Charge partitioning during the dissociation of protein complexes in the gas phase is influenced by many factors, such as interfacial interactions, protein flexibility, protein conformation, and dissociation methods. In the present work, two cysteine-containing homodimer proteins, β-lactoglobulin and α-lactalbumin, with the disulfide bonds intact and reduced, were used to gain insight into the charge partitioning behaviors of collision-induced dissociation (CID) and surface-induced dissociation (SID) processes. For these proteins, we find that restructuring dominates with CID and dissociation with symmetric charge partitioning dominates with SID, regardless of whether intramolecular disulfide bonds are oxidized or reduced. CID of the charge-reduced dimeric protein complex leads to a precursor with a slightly smaller collision cross section (CCS), greater stability, and more symmetrically distributed charges than the significantly expanded form produced by CID of the higher charged dimer. Collision-induced unfolding plots demonstrate that the unfolding-restructuring of the protein complexes initiates the charge migration of higher charge-state precursors. Overall, gas collisions reveal the charge-dependent restructuring/unfolding properties of the protein precursor, while surface collisions lead predominantly to more charge-symmetric monomer separation. CID's multiple low-energy collisions sequentially reorganize intra- and intermolecular bonds, while SID's large-step energy jump cleaves intermolecular interfacial bonds in preference to reorganizing intramolecular bonds. The activated population of precursors that have taken on energy without dissociating (populated in CID over a wide range of collision energies, populated in SID for only a narrow distribution of collision energies near the onset of dissociation) is expected to be restructured, regardless of the activation method.

A flexible inline size exclusion chromatography (SEC) and ion-exchange chromatography (IEX) method coupled to native mass spectrometry (nMS) method was developed for the analysis of protein aggregates and charge variants in a single run of biotherapeutics. This method can be applied to a broad spectrum of biologics such as human growth hormone and its soluble aggregates, the oligomer state distribution of liraglutide in solutions with different pH, and the native states of homodimers and heterodimers generated during a force stressed study of mixtures of two monoclonal antibodies. Tuning of both orbitrap and time of flight (TOF) mass spectrometers in native MS mode to accommodate this LC system is shown in this work for the analysis of biopharmaceutics. This SEC-IEX -UV-nMS system offers two-dimensional separation and two detection methods (ultraviolet absorbance and mass detection), providing an orthogonal characterization of complicated protein samples in a single method.

RNA-binding proteins contain intrinsically disordered regions whose functions in RNA recognition are poorly understood. The RNA chaperone Hfq is a homohexamer that contains six flexible C-terminal domains (CTDs). The effect of the CTDs on Hfq’s integrity and RNA binding has been challenging to study because of their sequence identity and inherent disorder. We used native mass spectrometry coupled with surface-induced dissociation and molecular dynamics simulations to disentangle the arrangement of the CTDs and their impact on the stability of Escherichia coli Hfq with and without RNA. The results show that the CTDs stabilize the Hfq hexamer through multiple interactions with the core and between CTDs. RNA binding perturbs this network of CTD interactions, destabilizing the Hfq ring. This destabilization is partially compensated by binding of RNAs that contact multiple surfaces of Hfq. By contrast, binding of short RNAs that only contact one or two subunits results in net destabilization of the complex. Together, the results show that a network of intrinsically disordered interactions integrate RNA contacts with the six subunits of Hfq. We propose that this CTD network raises the selectivity of RNA binding.

The design of modular protein logic for regulating protein function at the posttranscriptional level is a challenge for synthetic biology. Here, we describe the design of two-input AND, OR, NAND, NOR, XNOR, and NOT gates built from de novo-designed proteins. These gates regulate the association of arbitrary protein units ranging from split enzymes to transcriptional machinery in vitro, in yeast and in primary human T cells, where they control the expression of the TIM3 gene related to T cell exhaustion. Designed binding interaction cooperativity, confirmed by native mass spectrometry, makes the gates largely insensitive to stoichiometric imbalances in the inputs, and the modularity of the approach enables ready extension to three-input OR, AND, and disjunctive normal form gates. The modularity and cooperativity of the control elements, coupled with the ability to de novo design an essentially unlimited number of protein components, should enable the design of sophisticated posttranslational control logic over a wide range of biological functions.

Many iron-sulfur proteins involved in cluster trafficking form [2Fe-2S]-cluster-bridged complexes that are often challenging to characterize because of the inherent instability of the cluster at the interface. Herein, we illustrate the use of fast, online buffer exchange coupled to a native mass spectrometry (OBE nMS) method to characterize [2Fe-2S]-cluster-bridged proteins and their transient cluster-transfer intermediates. The use of this mechanistic and protein-characterization tool is demonstrated with holo glutaredoxin 5 (GLRX5) homodimer and holo GLRX5:BolA-like protein 3 (BOLA3) heterodimer. Using the OBE nMS method, cluster-transfer reactions between the holo-dimers and apo-ferredoxin (FDX2) are monitored, and intermediate [2Fe-2S] species, such as (FDX2:GLRX5:[2Fe-2S]:GSH) and (FDX2:BOLA3:GLRX5:[2Fe-2S]:GSH) are detected. The OBE nMS method is a robust technique for characterizing iron-sulfur-cluster-bridged protein complexes and transient iron-sulfur-cluster transfer intermediates.

It is important to assess the identity and purity of proteins and protein complexes during and after protein purification to ensure that samples are of sufficient quality for further biochemical and structural characterization, as well as for use in consumer products, chemical processes, and therapeutics. Native mass spectrometry (nMS) has become an important tool in protein analysis due to its ability to retain non-covalent interactions during measurements, making it possible to obtain protein structural information with high sensitivity and at high speed. Interferences from the presence of non-volatiles are typically alleviated by offline buffer exchange, which is timeconsuming and difficult to automate. We provide a protocol for rapid online buffer exchange (OBE) nMS to directly screen structural features of pre-purified proteins, protein complexes, or clarified cell lysates. Information obtained by OBE nMS can be used for fast (<5 min) quality control and can further guide protein expression and purification optimization.

The ability of naturally occurring proteins to change conformation in response to environmental changes is critical to biological function. Although there have been advances in the de novo design of stable proteins with a single, deep free-energy minimum, the design of conformational switches remains challenging. We present a general strategy to design pH-responsive protein conformational changes by precisely preorganizing histidine residues in buried hydrogen-bond networks. We design homotrimers and heterodimers that are stable above pH 6.5 but undergo cooperative, large-scale conformational changes when the pH is lowered and electrostatic and steric repulsion builds up as the network histidine residues become protonated. The transition pH and cooperativity can be controlled through the number of histidine-containing networks and the strength of the surrounding hydrophobic interactions. Upon disassembly, the designed proteins disrupt lipid membranes both in vitro and after being endocytosed in mammalian cells. Our results demonstrate that environmentally triggered conformational changes can now be programmed by de novo protein design.

Specificity of interactions between two DNA strands, or between protein and DNA, is often achieved by varying bases or side chains coming off the DNA or protein backbone-for example, the bases participating in Watson-Crick pairing in the double helix, or the side chains contacting DNA in TALEN-DNA complexes. By contrast, specificity of protein-protein interactions usually involves backbone shape complementarity(1), which is less modular and hence harder to generalize. Coiled-coil heterodimers are an exception, but the restricted geometry of interactions across the heterodimer interface (primarily at the heptad a and d positions(2)) limits the number of orthogonal pairs that can be created simply by varying side-chain interactions(3,4). Here we show that protein-protein interaction specificity can be achieved using extensive and modular side-chain hydrogen-bond networks. We used the Crick generating equations(5) to produce millions of four-helix backbones with varying degrees of supercoiling around a central axis, identified those accommodating extensive hydrogen-bond networks, and used Rosetta to connect pairs of helices with short loops and to optimize the remainder of the sequence. Of 97 such designs expressed in Escherichia coli, 65 formed constitutive heterodimers, and the crystal structures of four designs were in close agreement with the computational models and confirmed the designed hydrogen-bond networks. In cells, six heterodimers were fully orthogonal, and in vitro-following mixing of 32 chains from 16 heterodimer designs, denaturation in 5 M guanidine hydrochloride and reannealing-almost all of the interactions observed by native mass spectrometry were between the designed cognate pairs. The ability to design orthogonal protein heterodimers should enable sophisticated protein-based control logic for synthetic biology, and illustrates that nature has not fully explored the possibilities for programmable biomolecular interaction modalities.

The exon junction complex (EJC) deposited upstream of mRNA exon junctions shapes structure, composition and fate of spliced mRNA ribonucleoprotein particles (mRNPs). To achieve this, the EJC core nucleates assembly of a dynamic shell of peripheral proteins that function in diverse post-transcriptional processes. To illuminate consequences of EJC composition change, we purified EJCs from human cells via peripheral proteins RNPS1 and CASC3. We show that EJC originates as an SR-rich mega-dalton sized RNP that contains RNPS1 but lacks CASC3. After mRNP export to the cytoplasm and before translation, the EJC undergoes a remarkable compositional and structural remodeling into an SR-devoid monomeric complex that contains CASC3. Surprisingly, RNPS1 is important for nonsense-mediated mRNA decay (NMD) in general whereas CASC3 is needed for NMD of only select mRNAs. The promotion of switch to CASC3-EJC slows down NMD. Overall, the EJC compositional switch dramatically alters mRNP structure and specifies two distinct phases of EJC-dependent NMD.

Infrared multiple-photon dissociation (IRMPD) action spectroscopy was performed on the b2+ fragment ion from the protonated PPG tripeptide. Comparison of the experimental infrared spectrum with computed spectra for both oxazolone and diketopiperazine structures indicates that the majority of the fragment ion population has an oxazolone structure with the remainder having a diketopiperazine structure. This result is in contrast with a recent study of the IRMPD action spectrum of the PP b2+ fragment ion from PPP, which was found to be nearly 100% diketopiperazine (Martens et al. Int. J. Mass Spectrom. 2015, 377, 179). The diketopiperazine b2+ ion is thermodynamically more stable than the oxazolone but normally requires a trans/cis peptide bond isomerization in the dissociating peptide. Martens et al. showed through IRMPD action spectroscopy that the PPP precursor ion was in a conformation in which the first peptide bond is already in the cis conformation and thus it was energetically favorable to form the thermodynamically-favored diketopiperazine b2+ ion. In the present case, solution-phase NMR spectroscopy and gas-phase IRMPD action spectroscopy show that the PPG precursor ion has its first amide bond in a trans configuration suggesting that the third residue is playing an important role in both the structure of the peptide and the associated ring-closure barriers for oxazolone and diketopiperazine formation. Graphical Abstract ᅟ.

Tryptophan hydroxylase 1 (Tph-1), the principal enzyme for peripheral serotonin biosynthesis, provides a novel target to design anabolic agents for osteoporosis. Here, we present a design, synthesis of a novel series of ursolic acid derivatives under the guidance of docking technique, and bioevaluation of the derivatives using RBL2H3 cells and ovariectomized (OVX) rats. Of the compounds, 9a showed a potent inhibitory activity on serotonin biosynthesis. Further investigations revealed that 9a, as an efficient Tph-1 binder identified by SPR (estimated KD: 6.82 μM), suppressed the protein and mRNA expressions of Tph-1 and lowered serotonin contents in serum and gut without influence on brain serotonin. Moreover, oral administration of 9a elevated serum level of N-terminal propeptide of procollagen type 1 (P1NP), a bone formation marker, and improved bone microarchitecture without estrogenic side effects in ovariectomized rats. Collectively, 9a may serve as a new candidate for bone anabolic drug discovery.