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The gas phase is an idealized laboratory for the study of protein structure, from which it is possible to examine stable and transient forms of mass selected ions in the absence of bulk solvent. With ion mobility-mass spectrometry (IM-MS) apparatus built to operate at both cryogenic and elevated temperatures, we have examined the conformational transitions of the monomeric proteins: ubiquitin, lysozyme and alpha-synuclein as a function of temperature and in source activation. We rationalize the experimental observations with a temperature dependent framework model and comparison to known conformers. Data from ubiquitin shows unfolding transitions that proceed through diverse and highly elongated intermediate states, which converge to more compact structures. These findings contrast with data obtained from lysozyme – a protein where (un)-folding plasticity is restricted by four disulfide linkages, although this is alleviated in its reduced form. For structured proteins, collision activation of the protein ions in- source, enables subsequent “freezing” or thermal annealing of unfolding intermediates whereas disordered proteins restructure substantially at 250 K even without activation, indicating that cold denaturation can occur without solvent. These data are presented in the context of a toy model framework which describes the relative occupancy of the available conformational space.

Charge reduction in the gas phase provides a direct means of manipulating protein charge state, and when coupled to ion mobility mass spectrometry (IM-MS), it is possible to monitor the effect of charge on protein conformation in the absence of solution. Use of the electron transfer reagent 1,3-dicyanobenzene, coupled with IM-MS, allows us to monitor the effect of charge reduction on the conformation of two proteins deliberately chosen from opposite sides of the order to disorder continuum: bovine pancreatic trypsin inhibitor (BPTI) and beta casein. The ordered BPTI presents compact conformers for each of three charge states accompanied by narrow collision cross-section distributions (TWCCSDN2→He). Upon reduction of BPTI, irrespective of precursor charge state, the TWCCSN2→He decreases to a similar distribution as found for the nESI generated ion of identical charge. The behavior of beta casein upon charge reduction is more complex. It presents over a wide charge state range (9–28), and intermediate charge states (13–18) have broad TWCCSDN2→He with multiple conformations, where both compaction and rearrangement are seen. Further, we see that the TWCCSDN2→He of the latter charge states are even affected by the presence of radical anions. Overall, we conclude that the flexible nature of some proteins result in broad conformational distributions comprised of many families, even for single charge states, and the barrier between different states can be easily overcome by an alteration of the net charge. Graphical Abstractᅟ Electronic supplementary material The online version of this article (doi:10.1007/s13361-017-1692-1) contains supplementary material, which is available to authorized users.

A novel mass spectrometry approach is reported which investigate how ion-molecule charge reduction reactions between radical anions and protein cations modulate protein conformation. An electron transfer reagent (1,3-dicyanobenzene) transfers electrons to positively charged proteins and there are no observable products of dissociation. ETnoD product ions are detected as charged-reduced species with the same molecular weight as the precursor ion, and no significant evidence for proton transfer. We present collision cross section distributions of precursor and product ions before and after exposure to radical ions. Cytochrome c and myoglobin are examined as exemplar systems under both aqueous salt and denaturing conditions before and after exposure to radical anions. We consistently observe depletion of the more compact precursor ion conformers on exposure to the ETD reagent. Remarkably, by examining the collision cross section distributions of the product ions it can be seen that the addition of a single electron can cause a dramatic rearrangement in protein conformation for charge states that are highly populated when sprayed from salty aqueous conditions. Furthermore, a given net charge on an exposed precursor and product ion favours a preferred collision cross section distribution, indicating that the distribution of charge on proteins in the gas phase dictate their conformation. An exception is reported for the low charge state of cytochrome c where compaction was seen in the radical formed post reduction compared to the electrospray generated ion under ETnoD optimised conditions. We propose a model that postulates how electron transfer to conformation stabilising salt bridges may explain our observations.

Using Ubiquitin as an exemplar protein we examine the effect of net charge reduction post electrospray ionisation by exposure to the electron transfer reagent, 1,3-dicyanobenzene. We monitor the change in gas phase conformation of both precursor and products with ion mobility mass spectrometry (IM-MS). Dramatic conformational rearrangement is seen for low charge state ions upon exposure to the electron transfer reagent. Ions with low charge states sprayed from both native-like and denaturing solvent conditions undergo structural transitions to conformers with cross sections in the range measured for the native structure (TWCCSN2®He, 950-1000 Å2). Thus, we infer that some memory of the solution phase structure is retained in the gas phase. Intermediate structures are seen in the reduction of the [M+6H]6+ ion sprayed from both native-like and denaturing solvents. Further, the reduction pathway of this ion shows compaction to structures with a TWCCSN2®He centred at 1069 Å2 (5+) and 949 Å2 (4+) for ions originating from native-like and denaturing solvents respectively. We propose that charge reduction sites for radical anion localisation (to effect electron transfer) are not easily accessible in the case of ubiquitin molecules originating from native-like solution conditions. This highlights the importance of salt bridge interactions in maintaining the structural integrity of a protein in the gas phase. Most interestingly, two distinct conformer populations are seen for the 6+ charge-reduced product originating from the 7+ and, the 6+ exposed to radical anions (post ESI); we infer that these populations are intermediate in the refolding of ubiquitin in the gas phase, sometimes transient. Overall, we are able to monitor the refolding pathway of ubiquitin in the gas phase as its charge is reduced and show that charge-charge interactions play a significant role in the gas phase conformation adopted; whereby specific conformations are formed.

Plant and animal pathogenic bacteria can suppress host immunity by injecting type III secreted effector (T3SE) proteins into host cells. However, T3SEs can also elicit host immunity if the host has evolved a means to recognize the presence or activity of specific T3SEs. The diverse YopJ/HopZ/AvrRxv T3SE superfamily, which is found in both animal and plant pathogens, provides examples of T3SEs playing this dual role. The T3SE HopZ1a is an acetyltransferase carried by the phytopathogen Pseudomonas syringae that elicits effector-triggered immunity (ETI) when recognized in Arabidopsis thaliana by the nucleotide-binding leucine-rich repeat (NB-LRR) protein ZAR1. However, recognition of HopZ1a does not require any known ETI-related genes. Using a forward genetics approach, we identify a unique ETI-associated gene that is essential for ZAR1-mediated immunity. The hopZ-ETI-deficient1 (zed1) mutant is specifically impaired in the recognition of HopZ1a, but not the recognition of other unrelated T3SEs or in pattern recognition receptor (PRR)-triggered immunity. ZED1 directly interacts with both HopZ1a and ZAR1 and is acetylated on threonines 125 and 177 by HopZ1a. ZED1 is a nonfunctional kinase that forms part of small genomic cluster of kinases in Arabidopsis. We hypothesize that ZED1 acts as a decoy to lure HopZ1a to the ZAR1-resistance complex, resulting in ETI activation.

The Par complex is a conserved cell polarity regulator. Bazooka/Par-3 is scaffold for the complex and contains three PDZ domains in tandem. PDZ domains can act singly or synergistically to bind the C-termini of interacting proteins. Sequence comparisons among Drosophila Baz and its human and C. elegans Par-3 counterparts indicate a divergence of the peptide binding pocket of PDZ1 and greater conservation for the pockets of PDZ2 and PDZ3. However, it is unclear whether the domains from different species share peptide binding preferences, or if their tandem organization affects their peptide binding properties. To investigate these questions, we first used phage display screens to identify unique peptide binding profiles for each single PDZ domain of Baz. Comparisons with published phage display screens indicate that Baz and C. elegans PDZ2 bind to similar peptides, and that the peptide binding preferences of Baz PDZ3 are more similar to C. elegans versus human PDZ3. Next we quantified the peptide binding preferences of each Baz PDZ domain using single identified peptides in surface plasmon resonance assays. In these direct binding studies, each peptide had a binding preference for a single PDZ domain (although the peptide binding of PDZ2 was weakest and the least specific). PDZ1 and PDZ3 bound their peptides with dissociation constants in the nM range, whereas PDZ2-peptide binding was in the µM range. To test whether tandem PDZ domain organization affects peptide binding, we examined a fusion protein containing all three PDZ domains and their normal linker regions. The binding strengths of the PDZ-specific peptides to single PDZ domains and to the PDZ domain tandem were indistinguishable. Thus, the peptide binding pockets of each PDZ domain in Baz are not obviously affected by the presence of neighbouring PDZ domains, but act as isolated modules with specific in vitro peptide binding preferences.