Amino acid transport in thermophiles: characterization of an arginine-binding protein from Thermotoga maritima. 3. Conformational dynamics and stability

Influence of pH on indole-dependent heterodimeric interactions between Anopheles gambiae odorant-binding proteins OBP1 and OBP4

Insect Odorant Binding Proteins (OBPs) constitute important components of their olfactory apparatus, as they are essential for odor recognition. OBPs undergo conformational changes upon pH change, altering their interactions with odorants. Moreover, they can form heterodimers with novel binding characteristics. Anopheles gambiae OBP1 and OBP4 were found capable of forming heterodimers possibly involved in the specific perception of the attractant indole. In order to understand how these OBPs interact in the presence of indole and to investigate the likelihood of a pH-dependent heterodimerization mechanism, the crystal structures of OBP4 at pH 4.6 and 8.5 were determined. Structural comparison to each other and with the OBP4-indole complex (3Q8I, pH 6.85) revealed a flexible N-terminus and conformational changes in the α4-loop-α5 region at acidic pH. Fluorescence competition assays showed a weak binding of indole to OBP4 that become further impaired at acidic pH. Additional Molecular Dynamic and Differential Scanning Calorimetry studies displayed that the influence of pH on OBP4 stability is significant compared to the modest effect of indole. Furthermore, OBP1-OBP4 heterodimeric models were generated at pH 4.5, 6.5, and 8.5, and compared concerning their interface energy and cross-correlated motions in the absence and presence of indole. The results indicate that the increase in pH may induce the stabilization of OBP4 by increasing its helicity, thereby enabling indole binding at neutral pH that further stabilizes the protein and possibly promotes the creation of a binding site for OBP1. A decrease in interface stability and loss of correlated motions upon transition to acidic pH may provoke the heterodimeric dissociation allowing indole release. Finally, we propose a potential OBP1-OBP4 heterodimer formation/disruption mechanism induced by pH change and indole binding.

The structure of AgamOBP5 in complex with the natural insect repellents Carvacrol and Thymol: Crystallographic, fluorescence and thermodynamic binding studies

Among several proteins participating in the olfactory perception process of insects, Odorant Binding Proteins (OBPs) are today considered valid targets for the discovery of compounds that interfere with their host-detection behavior. The 3D structures of Anopheles gambiae mosquito AgamOBP1 in complex with the known synthetic repellents DEET and Icaridin have provided valuable information on the structural characteristics that govern their selective binding. However, no structure of a plant-derived repellent bound to an OBP has been available until now. Herein, we present the novel three-dimensional crystal structures of AgamOBP5 in complex with two natural phenolic monoterpenoid repellents, Carvacrol and Thymol, and the MPD molecule. Structural analysis revealed that both monoterpenoids occupy a binding site (Site-1) by adopting two alternative conformations. An additional Carvacrol was also bound to a secondary site (Site-2) near the central cavity entrance. A protein-ligand hydrogen-bond network supplemented by van der Waals interactions spans the entire binding cavity, bridging α4, α6, and α3 helices and stabilizing the overall structure. Fluorescence competition and Differential Scanning Calorimetry experiments verified the presence of two binding sites and the stabilization effect on AgamOBP5. While Carvacrol and Thymol bind to Site-1 with equal affinity in the submicromolar range, they exhibit a significantly lower and distinct binding capacity for Site-2 with Kd's of ~7 μΜ and ~18 μΜ, respectively. Finally, a comparison of AgamOBP5 complexes with the AgamOBP4-Indole structure revealed that variations of ligand-interacting aminoacids such as A109T, I72M, A112L, and A105T cause two structurally similar and homologous proteins to display different binding specificities.

The role of C-terminal helix in the conformational transition of an arginine binding protein

•

Probe the role of C-ter. helix (CTH) in conformational transition of TmArgBP.

•

Presence of CTH almost doubles the barrier to access the closed-state.

•

In the absence of CTH, the protein can fluctuate between the two conformations.

•

CTH not only constraints the open-state conformation but also guides in accessing it.

The thermotoga maritima arginine binding protein (TmArgBP) is a periplasmic binding protein that has a short helix at the C-terminal end (CTH), which is swapped between the two chains. We apply a coarse-grained structure-based model (SBM) and all-atom MD simulation on this protein to understand the mechanism and the role of CTH in the conformational transition. When the results of SBM simulations of TmArgBP in the presence and absence of CTH are compared, we find that CTH is strategically located at the back of the binding pocket restraining the open-state conformation thereby disengaging access to the closed-state. We also ran all-atom MD simulations of open-state TmArgBP with and without CTH and discovered that in the absence of CTH the protein could reach the closed-state within 250 ns, while in its presence, the protein remained predominantly in its open-state conformation. In the simulation started from unliganded closed-state conformation without CTH, the protein exhibited multiple transitions between the two states, suggesting CTH as an essential structural element to stabilize the open-state conformation. In another simulation that began with an unliganded closed-state conformation with CTH, the protein was able to access the open-state. In this simulation the CTH was observed to reorient itself to interact with the protein emphasizing its role in assisting the conformational change. Based on our findings, we believe that CTH not only acts as a structural element that constraints the protein in its open-state but it may also guide the protein back to its open-state conformation upon ligand unbinding.

Analogs of the Cathelicidin-Derived Antimicrobial Peptide PMAP-23 Exhibit Improved Stability and Antibacterial Activity

Antimicrobial peptides (AMPs) have gained interesting as a new type of antimicrobial agent. The cathelicidin-derived antimicrobial peptide PMAP-23 has broad-spectrum antibacterial activity, and to improve its antimicrobial activity, we used amino acid substitution at position 5 or 19 of PMAP-23 to design three analogs, named PMAP-23R (Leu⁵--Arg), PMAP-23I (Thr¹⁹--Ile), and PMAP-23RI (Leu⁵--Arg and Thr¹⁹--Ile). We found that the analog peptides exhibited higher stability and improved antibacterial activity compared with PMAP-23. Additionally, the analog peptides PMAP-23I and PMAP-23RI inhibited the growth of Shigella flexneri CICC 21534, whereas PMAP-23 and PMAP-23R exhibited no antibacterial activity against S. flexneri CICC 21534. Moreover, the peptide analogs showed negligible hemolysis and cytotoxicity. We also found that PMAP-23RI exerted impressive therapeutic effects on mice infected with Staphylococcus aureus ATCC 25923 and Salmonella enterica serovar Typhimurium SL1344. PMAP-23RI induced a greater reduction in pathological damage and a higher decrease in the bacterial gene copies in the lung and liver tissues and greatly reduced mouse mortality. In conclusion, the peptide analogs PMAP-23R, PMAP-23I, and PMAP-23RI enhanced the stability and antimicrobial activity of PMAP-23, but PMAP-23RI exhibits more promise as a new antimicrobial agent candidate for the treatment of bacterial infections.

Thermodynamic Evaluation of the Interactions between Anticancer Pt(II) Complexes and Model Proteins

In this work, we have analysed the binding of the Pt(II) complexes ([PtCl(4′-phenyl-2,2′:6′,2″-terpyridine)](CF₃SO₃) (1), [PtI(4′-phenyl-2,2′:6′,2″-terpyridine)](CF₃SO₃) (2) and [PtCl(1,3-di(2-pyridyl)benzene) (3)] with selected model proteins (hen egg-white lysozyme, HEWL, and ribonuclease A, RNase A). Platinum coordination compounds are intensively studied to develop improved anticancer agents. In this regard, a critical issue is the possible role of Pt-protein interactions in their mechanisms of action. Multiple techniques such as differential scanning calorimetry (DSC), electrospray ionization mass spectrometry (ESI-MS) and UV-Vis absorbance titrations were used to enlighten the details of the binding to the different biosubstrates. On the one hand, it may be concluded that the affinity of 3 for the proteins is low. On the other hand, 1 and 2 strongly bind them, but with major binding mode differences when switching from HEWL to RNase A. Both 1 and 2 bind to HEWL with a non-specific (DSC) and non-covalent (ESI-MS) binding mode, dominated by a 1:1 binding stoichiometry (UV-Vis). ESI-MS data indicate a protein-driven chloride loss that does not convert into a covalent bond, likely due to the unfavourable complexes’ geometries and steric hindrance. This result, together with the significant changes of the absorbance profiles of the complex upon interaction, suggest an electrostatic binding mode supported by some stacking interaction of the aromatic ligand. Very differently, in the case of RNase A, slow formation of covalent adducts occurs (DSC, ESI-MS). The reactivity is higher for the iodo-compound 2, in agreement with iodine lability higher than chlorine.

Guanidinium binding to proteins: The intriguing effects on the D1 and D2 domains of Thermotoga maritima Arginine Binding Protein and a comprehensive analysis of the Protein Data Bank

Thermotoga maritima Arginine Binding Protein has been extensively characterized because of its peculiar features and its possible use as a biosensor. In this characterization, deletion of the C-terminal helix to obtain the monomeric protein TmArgBP^20-233 and dissection of the monomer in its two domains, D1 and D2, have been performed. In the present study the stability of these three forms against guanidinium chloride is investigated by means of circular dichroism and differential scanning calorimetry measurements. All three proteins show a high conformational stability; moreover, D1 shows an unusual behavior in the presence of low concentrations of guanidinium chloride. This finding has led us to investigate a possible binding interaction by means of isothermal titration calorimetry and X-ray crystallography; the results indicate that D1 is able to bind the guanidinium ion (GuH⁺), due to its similarity with the arginine terminal moiety. The analysis of the structural and dynamic properties of the D1-GuH⁺ complex indicates that the protein binds the ligand through multiple and diversified interactions. An exhaustive survey of the binding modes of GuH⁺ to proteins indicates that this is a rather common feature. These observations provide interesting insights into the effects that GuH⁺ is able to induce in protein structures.

Antimicrobial peptide PMAP-37 analogs: Increasing the positive charge to enhance the antibacterial activity of PMAP-37

Bacterial resistance induced by the use of antibiotics has provided a chance for the development of antimicrobial peptides (AMPs), and modification of AMPs to enhance the antibacterial activity or stability has become a research focus. PMAP-37 is an AMP isolated from porcine myeloid marrow, and studies on its modification have not yet been reported. In this study, three PMAP-37 analogs named PMAP-37(F9-R), PMAP-37(F34-R), and PMAP-37(F9/34-R) were designed by residue substitution to enhance the positive charge. The antimicrobial activity of PMAP-37 and its analogs in vitro and in vivo were detected. The results showed that compared with PMAP-37, PMAP-37(F9-R) and PMAP-37(F9/34-R) exhibited antibacterial activity against S. flexneri CICC21534. Although PMAP-37(F34-R) had no antibacterial activity against S. flexneri CICC21534, its minimal inhibitory concentrations (MICs) were significantly lower than those of PMAP-37 against most bacterial strains. Besides, all PMAP-37 analogs were pH stable, retaining stable antibacterial activity after treatment with solution from pH 2 to pH 8/9. In addition, the PMAP-37 analogs displayed increased thermal stability, and PMAP-37(F34-R) retained >60% antibacterial activity after boiling for 2 hours. Furthermore, the PMAP-37 analogs exhibited impressive therapeutic efficacy in bacterial infections by reducing bacterial burden and inflammatory damage in the lung and liver, resulting in a reduction in mortality. Notably, the therapeutic effect of PMAP-37(F34-R) was comparable to that of ceftiofur sodium, and even superior to antibiotics in L. monocytogenes CICC21533 infection model. In conclusion, the PMAP-37(F34-R) may be a candidate for the treatment of bacterial infections in the clinic.

Domain communication in Thermotoga maritima Arginine Binding Protein unraveled through protein dissection

Substrate binding proteins represent a large protein family that plays fundamental roles in selective transportation of metabolites across membrane. The function of these proteins relies on the relative motions of their two domains. Insights into domain communication in this class of proteins have been here collected using Thermotoga maritima Arginine Binding Protein (TmArgBP) as model system. TmArgBP was dissected into two domains (D1 and D2) that were exhaustively characterized using a repertoire of different experimental and computational techniques. Indeed, stability, crystalline structure, ability to recognize the arginine substrate, and dynamics of the two individual domains have been here studied. Present data demonstrate that, although in the parent protein both D1 and D2 cooperate for the arginine anchoring; only D1 is intrinsically able to bind the substrate. The implications of this finding on the mechanism of arginine binding and release by TmArgBP have been discussed. Interestingly, both D1 and D2 retain the remarkable thermal/chemical stability of the parent protein. The analysis of the structural and dynamic properties of TmArgBP and of the individual domains highlights possible routes of domain communication. Finally, this study generated two interesting molecular tools, the two stable isolated domains that could be used in future investigations.

Domain swapping dissection in Thermotoga maritima arginine binding protein: How structural flexibility may compensate destabilization

Thermotoga maritima Arginine Binding Protein (TmArgBP) is a valuable candidate for arginine biosensing in diagnostics. This protein is endowed with unusual structural properties that include an extraordinary thermal/chemical stability, a domain swapped structure that undergoes large tertiary and quaternary structural transition, and the ability to form non-canonical oligomeric species. As the intrinsic stability of TmArgBP allows for extensive protein manipulations, we here dissected its structure in two parts: its main body deprived of the swapping fragment (TmArgBP^20-233) and the C-terminal peptide corresponding to the helical swapping element. Both elements have been characterized independently or in combination using a repertoire of biophysical/structural techniques. Present investigations clearly indicate that TmArgBP^20-233 represents a better scaffold for arginine sensing compared to the wild-type protein. Moreover, our data demonstrate that the ligand-free and the ligand-bound forms respond very differently to this helix deletion. This drastic perturbation has an important impact on the ligand-bound form of TmArgBP^20-233 stability whereas it barely affects its ligand-free state. The crystallographic structures of these forms provide a rationale to this puzzling observation. Indeed, the arginine-bound state is very rigid and virtually unchanged upon protein truncation. On the other hand, the flexible ligand-free TmArgBP^20-233 is able to adopt a novel state as a consequence of the helix deletion. Therefore, the flexibility of the ligand-free form endows this state with a remarkable robustness upon severe perturbations. In this scenario, TmArgBP dissection highlights an intriguing connection between destabilizing/stabilizing effects and the overall flexibility that could operate also in other proteins.

Engineering a switch-based biosensor for arginine using a Thermotoga maritima periplasmic binding protein

The Thermotoga maritima arginine-binding protein (TmArgBP) has been modified to create a reagentless fluorescent protein biosensor. Two design methods for biosensor construction are compared: 1) solvent accessibility of environmentally-sensitive probes and 2) fluorescence deactivation due to photo-induced electron transfer (PET). Nine single cysteine TmArgBP mutants were created and labeled with three different environmentally sensitive fluorescent probes. These mutants demonstrated limited changes in fluorescence emission upon the addition of arginine. In contrast, the PET-based biosensor provides significant enhancements over the traditional approach and provides a fluorescence quenching mechanism that was capable of providing quantitative detection of arginine. Site-directed mutagenesis of TmArgBP was used to create attachment points for the fluorescent probe (K145C) and for an internal aromatic residue (D18X) to serve as the PET quencher. Both tyrosine and tryptophan, but not phenylalanine, were able to quench the emission of the fluorescent probe by more than 80% upon the addition of arginine. The dissociation constant for arginine ranged from 0.87 to 1.5 μM across the different sensors. This PET-based strategy provides a simple and broadly applicable approach for the analytical detection of small molecules that may be applied to any protein that exhibits conformational switching in a ligand dependent manner.

Proline 235 plays a key role in the regulation of the oligomeric states of Thermotoga maritima Arginine Binding Protein

The Arginine Binding Protein isolated from Thermotoga maritima (TmArgBP) is a protein endowed with several peculiar properties. We have previously shown that TmArgBP dimerization is a consequence of the swapping of the C-terminal helix. Here we explored the structural determinants of TmArgBP domain swapping and oligomerization. In particular, we report a mutational analysis of the residue Pro235, which is located in the hinge region of the swapping dimer. This residue was either replaced with a Gly-Lys dipeptide (TmArgBP(P235GK)) or a Gly residue (TmArgBP(P235G)). Different forms of these mutants were generated and extensively characterized using biophysical techniques. For both TmArgBP(P235GK) and TmArgBP(P235G) mutants, the occurrence of multiple oligomerization states (monomers, dimers and trimers) was detected. The formation of well-folded monomeric forms for these mutants indicates that the dimerization through C-terminal domain swapping observed in wild-type TmArgBP is driven by conformational restraints imposed by the presence of Pro235 in the hinge region. Molecular dynamics studies corroborate this observation by showing that Gly235 assumes conformational states forbidden for Pro residues in the TmArgBP(P235G) monomer. Unexpectedly, the trimeric forms present: (a) peculiar circular dichroism spectra, (b) a great susceptibility to heating, and (c) the ability to bind the Thioflavin T dye. The present findings clearly demonstrate that single-point mutations have an important impact on the TmArgBP oligomerization process. In a wider context, they also indicate that proteins endowed with an intrinsic propensity to swap have an easy access to states with altered structural and, possibly, functional properties.

A loose domain swapping organization confers a remarkable stability to the dimeric structure of the arginine binding protein from Thermotoga maritima

The arginine binding protein from Thermatoga maritima (TmArgBP), a substrate binding protein (SBP) involved in the ABC system of solute transport, presents a number of remarkable properties. These include an extraordinary stability to temperature and chemical denaturants and the tendency to form multimeric structures, an uncommon feature among SBPs involved in solute transport. Here we report a biophysical and structural characterization of the TmArgBP dimer. Our data indicate that the dimer of the protein is endowed with a remarkable stability since its full dissociation requires high temperature as well as SDS and urea at high concentrations. In order to elucidate the atomic level structural properties of this intriguing protein, we determined the crystallographic structures of the apo and the arginine-bound forms of TmArgBP using MAD and SAD methods, respectively. The comparison of the liganded and unliganded models demonstrates that TmArgBP tertiary structure undergoes a very large structural re-organization upon arginine binding. This transition follows the Venus Fly-trap mechanism, although the entity of the re-organization observed in TmArgBP is larger than that observed in homologous proteins. Intriguingly, TmArgBP dimerizes through the swapping of the C-terminal helix. This dimer is stabilized exclusively by the interactions established by the swapping helix. Therefore, the TmArgBP dimer combines a high level of stability and conformational freedom. The structure of the TmArgBP dimer represents an uncommon example of large tertiary structure variations amplified at quaternary structure level by domain swapping. Although the biological relevance of the dimer needs further assessments, molecular modelling suggests that the two TmArgBP subunits may simultaneously interact with two distinct ABC transporters. Moreover, the present protein structures provide some clues about the determinants of the extraordinary stability of the biomolecule. The availability of an accurate 3D model represents a powerful tool for the design of new TmArgBP suited for biotechnological applications.

A reciprocating twin-channel model for ABC transporters

ABC transporters comprise a large, diverse, and ubiquitous superfamily of membrane active transporters. Their core architecture is a dimer of dimers, comprising two transmembrane (TM) domains that bind substrate, and two ATP-binding cassettes, which use the cell's energy currency to couple substrate translocation to ATP hydrolysis. Despite the availability of over a dozen resolved structures and a wealth of biochemical and biophysical data, this field is bedeviled by controversy and long-standing mechanistic questions remain unresolved. The prevailing paradigm for the ABC transport mechanism is the Switch Model, in which the ATP-binding cassettes dimerize upon binding two ATP molecules, and thence dissociate upon sequential ATP hydrolysis. This cycle of nucleotide-binding domain (NBD) dimerization and dissociation is coupled to a switch between inward- or outward facing conformations of a single TM channel; this alternating access enables substrate binding on one face of the membrane and its release at the other. Notwithstanding widespread acceptance of the Switch Model, there is substantial evidence that the NBDs do not separate very much, if at all, and thus physical separation of the ATP cassettes observed in crystallographic structures may be an artefact. An alternative Constant Contact Model has been proposed, in which ATP hydrolysis occurs alternately at the two ATP-binding sites, with one of the sites remaining closed and containing occluded nucleotide at all times. In this model, the cassettes remain in contact and the active sites swing open in an alternately seesawing motion. Whilst the concept of NBD association/dissociation in the Switch Model is naturally compatible with a single alternating-access channel, the asymmetric functioning proposed by the Constant Contact model suggests an alternating or reciprocating function in the TMDs. Here, a new model for the function of ABC transporters is proposed in which the sequence of ATP binding, hydrolysis, and product release in each active site is directly coupled to the analogous sequence of substrate binding, translocation and release in one of two functionally separate substrate translocation pathways. Each translocation pathway functions 180° out of phase. A wide and diverse selection of data for both ABC importers and exporters is examined, and the ability of the Switch and Reciprocating Models to explain the data is compared and contrasted. This analysis shows that not only can the Reciprocating Model readily explain the data; it also suggests straightforward explanations for the function of a number of atypical ABC transporters. This study represents the most coherent and complete attempt at an all-encompassing scheme to explain how these important proteins work, one that is consistent with sound biochemical and biophysical evidence.

Crystal and solution studies of the "Plus-C" odorant-binding protein 48 from Anopheles gambiae: control of binding specificity through three-dimensional domain swapping

Much physiological and behavioral evidence has been provided suggesting that insect odorant-binding proteins (OBPs) are indispensable for odorant recognition and thus are appealing targets for structure-based discovery and design of novel host-seeking disruptors. Despite the fact that more than 60 putative OBP-encoding genes have been identified in the malaria vector Anopheles gambiae, the crystal structures of only six of them are known. It is therefore clear that OBP structure determination constitutes the bottleneck for structure-based approaches to mosquito repellent/attractant discovery. Here, we describe the three-dimensional structure of an A. gambiae "Plus-C" group OBP (AgamOBP48), which exhibits the second highest expression levels in female antennae. This structure represents the first example of a three-dimensional domain-swapped dimer in dipteran species. A combined binding site is formed at the dimer interface by equal contribution of each monomer. Structural comparisons with the monomeric AgamOBP47 revealed that the major structural difference between the two Plus-C proteins localizes in their N- and C-terminal regions, and their concerted conformational change may account for monomer-swapped dimer conversion and furthermore the formation of novel binding pockets. Using a combination of gel filtration chromatography, differential scanning calorimetry, and analytical ultracentrifugation, we demonstrate the AgamOBP48 dimerization in solution. Eventually, molecular modeling calculations were used to predict the binding mode of the most potent synthetic ligand of AgamOBP48 known so far, discovered by ligand- and structure-based virtual screening. The structure-aided identification of multiple OBP binders represents a powerful tool to be employed in the effort to control transmission of the vector-borne diseases.

Periplasmic Binding Proteins in Thermophiles: Characterization and Potential Application of an Arginine-Binding Protein from Thermotoga maritima: A Brief Thermo-Story

Arginine-binding protein from the extremophile Thermotoga maritima is a 27.7 kDa protein possessing the typical two-domain structure of the periplasmic binding proteins family. The protein is characterized by a very high specificity and affinity to bind to arginine, also at high temperatures. Due to its features, this protein could be taken into account as a potential candidate for the design of a biosensor for arginine. It is important to investigate the stability of proteins when they are used for biotechnological applications. In this article, we review the structural and functional features of an arginine-binding protein from the extremophile Thermotoga maritima with a particular eye on its potential biotechnological applications.