Crystal structure of the Yersinia type III secretion protein YscE

The plague-causing bacterium Yersinia pestis utilizes a contact-dependent (type III) secretion system (T3SS) to transport virulence factors from the bacterial cytosol directly into the interior of mammalian cells where they interfere with signal transduction pathways that mediate phagocytosis and the inflammatory response. The type III secretion apparatus is composed of 20-25 different Yersinia secretion (Ysc) proteins. We report here the structure of YscE, the smallest Ysc protein, which is a dimer in solution. The probable mode of oligomerization is discussed.

Selection and characterization of Yersinia pestis YopN mutants that constitutively block Yop secretion

Secretion of Yop effector proteins by the Yersinia pestis plasmid pCD1-encoded type III secretion system (T3SS) is regulated in response to specific environmental signals. Yop secretion is activated by contact with a eukaryotic cell or by growth at 37 degrees C in the absence of calcium. The secreted YopN protein, the SycN/YscB chaperone and TyeA form a cytosolic YopN/SycN/YscB/TyeA complex that is required to prevent Yop secretion in the presence of calcium and prior to contact with a eukaryotic cell. The mechanism by which these proteins prevent secretion and the subcellular location where the block in secretion occurs are not known. To further investigate both the mechanism and location of the YopN-dependent block, we isolated and characterized several YopN mutants that constitutively block Yop secretion. All the identified amino-acid substitutions that resulted in a constitutive block in Yop secretion mapped to a central domain of YopN that is not directly involved in the interaction with the SycN/YscB chaperone or TyeA. The YopN mutants required an intact TyeA-binding domain and TyeA to block secretion, but did not require an N-terminal secretion signal, an intact chaperone-binding domain or the SycN/YscB chaperone. These results suggest that a C-terminal domain of YopN complexed with TyeA blocks Yop secretion from a cytosolic, not an extracellular, location. A hypothetical model for how the YopN/SycN/YscB/TyeA complex regulates Yop secretion is presented.

Synthesis of tripeptides as potent Yersinia protein tyrosine phosphatase inhibitors

We report the synthesis of a series of monoanionic phosphotyrosyl (pTyr) mimetic-containing tripeptides based on 'Fmoc-Glu(OBn)-Xxx-Leu-amide' (where Xxx = pTyr mimetic) and their N-terminally modified derivatives. The inhibitory potencies of compounds were tested against YopH and human PTP1B enzymes. Several compounds exhibited noteworthy activity against both YopH and PTP1B. Among the N-terminally modified analogues, 5-methylindole derivative 30 was found to be the best moiety to replace base-labile Fmoc group. A mode of binding with YopH is proposed for tripeptides 21, 30, and 31.

Crystal Structures of a High-affinity Macrocyclic Peptide Mimetic in Complex with the Grb2 SH2 Domain

The high-affinity binding of the growth factor receptor-bound protein 2 (Grb2) SH2 domain to tyrosine-phosphorylated cytosolic domains of receptor tyrosine kinases (RTKs) is an attractive target for therapeutic intervention in many types of cancer. We report here two crystal forms of a complex between the Grb2 SH2 domain and a potent non-phosphorus-containing macrocyclic peptide mimetic that exhibits significant anti-proliferative effects against erbB-2-dependent breast cancers. This agent represents a "second generation" inhibitor with greatly improved binding affinity and bio-availability compared to its open-chain counterpart. The structures were determined at 2.0A and 1.8A with one and two domain-swapped dimers per asymmetric unit, respectively. The mode of binding and specific interactions between the protein and the inhibitor provide insight into the high potency of this class of macrocylic compounds and may aid in further optimization as part of the iterative rational drug design process.

Crystal structure of the protease-resistant core domain of Yersinia pestis virulence factor YopR

Yersinia pestis, the causative agent of the plague, employs a type III secretion system (T3SS) to secrete and translocate virulence factors into to the cytoplasm of mammalian host cells. One of the secreted virulence factors is YopR. Little is known about the function of YopR other than that it is secreted into the extracellular milieu during the early stages of infection and that it contributes to virulence. Hoping to gain some insight into the function of YopR, we determined the crystal structure of its protease-resistant core domain, which consists of residues 38-149 out of 165 amino acids. The core domain is composed of five alpha-helices that display unexpected structural similarity with one domain of YopN, a central regulator of type III secretion in Y. pestis. This finding raises the possibility that YopR may play a role in the regulation of type III secretion.

Making the most of affinity tags

Proteins do not naturally lend themselves to high-throughput analysis because of their diverse physiochemical properties. Consequently, affinity tags have become indispensable tools for structural and functional proteomics initiatives. Although originally developed to facilitate the detection and purification of recombinant proteins, in recent years it has become clear that affinity tags can have a positive impact on the yield, solubility and even the folding of their fusion partners. However, no single affinity tag is optimal with respect to all of these parameters; each has its strengths and weaknesses. Therefore, combinatorial tagging might be the only way to harness the full potential of affinity tags in a high-throughput setting.

Efficient site-specific processing of fusion proteins by tobacco vein mottling virus protease in vivo and in vitro

Affinity tags are widely used as vehicles for the production of recombinant proteins. Yet, because of concerns about their potential to interfere with the activity or structure of proteins, it is almost always desirable to remove them from the target protein. The proteases that are most often used to cleave fusion proteins are factor Xa, enterokinase, and thrombin, yet the literature is replete with reports of fusion proteins that were cleaved by these proteases at locations other than the designed site. It is becoming increasingly evident that certain viral proteases have more stringent sequence specificity. These proteases adopt a trypsin-like fold but possess an unconventional catalytic triad in which Cys replaces Ser. The tobacco etch virus (TEV) protease is the best-characterized enzyme of this type. TEV protease cleaves the sequence ENLYFQG/S between QG or QS with high specificity. The tobacco vein mottling virus (TVMV) protease is a close relative of TEV protease with a distinct sequence specificity (ETVRFQG/S). We show that, like TEV protease, TVMV protease can be used to cleave fusion proteins with high specificity in vitro and in vivo. We compared the catalytic activity of the two enzymes as a function of temperature and ionic strength, using an MBP-NusG fusion protein as a model substrate. The behavior of TVMV protease was very similar to that of TEV protease. Its catalytic activity was greatest in the absence of NaCl, but diminished only threefold with increasing salt up to 200 mM. We found that the optimum temperatures of the two enzymes are nearly the same and that they differ only two-fold in catalytic efficiency, both at room temperature and 4 degrees C. Hence, TVMV protease may be a useful alternative to TEV protease when a recombinant protein happens to contain a sequence that is similar to a TEV protease recognition site or for protein expression strategies that involve the use of more than one protease.

Comparison of the substrate specificity of two potyvirus proteases

The substrate specificity of the nuclear inclusion protein a (NIa) proteolytic enzymes from two potyviruses, the tobacco etch virus (TEV) and tobacco vein mottling virus (TVMV), was compared using oligopeptide substrates. Mutations were introduced into TEV protease in an effort to identify key determinants of substrate specificity. The specificity of the mutant enzymes was assessed by using peptides with complementary substitutions. The crystal structure of TEV protease and a homology model of TVMV protease were used to interpret the kinetic data. A comparison of the two structures and the experimental data suggested that the differences in the specificity of the two enzymes may be mainly due to the variation in their S4 and S3 binding subsites. Two key residues predicted to be important for these differences were replaced in TEV protease with the corresponding residues of TVMV protease. Kinetic analyses of the mutants confirmed that these residues play a role in the specificity of the two enzymes. Additional residues in the substrate-binding subsites of TEV protease were also mutated in an effort to alter the specificity of the enzyme.

Structural basis for the function of stringent starvation protein a as a transcription factor

Stringent starvation protein A (SspA) of Escherichia coli is an RNA polymerase-associated transcriptional activator for the lytic development of phage P1 and is essential for stationary phase-induced acid tolerance of E. coli. We report the crystal structure of Yersinia pestis SspA, which is 83% identical to E. coli SspA in amino acid sequence and is functionally complementary in supporting the lytic growth of phage P1 and acid resistance of an E. coli sspA mutant. The structure reveals that SspA assumes the characteristic fold of glutathione S-transferase (GST). However, SspA lacks GST activity and does not bind glutathione. Three regions of SspA are flexible, the N and C termini and the alpha2-helix. The structure also reveals a conserved surface-exposed pocket composed of residues from a loop between helices alpha3 and alpha4. The functional roles of these structural features were investigated by assessing the ability of deletion and site-directed mutants to confer acid resistance of E. coli and to activate transcription from a phage P1 late promoter, thereby supporting the lytic growth of phage P1. The results indicate that the flexible regions are not critical for SspA function, whereas the surface pocket is important for both transcriptional activation of the phage P1 late promoter and acid resistance of E. coli. The size, shape, and property of the pocket suggest that it mediates protein-protein interactions. SspA orthologs from Y. pestis, Vibrio cholerae, and Pseudomonas aeruginosa are all functional in acid resistance of E. coli, whereas only Y. pestis SspA supports phage P1 growth.

Three-dimensional structure of a macromolecular assembly that regulates type III secretion in Yersinia pestis

Yersinia pestis, the causative agent of plague, utilizes a type III secretion system (T3SS) to inject effector proteins directly into the cytosol of mammalian cells where they interfere with signal transduction pathways that regulate actin cytoskeleton dynamics and inflammation, thereby enabling the bacterium to avoid engulfment and destruction by macrophages. Type III secretion normally does not occur in the absence of close contact with eukaryotic cells. Negative regulation is mediated in part by a multiprotein complex that has been proposed to act as a physical impediment to type III secretion by blocking the entrance to the secretion apparatus prior to contact with mammalian cells. This complex is composed of YopN, its heterodimeric secretion chaperone SycN-YscB, and TyeA. Here, we report two crystal structures of YopN in complex with its heterodimeric secretion chaperone SycN-YscB and the co-regulatory protein TyeA, respectively. By merging these two overlapping structures, it was possible to construct a credible theoretical model of the YopN-SycN-YscB-TyeA complex. The modeled assembly features the secretion signaling elements of YopN at one end of an elongated structure and the secretion regulating TyeA binding site at the other. A patch of highly conserved residues on the surface of the C-terminal alpha-helix of TyeA may mediate its interaction with structural components of the secretion apparatus. Conserved arginine residues that reside inside a prominent cavity at the dimer interface of SycN-YscB were mutated in order to investigate whether they play a role in targeting the YopN-chaperone complex to the type III secretion apparatus. One of the mutants exhibited a phenotype that is consistent with this hypothesis.

Involvement of Domain II in Toxicity of Anthrax Lethal Factor

Anthrax lethal factor (LF) is a Zn2+ -metalloprotease that cleaves and inactivates mitogen-activated protein kinase kinases (MEKs). We have used site-directed mutagenesis to identify a cluster of residues in domain II of LF that lie outside the active site and are required for cellular proteolytic activity toward MEKs. Alanine substituted for Leu293, Lys294, Leu514, Asn516, or Arg491 caused a 10-50-fold reduction in LF toxicity. Further, whereas pairwise substitution of alanine for Leu514 and either Leu293, Lys294, or Arg491 completely abrogated LF toxicity, pairwise mutation of Leu514 and Asn516 resulted in toxicity comparable with N516A alone. The introduction of these mutations reduced LF-mediated cleavage of MEK2 in cell-based assays but altered neither the ability of LF to bind protective antigen nor its ability to translocate across a membrane. Interestingly, direct in vitro measurement of LF activity indicated that decreased toxicity was not always accompanied by reduced proteolytic activity. However, mutations in this region significantly reduced the ability of LF to competitively inhibit B-Raf phosphorylation of MEK. These results provide evidence that elements of domain II are involved in the association of LF into productive complex with MEKs.

A pivotal role for reductive methylation in the de novo crystallization of a ternary complex composed of Yersinia pestis virulence factors YopN, SycN and YscB

Structural studies of a ternary complex composed of the Yersina pestis virulence factors YopN, SycN and YscB were initially hampered by poor solubility of the individual proteins. Co-expression of all three proteins in Escherichia coli yielded a well behaved complex, but this sample proved to be recalcitrant to crystallization. As crystallization efforts remained fruitless, even after the proteolysis-guided engineering of a truncated YopN polypeptide, reductive methylation of lysine residues was employed to alter the surface properties of the complex. The methylated complex yielded crystals that diffracted X-rays to a maximal resolution of 1.8 A. The potential utility of reductive methylation as a remedial strategy for high-throughput structural biology was further underscored by the successful modification of a selenomethionine-substituted sample.

The structure of Yersinia pestis V-antigen, an essential virulence factor and mediator of immunity against plague

The LcrV protein (V-antigen) is a multifunctional virulence factor in Yersinia pestis, the causative agent of plague. LcrV regulates the translocation of cytotoxic effector proteins from the bacterium into the cytosol of mammalian cells via a type III secretion system, possesses antihost activities of its own, and is also an active and passive mediator of resistance to disease. Although a crystal structure of this protein has been actively sought for better understanding of its role in pathogenesis, the wild-type LcrV was found to be recalcitrant to crystallization. We employed a surface entropy reduction mutagenesis strategy to obtain crystals of LcrV that diffract to 2.2 A and determined its structure. The refined model reveals a dumbbell-like molecule with a novel fold that includes an unexpected coiled-coil motif, and provides a detailed three-dimensional roadmap for exploring structure-function relationships in this essential virulence determinant.

Structure of the Yersinia pestis type III secretion chaperone SycH in complex with a stable fragment of YscM2

Pathogenic Yersinia species use a type III secretion system to inject cytotoxic effector proteins directly into the cytosol of mammalian cells, where they neutralize the innate immune response by interfering with the signal-transduction pathways that control phagocytosis and inflammation. To be exported efficiently, some effectors must transiently associate with cognate cytoplasmic secretion chaperones. SycH is the chaperone for YopH, a potent eukaryotic-like protein tyrosine phosphatase that is essential for virulence. SycH also binds two negative regulators of type III secretion, YscM1 and YscM2, both of which share significant sequence homology with the chaperone-binding domain of YopH. Here, the structure of a complex between SycH and a stable fragment of YscM2 that was designed on the basis of limited proteolysis experiments is presented. The overall fold of SycH is very similar to the structures of other homodimeric secretion chaperones that have been determined to date. YscM2 wraps around SycH in an extended fashion, with some secondary but no tertiary structure, assuming a conformation distinct from the globular fold that it is predicted to adopt in the absence of SycH.

Novel protein-protein interactions of the Yersinia pestis type III secretion system elucidated with a matrix analysis by surface plasmon resonance and mass spectrometry

Binary complexes formed by components of the Yersinia pestis type III secretion system were investigated by surface plasmon resonance (SPR) and matrix-assisted laser desorption time-of-flight mass spectrometry. Pairwise interactions between 15 recombinant Yersinia outer proteins (Yops), regulators, and chaperones were first identified by SPR. Mass spectrometry confirmed over 80% of the protein-protein interactions suggested by SPR, and new binding partners were further characterized. The Yop secretion protein (Ysc) M2 of Yersinia enterocolitica and LcrQ of Y. pestis, formerly described as ligands only for the specific Yop chaperone (Syc) H, formed stable complexes with SycE. Additional previously unreported complexes of YscE with the translocation regulator protein TyeA and the thermal regulator protein YmoA and multiple potential protein contacts by YscE, YopK, YopH, and LcrH were also identified. Because only stably folded proteins were examined, the interactions we identified are likely to occur either before or after transfer through the injectosome to mammalian host cells and may have relevance to understanding disease processes initiated by the plague bacterium.

High-resolution structure of the Yersinia pestis protein tyrosine phosphatase YopH in complex with a phosphotyrosyl mimetic-containing hexapeptide

Yersinia pestis, the causative agent of bubonic plague, secretes a eukaryotic-like protein tyrosine phosphatase (PTPase) termed Yersinia outer protein H (YopH) that is essential for virulence. We have determined, for the first time, the crystal structure of the YopH PTPase domain in complex with a nonhydrolyzable substrate analogue, the hexapeptide mimetic Ac-DADE-F(2)Pmp-L-NH(2). As anticipated, the mode of ligand binding in the active site is similar to the way in which the corresponding phosphohexapeptide binds to the structurally homologous human PTP1B. Unexpectedly, however, the crystal structure also revealed a second substrate-binding site in YopH that is not present in PTP1B. The mode of binding and structural conformation of the hexapeptide analogue is quite different in the two sites. Although the biological function of the second substrate-binding site remains to be investigated, the structure of a substrate analogue in the active site of Y. pestis YopH opens the door for the structure-based design and optimization of therapeutic countermeasures to combat this potential agent of bioterrorism.

Noncatalytic Assembly of Ribonuclease III with Double-Stranded RNA

Ribonuclease III (RNase III) represents a family of double-stranded RNA (dsRNA) endonucleases. The simplest bacterial enzyme contains an endonuclease domain (endoND) and a dsRNA binding domain (dsRBD). RNase III can affect RNA structure and gene expression in either of two ways: as a dsRNA-processing enzyme that cleaves dsRNA, or as a dsRNA binding protein that binds but does not cleave dsRNA. We previously determined the endoND structure of Aquifex aeolicus RNase III (Aa-RNase III) and modeled a catalytic complex of full-length Aa-RNase III with dsRNA. Here, we present the crystal structure of Aa-RNase III in complex with dsRNA, revealing a noncatalytic assembly. The major differences between the two functional forms of RNase III.dsRNA are the conformation of the protein and the orientation and location of dsRNA. The flexibility of a 7 residue linker between the endoND and dsRBD enables the transition between these two forms.

Similar modes of polypeptide recognition by export chaperones in flagellar biosynthesis and type III secretion

Assembly of the bacterial flagellum and type III secretion in pathogenic bacteria require cytosolic export chaperones that interact with mobile components to facilitate their secretion. Although their amino acid sequences are not conserved, the structures of several type III secretion chaperones revealed striking similarities between their folds and modes of substrate recognition. Here, we report the first crystallographic structure of a flagellar export chaperone, Aquifex aeolicus FliS. FliS adopts a novel fold that is clearly distinct from those of the type III secretion chaperones, indicating that they do not share a common evolutionary origin. However, the structure of FliS in complex with a fragment of FliC (flagellin) reveals that, like the type III secretion chaperones, flagellar export chaperones bind their target proteins in extended conformation and suggests that this mode of recognition may be widely used in bacteria.

[Tobacco etch virus proteinase: crystal structure of the active enzyme and its inactive mutant]

Tobacco Etch Virus Protease (TEV protease) is widely used as a tool for separation of recombinant target proteins from their fusion partners. The crystal structures of two mutants of TEV protease, active autolysis-resistant mutant TEV-S219D in complex with the proteolysis product, and inactive mutant TEV-C151A in complex with a substrate, have been determined at 1.8 and 2.2 A resolution, respectively. The active sites of both mutants, including their oxyanion holes, have identical structures. The C-terminal residues 217-221 of the enzyme are involved in formation of the binding pockets S3-S6. This indicates that the autolysis of the peptide bond Met218-Ser219 exerts a strong effect on the fine-tuning of the substrate in the enzyme active site, which results in considerable decrease in the enzymatic activity.

Differential effects of supplementary affinity tags on the solubility of MBP fusion proteins

It is difficult to imagine any strategy for high-throughput protein expression and purification that does not involve genetically engineered affinity tags. Because of its ability to enhance the solubility and promote the proper folding of its fusion partners, Escherichia coli maltose-binding protein (MBP) is a particularly useful affinity tag. However, not all MBP fusion proteins bind efficiently to amylose resin, and even when they do it is usually not possible to obtain a sample of adequate purity after a single affinity step. To address this problem, we endeavored to incorporate supplemental affinity tags within the framework of an MBP fusion protein. We show that both the nature of the supplemental tags and their location can influence the ability of MBP to promote the solubility of its fusion partners. The most promising configurations for high-throughput protein expression and purification appear to be a fusion protein with a biotin acceptor peptide (BAP) on the N-terminus of MBP and/or a hexahistidine tag (His-tag) on the C-terminus of the passenger protein.

Tripeptide inhibitors of Yersinia protein-tyrosine phosphatase

The protein-tyrosine phosphatase (PTP) 'YopH' is a virulence factor of Yersinia pestis, the causative agent of plague. Potential use of Yersinia as a bioterrorism agent renders YopH inhibitors of therapeutic importance. Previously, we had examined the inhibitory potencies of a variety of phosphotyrosyl (pTyr) mimetics against the human PTP1B enzyme by displaying them in the EGFR-derived hexapeptide sequence, 'Ac-Asp-Ala-Asp-Glu-Xxx-Leu-amide', where Xxx=pTyr mimetic. The poor inhibitory potencies of certain of these pTyr mimetics were attributed to restricted orientation within the PTP1B catalytic pocket incurred by extensive peripheral interaction of the hexapeptide platform. Utilizing the smaller tripeptide platform, 'Fmoc-Glu-Xxx-Leu-amide' we demonstrate herein that several of the low affinity hexapeptide-expressed pTyr mimetics exhibit high PTP1B affinity within the context of the tripeptide platform. Of particular note, the mono-anionic 4-(carboxydifluoromethyl)Phe residue exhibits affinity equivalent to the di-anionic F(2)Pmp residue, which had previously been among the most potent PTP-binding motifs. Against YopH, it was found that all tripeptides having Glu residues with an unprotected side chain carboxyl were inactive. Alternatively, in their Glu-OBn ester forms, several of the tripeptides exhibited good YopH affinity with the mono-anionic peptide, Fmoc-Glu(OBn)-Xxx-Leu-amide, where Xxx=4-(carboxymethyloxy)Phe providing an IC(50) value of 2.8 microM. One concern with such inhibitors is that they may potentially function by non-specific mechanisms. Studies with representative inhibitors, while failing to provide evidence of a non-specific promiscuous mode of inhibition, did indicate that non-classical inhibition may be involved.

Characterization of four orthologs of stringent starvation protein A

Orthologous proteins can be beneficial for X-ray crystallographic studies when a protein from an organism of choice fails to crystallize or the crystals are not suitable for structure determination. Their amino-acid sequences should be similar enough that they will share the same fold, but different enough so that they may crystallize under alternative conditions and diffract to higher resolution. This multi-species approach was employed to obtain diffraction-quality crystals of the RNA polymerase (RNAP) associated stringent starvation protein A (SspA). Although Escherichia coli SspA could be crystallized, the crystals failed to diffract well enough for structure determination. Therefore, SspA proteins from Yersinia pestis, Vibrio cholerae and Pseudomonas aeruginosa were cloned, expressed, purified and subjected to crystallization trials. The V. cholerae SspA protein failed to crystallize under any conditions tested and the P. aeruginosa SspA protein did not form crystals suitable for data collection. On the other hand, Y. pestis SspA crystallized readily and the crystals diffracted to 2.0 A.

A spring-loaded state of NusG in its functional cycle is suggested by X-ray crystallography and supported by site-directed mutants

Transcription factor NusG is present in all prokaryotes, and orthologous proteins have also been identified in yeast and humans. NusG contains a 27-residue KOW motif, found in ribosomal protein L24 where it interacts with rRNA. NusG in Escherichia coli (EcNusG) is an essential protein and functions as a regulator of Rho-dependent transcription termination, phage lambda N and rRNA transcription antitermination, and phage HK022 Nun termination. Relative to EcNusG, Aquifex aeolicus NusG (AaNusG) and several other bacterial NusG proteins contain a variable insertion sequence of approximately 70 residues in the central region of the molecule. Recently, crystal structures of AaNusG in space groups P2(1) and I222 have been reported; the authors conclude that there are no conserved dimers among the contacting molecules in the crystals [Steiner, T., Kaiser, J. T., Marinkovic, S., Huber, R., and Wahl, M. C. (2002) EMBO J. 21, 4641-4653]. We have independently determined the structures of AaNusG also in two crystal forms, P2(1) and C222(1), and surprisingly found that AaNusG molecules form domain-swapped dimers in both crystals. Additionally, polymerization is also observed in the P2(1) crystal. A unique "ball-and-socket" junction dominates the intermolecular interactions within both oligomers. We believe that this interaction is a clue to the function of the molecule and propose a spring-loaded state in the functional cycle of NusG. The importance of the ball-and-socket junction for the function of NusG is supported by the functional analysis of site-directed mutants.

Maltodextrin-binding proteins from diverse bacteria and archaea are potent solubility enhancers

Escherichia coli maltose-binding protein (MBP) is frequently used as an affinity tag to facilitate the purification of recombinant proteins. An important additional attribute of MBP is its remarkable ability to enhance the solubility of its fusion partners. MBPs are present in a wide variety of microorganisms including both mesophilic and thermophilic bacteria and archaea. In the present study, we compared the ability of MBPs from six diverse microorganisms (E. coli, Pyrococcus furiosus, Thermococcus litoralis, Vibrio cholerae, Thermotoga maritima, and Yersinia pestis) to promote the solubility of eight different aggregation-prone proteins in E. coli. In contrast to glutathione S-transferase (GST), all of these MBPs proved to be effective solubility enhancers and some of them were even more potent solubilizing agents than E. coli MBP.