Mass spectrometry captures biased signalling and allosteric modulation of a G-protein-coupled receptor

G-protein-coupled receptors signal through cognate G proteins. Despite the widespread importance of these receptors, their regulatory mechanisms for G-protein selectivity are not fully understood. Here we present a native mass spectrometry-based approach to interrogate both biased signalling and allosteric modulation of the β1-adrenergic receptor in response to various ligands. By simultaneously capturing the effects of ligand binding and receptor coupling to different G proteins, we probed the relative importance of specific interactions with the receptor through systematic changes in 14 ligands, including isoprenaline derivatives, full and partial agonists, and antagonists. We observed enhanced dynamics of the intracellular loop 3 in the presence of isoprenaline, which is capable of acting as a biased agonist. We also show here that endogenous zinc ions augment the binding in receptor-Gs complexes and propose a zinc ion-binding hotspot at the TM5/TM6 intracellular interface of the receptor-Gs complex. Further interrogation led us to propose a mechanism in which zinc ions facilitate a structural transition of the intermediate complex towards the stable state.

Charge Engineering Reveals the Roles of Ionizable Side Chains in Electrospray Ionization Mass Spectrometry

In solution, the charge of a protein is intricately linked to its stability, but electrospray ionization distorts this connection, potentially limiting the ability of native mass spectrometry to inform about protein structure and dynamics. How the behavior of intact proteins in the gas phase depends on the presence and distribution of ionizable surface residues has been difficult to answer because multiple chargeable sites are present in virtually all proteins. Turning to protein engineering, we show that ionizable side chains are completely dispensable for charging under native conditions, but if present, they are preferential protonation sites. The absence of ionizable side chains results in identical charge state distributions under native-like and denaturing conditions, while coexisting conformers can be distinguished using ion mobility separation. An excess of ionizable side chains, on the other hand, effectively modulates protein ion stability. In fact, moving a single ionizable group can dramatically alter the gas-phase conformation of a protein ion. We conclude that although the sum of the charges is governed solely by Coulombic terms, their locations affect the stability of the protein in the gas phase.

Bifurcated binding of the OmpF receptor underpins import of the bacteriocin colicin N into Escherichia coli

Colicins are Escherichia coli-specific bacteriocins that translocate across the outer bacterial membrane by a poorly understood mechanism. Group A colicins typically parasitize the proton-motive force-linked Tol system in the inner membrane via porins after first binding an outer membrane protein receptor. Recent studies have suggested that the pore-forming group A colicin N (ColN) instead uses lipopolysaccharide as a receptor. Contrary to this prevailing view, using diffusion-precipitation assays, native state MS, isothermal titration calorimetry, single-channel conductance measurements in planar lipid bilayers, and in vivo fluorescence imaging, we demonstrate here that ColN uses OmpF both as its receptor and translocator. This dual function is achieved by ColN having multiple distinct OmpF-binding sites, one located within its central globular domain and another within its disordered N terminus. We observed that the ColN globular domain associates with the extracellular surface of OmpF and that lipopolysaccharide (LPS) enhances this binding. Approximately 90 amino acids of ColN then translocate through the porin, enabling the ColN N terminus to localize within the lumen of an OmpF subunit from the periplasmic side of the membrane, a binding mode reminiscent of that observed for the nuclease colicin E9. We conclude that bifurcated engagement of porins is intrinsic to the import mechanism of group A colicins.

A Mass-Spectrometry-Based Approach to Distinguish Annular and Specific Lipid Binding to Membrane Proteins

Membrane proteins engage in a variety of contacts with their surrounding lipids, but distinguishing between specifically bound lipids, and non‐specific, annular interactions is a challenging problem. Applying native mass spectrometry to three membrane protein complexes with different lipid‐binding properties, we explore the ability of detergents to compete with lipids bound in different environments. We show that lipids in annular positions on the presenilin homologue protease are subject to constant exchange with detergent. By contrast, detergent‐resistant lipids bound at the dimer interface in the leucine transporter show decreased k_off rates in molecular dynamics simulations. Turning to the lipid flippase MurJ, we find that addition of the natural substrate lipid‐II results in the formation of a 1:1 protein–lipid complex, where the lipid cannot be displaced by detergent from the highly protected active site. In summary, we distinguish annular from non‐annular lipids based on their exchange rates in solution.

Identifying key membrane protein lipid interactions using mass spectrometry

With the recent success in determining membrane protein structures, further detailed understanding of the identity and function of the bound lipidome is essential. Using an approach that combines high-energy native mass spectrometry (HE-nMS) and solution-phase lipid profiling, this protocol can be used to determine the identity of the endogenous lipids that directly interact with a protein. Furthermore, this method can identify systems in which such lipid binding has a major role in regulating the oligomeric assembly of membrane proteins. The protocol begins with recording of the native mass spectrum of the protein of interest, under successive delipidation conditions, to determine whether delipidation leads to disruption of the oligomeric state. Subsequently, we propose using a bipronged strategy: first, an HE-nMS platform is used that allows dissociation of the detergent micelle at the front end of the instrument. This allows for isolation of the protein-lipid complex at the quadrupole and successive fragmentation at the collision cell, which leads to identification of the bound lipid masses. Next, simultaneous coupling of this with in-solution LC-MS/MS-based identification of extracted lipids reveals the complete identity of the interacting lipidome that copurifies with the proteins. Assimilation of the results of these two sets of experiments divulges the complete identity of the set of lipids that directly interact with the membrane protein of interest, and can further delineate its role in maintaining the oligomeric state of the protein. The entire procedure takes 2 d to complete.

Cotranslational protein assembly imposes evolutionary constraints on homomeric proteins

Cotranslational protein folding can facilitate rapid formation of functional structures. However, it can also cause premature assembly of protein complexes, if two interacting nascent chains are in close proximity. By analyzing known protein structures, we show that homomeric protein contacts are enriched toward the C termini of polypeptide chains across diverse proteomes. We hypothesize that this is the result of evolutionary constraints for folding to occur before assembly. Using high-throughput imaging of protein homomers in Escherichia coli and engineered protein constructs with N- and C-terminal oligomerization domains, we show that, indeed, proteins with C-terminal homomeric interface residues consistently assemble more efficiently than those with N-terminal interface residues. Using in vivo, in vitro and in silico experiments, we identify features that govern successful assembly of homomers, which have implications for protein design and expression optimization.

Native Desorption Electrospray Ionization Liberates Soluble and Membrane Protein Complexes from Surfaces

Mass spectrometry (MS) applications for intact protein complexes typically require electrospray (ES) ionization and have not been achieved via direct desorption from surfaces. Desorption ES ionization (DESI) MS has however transformed the study of tissue surfaces through release and characterisation of small molecules. Motivated by the desire to screen for ligand binding to intact protein complexes we report the development of a native DESI platform. By establishing conditions that preserve non-covalent interactions we exploit the surface to capture a rapid turnover enzyme-substrate complex and to optimise detergents for membrane protein study. We demonstrate binding of lipids and drugs to membrane proteins deposited on surfaces and selectivity from a mix of related agonists for specific binding to a GPCR. Overall therefore we introduce this native DESI platform with the potential for high-throughput ligand screening of some of the most challenging drug targets including GPCRs.

Ligand binding to a G protein-coupled receptor captured in a mass spectrometer

G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors belong to the largest family of membrane-embedded cell surface proteins and are involved in a diverse array of physiological processes. Despite progress in the mass spectrometry of membrane protein complexes, G protein-coupled receptors have remained intractable because of their low yield and instability after extraction from cell membranes. We established conditions in the mass spectrometer that preserve noncovalent ligand binding to the human purinergic receptor P2Y1. Results established differing affinities for nucleotides and the drug MRS2500 and link antagonist binding with the absence of receptor phosphorylation. Overall, therefore, our results are consistent with drug binding, preventing the conformational changes that facilitate downstream signaling. More generally, we highlight opportunities for mass spectrometry to probe effects of ligand binding on G protein-coupled receptors.

The Tetrameric Plant Lectin BanLec Neutralizes HIV through Bidentate Binding to Specific Viral Glycans

Select lectins have powerful anti-viral properties that effectively neutralize HIV-1 by targeting the dense glycan shield on the virus. Here, we reveal the mechanism by which one of the most potent lectins, BanLec, achieves its inhibition. We identify that BanLec recognizes a subset of high-mannose glycans via bidentate interactions spanning the two binding sites present on each BanLec monomer that were previously considered separate carbohydrate recognition domains. We show that both sites are required for high-affinity glycan binding and virus neutralization. Unexpectedly we find that BanLec adopts a tetrameric stoichiometry in solution whereby the glycan-binding sites are positioned to optimally target glycosylated viral spikes. The tetrameric architecture, together with bidentate binding to individual glycans, leads to layers of multivalency that drive viral neutralization through enhanced avidity effects. These structural insights will prove useful in engineering successful lectin therapeutics targeting the dense glycan shield of HIV.

Negative Ions Enhance Survival of Membrane Protein Complexes

Membrane protein complexes are commonly introduced to the mass spectrometer solubilized in detergent micelles. The collisional activation used to remove the detergent, however, often causes protein unfolding and dissociation. As in the case for soluble proteins, electrospray in the positive ion mode is most commonly used for the study of membrane proteins. Here we show several distinct advantages of employing the negative ion mode. Negative polarity can yield lower average charge states for membrane proteins solubilized in saccharide detergents, with enhanced peak resolution and reduced adduct formation. Most importantly, we demonstrate that negative ion mode electrospray ionization (ESI) minimizes subunit dissociation in the gas phase, allowing access to biologically relevant oligomeric states. Together, these properties mean that intact membrane protein ions can be generated in a greater range of solubilizing detergents. The formation of negative ions, therefore, greatly expands the possibilities of using mass spectrometry on this intractable class of protein. Graphical Abstract ᅟ.

A combined computational and structural model of the full-length human prolactin receptor

The prolactin receptor is an archetype member of the class I cytokine receptor family, comprising receptors with fundamental functions in biology as well as key drug targets. Structurally, each of these receptors represent an intriguing diversity, providing an exceptionally challenging target for structural biology. Here, we access the molecular architecture of the monomeric human prolactin receptor by combining experimental and computational efforts. We solve the NMR structure of its transmembrane domain in micelles and collect structural data on overlapping fragments of the receptor with small-angle X-ray scattering, native mass spectrometry and NMR spectroscopy. Along with previously published data, these are integrated by molecular modelling to generate a full receptor structure. The result provides the first full view of a class I cytokine receptor, exemplifying the architecture of more than 40 different receptor chains, and reveals that the extracellular domain is merely the tip of a molecular iceberg.

The human Na(+)/H(+) exchanger 1 is a membrane scaffold protein for extracellular signal-regulated kinase 2

Background

Extracellular signal-regulated kinase 2 (ERK2) is an S/T kinase with more than 200 known substrates, and with critical roles in regulation of cell growth and differentiation and currently no membrane proteins have been linked to ERK2 scaffolding.

Methods and results

Here, we identify the human Na⁺/H⁺ exchanger 1 (hNHE1) as a membrane scaffold protein for ERK2 and show direct hNHE1-ERK1/2 interaction in cellular contexts. Using nuclear magnetic resonance (NMR) spectroscopy and immunofluorescence analysis we demonstrate that ERK2 scaffolding by hNHE1 occurs by one of three D-domains and by two non-canonical F-sites located in the disordered intracellular tail of hNHE1, mutation of which reduced cellular hNHE1-ERK1/2 co-localization, as well as reduced cellular ERK1/2 activation. Time-resolved NMR spectroscopy revealed that ERK2 phosphorylated the disordered tail of hNHE1 at six sites in vitro, in a distinct temporal order, with the phosphorylation rates at the individual sites being modulated by the docking sites in a distant dependent manner.

Conclusions

This work characterizes a new type of scaffolding complex, which we term a “shuffle complex”, between the disordered hNHE1-tail and ERK2, and provides a molecular mechanism for the important ERK2 scaffolding function of the membrane protein hNHE1, which regulates the phosphorylation of both hNHE1 and ERK2.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0252-7) contains supplementary material, which is available to authorized users.

Controlling release, unfolding and dissociation of membrane protein complexes in the gas phase through collisional cooling

Mass spectrometry of intact membrane protein complexes requires removal of the detergent micelle by collisional activation. We demonstrate that the necessary energy can be obtained by adjusting the degree of collisional cooling in the ion source. This enables us to extend the energy regime for dissociation of membrane protein complexes.

Structure and function of the Escherichia coli Tol-Pal stator protein TolR

TolR is a 15-kDa inner membrane protein subunit of the Tol-Pal complex in Gram-negative bacteria, and its function is poorly understood. Tol-Pal is recruited to cell division sites where it is involved in maintaining the integrity of the outer membrane. TolR is related to MotB, the peptidoglycan (PG)-binding stator protein from the flagellum, suggesting it might serve a similar role in Tol-Pal. The only structure thus far reported for TolR is of the periplasmic domain from Haemophilus influenzae in which N- and C-terminal residues had been deleted (TolR(62–133), Escherichia coli numbering). H. influenzae TolR(62–133) is a symmetrical dimer with a large deep cleft at the dimer interface. Here, we present the 1.7-Å crystal structure of the intact periplasmic domain of E. coli TolR (TolR(36–142)). E. coli TolR(36–142) is also dimeric, but the architecture of the dimer is radically different from that of TolR(62–133) due to the intertwining of its N and C termini. TolR monomers are rotated ∼180° relative to each other as a result of this strand swapping, obliterating the putative PG-binding groove seen in TolR(62–133). We found that removal of the strand-swapped regions (TolR(60–133)) exposes cryptic PG binding activity that is absent in the full-length domain. We conclude that to function as a stator in the Tol-Pal complex dimeric TolR must undergo large scale structural remodeling reminiscent of that proposed for MotB, where the N- and C-terminal sequences unfold in order for the protein to both reach and bind the PG layer ∼90 Å away from the inner membrane.

Mass spectrometry quantifies protein interactions--from molecular chaperones to membrane porins

Proteins possess an intimate relationship between their structure and function, with folded protein structures generating recognition motifs for the binding of ligands and other proteins. Mass spectrometry (MS) can provide information on a number of levels of protein structure, from the primary amino acid sequence to its three-dimensional fold and quaternary interactions. Given that MS is a gas-phase technique, with its foundations in analytical chemistry, it is perhaps counter-intuitive to use it to study the structure and non-covalent interactions of proteins that form in solution. Herein we show, however, that MS can go beyond simply preserving protein interactions in the gas phase by providing new insight into dynamic interaction networks, dissociation mechanisms, and the cooperativity of ligand binding. We consider potential pitfalls in data interpretation and place particular emphasis on recent studies that revealed quantitative information about dynamic protein interactions, in both soluble and membrane-embedded assemblies.

Charge reduction stabilizes intact membrane protein complexes for mass spectrometry

The study of intact soluble protein assemblies by means of mass spectrometry is providing invaluable contributions to structural biology and biochemistry. A recent breakthrough has enabled similar study of membrane protein complexes, following their release from detergent micelles in the gas phase. Careful optimization of mass spectrometry conditions, particularly with respect to energy regimes, is essential for maintaining compact folded states as detergent is removed. However, many of the saccharide detergents widely employed in structural biology can cause unfolding of membrane proteins in the gas phase. Here, we investigate the potential of charge reduction by introducing three membrane protein complexes from saccharide detergents and show how reducing their overall charge enables generation of compact states, as evidenced by ion mobility mass spectrometry. We find that charge reduction stabilizes the oligomeric state and enhances the stability of lipid-bound complexes. This finding is significant since maintaining native-like membrane proteins enables ligand binding to be assessed from a range of detergents that retain solubility while protecting the overall fold.

Evidence for the preservation of native inter- and intra-molecular hydrogen bonds in the desolvated FK-binding protein·FK506 complex produced by electrospray ionization

It is now well established that electrospray ionization (ESI) is capable of introducing noncovalent protein assemblies into a desolvated environment, thereby allowing their analysis by mass spectrometry. The degree to which native interactions from the solution phase are preserved in this environment is less clear. Site-directed mutagenesis of FK506-binding protein (FKBP) has been employed to probe specific intra- and inter-molecular interactions within the complex between FKBP and its ligand FK506. Collisional activation of wild-type and mutant-FKBP•FK506 ions, generated by ESI, demonstrated that removal of native protein-ligand interactions formed between residues Asp37, Tyr82, and FK506 significantly destabilized the complex. Mutation of Arg42 to Ala42, or Tyr26 to Phe26 also resulted in lower energy dissociation of the FKBP·FK506 complex. Although these residues do not form direct H-bonds to FK506, they interact with Asp37, ensuring its correct orientation to associate with the ligand. Comparison with solution-based affinity measurements of these mutants has been discussed, including the stabilization afforded by ordered water molecules. Ion mobility spectrometry (IMS) has been employed to provide gas-phase structural information on the unfolding of the complexes. The [M + 6H](6+) complexes of the wild-type and mutants have been shown to resist unfolding and retain compact conformations. However, removal of the basic Arg42 residue was found to induce significant structural weakening of the [M + 7H](7+) complex when raised to dissociation-level energies. Overall, destabilization of the FKBP·FK506 complex, resulting from targeted removal of specific H-bonds, provides evidence for the preservation of these interactions in the desolvated wild-type complex.

Alkali metal cation-induced destabilization of gas-phase protein-ligand complexes: consequences and prevention

Electrospray ionization, now a well established technique for studying noncovalent protein-ligand interactions, is prone to production of alkali metal adducts. Here it is shown that this adduction significantly destabilizes the interactions between two model proteins and their ligands and that destabilization correlates with cation size. For both the [FKBP·FK506] and [lysozyme·NAG(n)] systems, dissociation of the metalated complex occurs at markedly lower collision energies than their purely protonated equivalents. Dependency upon size of the metal(+) demonstrates the importance of electrostatic charge density during the dissociation process. Differences in the gas phase basicities (GBapp) of the multiply charged protein ions and proton and sodium affinities of the ligands explain the observed charge partitioning during dissociation of the complexes. Ion mobility-mass spectrometry measurements demonstrate that metal cation adduction does not induce a significant increase in unfolding of the polypeptides, indicating that this is not the principal mechanism responsible for destabilization. Destabilizing effects can be largely reduced by exposing the electrospray to solvent (e.g., acetonitrile) vapor, a method that acts to reduce the amount of adduct formation as well as decrease the charge states of the resulting ions. This approach leads to more accurate determination of apparent K(D)s in the presence of trace alkali metals.

Collision induced unfolding of protein ions in the gas phase studied by ion mobility-mass spectrometry: the effect of ligand binding on conformational stability

Ion mobility spectrometry, with subsequent mass spectrometric detection, has been employed to study the stability of compact protein conformations of FK-binding protein, hen egg-white lysozyme, and horse heart myoglobin in the presence and absence of bound ligands. Protein ions, generated by electrospray ionization from ammonium acetate buffer, were activated by collision with argon gas to induce unfolding of their compact structures. The collisional cross sections (Omega) of folded and unfolded conformations were measured in the T-Wave mobility cell of a Waters Synapt HDMS (Waters, Altrincham, UK) employing a calibration against literature values for a range of protein standards. In the absence of activation, collisional cross section measurements were found to be consistent with those predicted for folded protein structures. Under conditions of defined collisional activation energies partially unfolded conformations were produced. The degree of unfolding and dissociation induced by these defined collision energies are related to the stability of noncovalent intra- and intermolecular interactions within protein complexes. These findings highlight the additional conformational stability of protein ions in the gas phase resulting from ligand binding.