCs) are antibody-based therapeutics that have proven to be highly effective cancer treatment platforms. They are composed of monoclonal antibodies conjugated with highly potent drugs via chemical linkers. Compared to cysteine-targeted chemistries, conjugation at native lysine residues can lead to a higher degree of structural heterogeneity, and thus it is important to evaluate the impact of conjugation on antibody conformation. Here, we present a workflow involving native ion mobility (IM)-MS and gas-phase unfolding for the structural characterization of lysine-linked monoclonal antibody (mAb)-biotin conjugates. Following the determination of conjugation states via denaturing Liquid Chromatography-Mass Spectrometry (LC-MS) measurements, we performed both size exclusion chromatography (SEC) and native IM-MS measurements in order to compare the structures of biotinylated and unmodified IgG1 molecules. Hydrodynamic radii (Rh) and collision cross-sectional (CCS) values were insufficient to distinguish the conformational changes in these antibody-biotin conjugates owing to their flexible structures and limited instrument resolution. In contrast, collision induced unfolding (CIU) analyses were able to detect subtle structural and stability differences in the mAb upon biotin conjugation, exhibiting a sensitivity to mAb conjugation that exceeds native MS analysis alone. Destabilization of mAb-biotin conjugates was detected by both CIU and differential scanning calorimetry (DSC) data, suggesting a previously unknown correlation between the two measurement tools. We conclude by discussing the impact of IM-MS and CIU technologies on the future of ADC development pipelines.

Localization of Protein Complex Bound Ligands by Surface-Induced Dissociation High-Resolution Mass Spectrometry

Surface-induced dissociation (SID) is a powerful means of deciphering protein complex quaternary structures due to its capability of yielding dissociation products that reflect the native structures of protein complexes in solution. Here we explore the suitability of SID to locate the ligand binding sites in protein complexes. We studied C-reactive protein (CRP) pentamer, which contains a ligand binding site within each subunit, and cholera toxin B (CTB) pentamer, which contains a ligand binding site between each adjacent subunit. SID dissects ligand-bound CRP into subcomplexes with each subunit carrying predominantly one ligand. In contrast, SID of ligand-bound CTB results in the generation of subcomplexes with a ligand distribution reflective of two subunits contributing to each ligand binding site. SID thus has potential application in localizing sites of small ligand binding for multisubunit protein-ligand complexes.

Signature of Antibody Domain Exchange by Native Mass Spectrometry and Collision-Induced Unfolding

The development of domain-exchanged antibodies offers a route to high-affinity targeting to clustered multivalent epitopes, such as those associated with viral infections and many cancers. One strategy to generate these antibodies is to introduce mutations into target antibodies to drive domain exchange using the only known naturally occurring domain-exchanged anti-HIV (anti-human immunodeficiency virus) IgG1 antibody, 2G12 , as a template. Here, we show that domain exchange can be sensitively monitored by ion-mobility mass spectrometry and gas-phase collision-induced unfolding. Using native 2G12 and a mutated form that disrupts domain exchange such that it has a canonical IgG1 architecture ( 2G12 I19R ), we show that the two forms can be readily distinguished by their unfolding profiles. Importantly, the same signature of domain exchange is observed for both intact antibody and isolated Fab fragments. The development of a mass spectrometric method to detect antibody domain exchange will enable rapid screening and selection of candidate antibodies engineered to exhibit this and other unusual quaternary antibody architectures.