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

Destabilization of the D2 domain of Thermotoga maritima arginine binding protein induced by guanidinium thiocyanate and its counteraction by stabilizing agents

D2 is a structural and cooperative domain of Thermotoga maritima Arginine Binding Protein, that possesses a remarkable conformational stability, with a denaturation temperature of 102.6°C, at pH 7.4. The addition of potassium thiocyanate causes a significant decrease in the D2 denaturation temperature. The interactions of thiocyanate ions with D2 have been studied by means of isothermal titration calorimetry measurements and molecular dynamics simulations. It emerged that: (a) 20-30 thiocyanate ions interact with the D2 surface and are present in its first solvation shell; (b) each of them makes several contacts with protein groups, both polar and nonpolar ones. The addition of guanidinium thiocyanate causes a marked destabilization of the D2 native state, because both the ions are denaturing agents. However, on adding to the solution containing D2 and guanidinium thiocyanate a stabilizing agent, such as TMAO, sucrose or sodium sulfate, a significant increase in denaturation temperature occurs. The present results confirm that counteraction is a general phenomenon for globular proteins.

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

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

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

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

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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.

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.

Development of a Protein Scaffold for Arginine Sensing Generated through the Dissection of the Arginine-Binding Protein from Thermotoga maritima

Arginine is one of the most important nutrients of living organisms as it plays a major role in important biological pathways. However, the accumulation of arginine as consequence of metabolic defects causes hyperargininemia, an autosomal recessive disorder. Therefore, the efficient detection of the arginine is a field of relevant biomedical/biotechnological interest. Here, we developed protein variants suitable for arginine sensing by mutating and dissecting the multimeric and multidomain structure of Thermotoga maritima arginine-binding protein (TmArgBP). Indeed, previous studies have shown that TmArgBP domain-swapped structure can be manipulated to generate simplified monomeric and single domain scaffolds. On both these stable scaffolds, to measure tryptophan fluorescence variations associated with the arginine binding, a Phe residue of the ligand binding pocket was mutated to Trp. Upon arginine binding, both mutants displayed a clear variation of the Trp fluorescence. Notably, the single domain scaffold variant exhibited a good affinity (~3 µM) for the ligand. Moreover, the arginine binding to this variant could be easily reverted under very mild conditions. Atomic-level data on the recognition process between the scaffold and the arginine were obtained through the determination of the crystal structure of the adduct. Collectively, present data indicate that TmArgBP scaffolds represent promising candidates for developing arginine biosensors.

On the extraordinary pressure stability of the Thermotoga maritima arginine binding protein and its folded fragments - a high-pressure FTIR spectroscopy study

The arginine binding protein from T. maritima (ArgBP) exhibits several distinctive biophysical and structural properties. Here we show that ArgBP is also endowed with a ramarkable pressure stability as it undergoes minor structural changes only, even at 10 kbar. A similar stability is also observed for its folded fragments (truncated monomer and individual domains). A survey of literature data on the pressure stability of proteins highlights the uncommon behavior of ArgBP.

The non-swapped monomeric structure of the arginine-binding protein from Thermotoga maritima

Domain swapping is a widespread oligomerization process that is observed in a large variety of protein families. In the large superfamily of substrate-binding proteins, non-monomeric members have rarely been reported. The arginine-binding protein from Thermotoga maritima (TmArgBP), a protein endowed with a number of unusual properties, presents a domain-swapped structure in its dimeric native state in which the two polypeptide chains mutually exchange their C-terminal helices. It has previously been shown that mutations in the region connecting the last two helices of the TmArgBP structure lead to the formation of a variety of oligomeric states (monomers, dimers, trimers and larger aggregates). With the aim of defining the structural determinants of domain swapping in TmArgBP, the monomeric form of the P235GK mutant has been structurally characterized. Analysis of this arginine-bound structure indicates that it consists of a closed monomer with its C-terminal helix folded against the rest of the protein, as typically observed for substrate-binding proteins. Notably, the two terminal helices are joined by a single nonhelical residue (Gly235). Collectively, the present findings indicate that extending the hinge region and conferring it with more conformational freedom makes the formation of a closed TmArgBP monomer possible. On the other hand, the short connection between the helices may explain the tendency of the protein to also adopt alternative oligomeric states (dimers, trimers and larger aggregates). The data reported here highlight the importance of evolutionary control to avoid the uncontrolled formation of heterogeneous and potentially harmful oligomeric species through domain swapping.

The characterization of Thermotoga maritima Arginine Binding Protein variants demonstrates that minimal local strains have an important impact on protein stability

The Ramachandran plot is a versatile and valuable tool that provides fundamental information for protein structure determination, prediction, and validation. The structural/thermodynamic effects produced by forcing a residue to adopt a conformation predicted to be forbidden were here explored using Thermotoga maritima Arginine Binding Protein (TmArgBP) as model. Specifically, we mutated TmArgBP Gly52 that assumes a conformation believed to be strictly disallowed for non-Gly residues. Surprisingly, the crystallographic characterization of Gly52Ala TmArgBP indicates that the structural context forces the residue to adopt a non-canonical conformation never observed in any of the high-medium resolution PDB structures. Interestingly, the inspection of this high resolution structure demonstrates that only minor alterations occur. Nevertheless, experiments indicate that Gly52 replacements in TmArgBP produce destabilizations comparable to those observed upon protein truncation or dissection in domains. Notably, we show that force-fields commonly used in computational biology do not reproduce this non-canonical state. Using TmArgBP as model system we here demonstrate that the structural context may force residues to adopt conformations believed to be strictly forbidden and that barely detectable alterations produce major destabilizations. Present findings highlight the role of subtle strains in governing protein stability. A full understanding of these phenomena is essential for an exhaustive comprehension of the factors regulating protein structures.

Structure, stability and aggregation propensity of a Ribonuclease A-Onconase chimera

Structural roles of loop regions are frequently overlooked in proteins. Nevertheless, they may be key players in the definition of protein topology and in the self-assembly processes occurring through domain swapping. We here investigate the effects on structure and stability of replacing the loop connecting the last two β-strands of RNase A with the corresponding region of the more thermostable Onconase. The crystal structure of this chimeric variant (RNaseA-ONC) shows that its terminal loop size better adheres to the topological rules for the design of stabilized proteins, proposed by Baker and coworkers [43]. Indeed, RNaseA-ONC displays a thermal stability close to that of RNase A, despite the lack of Pro at position 114, which, due to its propensity to favor a cis peptide bond, has been identified as an important stabilizing factor of the native protein. Accordingly, RNaseA-ONC is significantly more stable than RNase A variants lacking Pro114; RNaseA-ONC also displays a higher propensity to form oligomers in native conditions when compared to either RNase A or Onconase. This finding demonstrates that modifications of terminal loops should to be carefully controlled in terms of size and sequence to avoid unwanted and/or potentially harmful aggregation processes.

Structure determination of UL49.5 transmembrane protein from bovine herpesvirus 1 by NMR spectroscopy and molecular dynamics

The transporter associated with antigen processing (TAP) directly participates in the immune response as a key component of the cytosolic peptide to major histocompatibility complex (MHC) class I protein loading machinery. This makes TAP an important target for viruses avoiding recognition by CD8+ T lymphocytes. Its activity can be suppressed by the UL49.5 protein produced by bovine herpesvirus 1, although the mechanism of this inhibition has not been understood so far.

Therefore, the main goal of our study was to investigate the 3D structure of bovine herpesvirus 1 - encoded UL49.5 protein. The final structure of the inhibitor was established using circular dichroism (CD), 2D nuclear magnetic resonance (NMR), and molecular dynamics (MD) in membrane mimetic environments. In NMR studies, UL49.5 was represented by two fragments: the extracellular region (residues 1–35) and the transmembrane-intracellular fragment (residues 36–75), displaying various functions during viral invasion. After the empirical structure determination, a molecular docking procedure was used to predict the complex of UL49.5 with the TAP heterodimer.

Our results revealed that UL49.5 adopted a highly flexible membrane-proximal helical structure in the extracellular part. In the transmembrane region, we observed two short α-helices. Furthermore, the cytoplasmic part had an unordered structure. Finally, we propose three different orientations of UL49.5 in the complex with TAP. Our studies provide, for the first time, the experimental structural information on UL49.5 and structure-based insight in its mechanism of action which might be helpful in designing new drugs against viral infections.

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The UL49.5 viral protein forms a helical structure in the biological membrane

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Our NMR-based 3D structure of UL49.5 differs from the theoretical predictions

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Apart from the protruding N-terminal helix the structure is buried in the membrane

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Attention should be paid to the turns in the external and transmembrane domains

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Molecular docking proposes three possible structures of the UL49.5/TAP complexes

Exploring the Roles of Proline in Three-Dimensional Domain Swapping from Structure Analysis and Molecular Dynamics Simulations

Three-dimensional (3D) domain swapping is a mechanism to form protein oligomers. It has been proposed that several factors, including proline residues in the hinge region, may affect the occurrence of 3D domain swapping. Although introducing prolines into the hinge region has been found to promote domain swapping for some proteins, the opposite effect has also been observed in several studies. So far, how proline affects 3D domain swapping remains elusive. In this work, based on a large set of 3D domain-swapped structures, we performed a systematic analysis to explore the correlation between the presence of proline in the hinge region and the occurrence of 3D domain swapping. We further analyzed the conformations of proline and pre-proline residues to investigate the roles of proline in 3D domain swapping. We found that more than 40% of the domain-swapped structures contained proline residues in the hinge region. Unexpectedly, conformational transitions of proline residues were rarely observed upon domain swapping. Our analyses showed that hinge regions containing proline residues preferred more extended conformations, which may be beneficial for the occurrence of domain swapping by facilitating opening of the exchanged segments.

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.

Can Mutational Analysis Be Used To Assist Structure Determination of Peptides?

Mutational analysis is widely used to study the relationship between sequence and structure of proteins and peptides. It is often assumed that substituting a proline with another amino acid "locks" the peptide bond in the trans conformation, allowing only a subset of the initial molecular geometries to be observed. To test this assumption, we assess the result of substituting two prolines in the bradykinin sequence with alanine using field-asymmetric ion mobility spectrometry combined with cryogenic ion spectroscopy in the gas phase. While the structure of the mutant coincides with a part of the conformational space of the original peptide, the higher flexibility of the alanine backbone compared to proline allows it to access additional structures. We conclude that proline-to-nonproline substitutions are helpful to assign structures, but they should be used in conjunction with spectroscopic techniques that allow detailed comparison of the structures of the mutant and the native peptide.