Structural analysis of the BisI family of modification dependent restriction endonucleases.

The BisI family of restriction endonucleases is unique in requiring multiple methylated or hydroxymethylated cytosine residues within a short recognition sequence (GCNGC), and in cleaving directly within this sequence, rather than at a distance. Here, we report that the number of modified cytosines that are required for cleavage can be tuned by the salt concentration. We present crystal structures of two members of the BisI family, NhoI and Eco15I_Ntd (N-terminal domain of Eco15I), in the absence of DNA and in specific complexes with tetra-methylated GCNGC target DNA. The structures show that NhoI and Eco15I_Ntd sense modified cytosine bases in the context of double-stranded DNA (dsDNA) without base flipping. In the co-crystal structures of NhoI and Eco15I_Ntd with DNA, the internal methyl groups (G5mCNGC) interact with the side chains of an (H/R)(V/I/T/M) di-amino acid motif near the C-terminus of the distal enzyme subunit and arginine residue from the proximal subunit. The external methyl groups (GCNG5mC) interact with the proximal enzyme subunit, mostly through main chain contacts. Surface plasmon resonance analysis for Eco15I_Ntd shows that the internal and external methyl binding pockets contribute about equally to sensing of cytosine methyl groups.

Conformation-dependent restraints for polynucleotides: the sugar moiety.

Stereochemical restraints are commonly used to aid the refinement of macromolecular structures obtained by experimental methods at lower resolution. The standard restraint library for nucleic acids has not been updated for over two decades and needs revision. In this paper, geometrical restraints for nucleic acids sugars are derived using information from high-resolution crystal structures in the Cambridge Structural Database. In contrast to the existing restraints, this work shows that different parts of the sugar moiety form groups of covalent geometry dependent on various chemical and conformational factors, such as the type of ribose or the attached nucleobase, and ring puckering or rotamers of the glycosidic (χ) or side-chain (γ) torsion angles. Moreover, the geometry of the glycosidic link and the endocyclic ribose bond angles are functionally dependent on χ and sugar pucker amplitude (τm), respectively. The proposed restraints have been positively validated against data from the Nucleic Acid Database, compared with an ultrahigh-resolution Z-DNA structure in the Protein Data Bank, and tested by re-refining hundreds of crystal structures in the Protein Data Bank. The conformation-dependent sugar restraints presented in this work are publicly available in REFMAC, PHENIX and SHELXL format through a dedicated RestraintLib web server with an API function.

Dynamer and Metallodynamer Interconversion: An Alternative View to Metal Ion Complexation.

A bifunctional molecule containing both a bidentate binding site for metal ions and an aminopyrimidine H-bond donor-acceptor site has been synthesized, and its properties, in its free and coordinated forms, have been established in solution and in the solid state by analytical and spectroscopic methods as well as by X-ray structure determinations. Structural characterization has shown that it forms a one-dimensional H-bonded polymeric assembly in the solid state, while spectroscopic measurements indicate that it also aggregates in solution. The reaction of a simple Fe(II) salt with this assembly results in the emergence of two geometrical isomers of the complex: [FeL3](BF4)2·9H2O-C1 (meridional, mer) and [FeL3]2(SiF6)(BF4)2·12H2O-C2 (facial, fac). While, complex C1 in the solid state generates a one-dimensional H-bonded polymer involving just two ligands on each Fe center, with the chirality of the complex units alternating along the polymer chain, the structure of complex C2 shows NH···N interactions seen in both the ligand and mer complex (C1) structures to be completely absent. Physicochemical properties of the free and complexed ligand differ substantially.

Self-Assembly and Ordering of Peptide-Based Cavitands in Water and DMSO: The Power of Hydrophobic Effects Combined with Neutral Hydrogen Bonds.

Directional self-assembly of uncharged molecules in water is a major challenge in supramolecular chemistry. Herein, it is demonstrated that peptide-based cavitands wrap around a hydrophobic core (fullerene C60 ) by a combination of the hydrophobic effect and hydrogen-bonding interactions to form highly ordered three-component complexes in water that resemble the molten-globule stage of protein folding. The complexes were characterized by DOSY NMR spectroscopy, small-angle X-ray scattering, and circular dichroism, and their structures were confirmed by X-ray crystallography. Enhancement of the CD signals by nearly one order of magnitude and increased hydrolytic stability of hydrazone bonds of the complexes relative to the nonassembled species were observed. In contrast, DMSO and DMSO/water mixtures were found to be highly disintegrative for these complexes. Interestingly, some cavitands can only be synthesized in the presence of the hydrophobic template followed by disassembly of the complexes.

Health hazards related to conidia of Cladosporium-biological air pollutants in Poland, central Europe.

The spores of Cladosporium Link. are often present in the air in high quantities and produce many allergenic proteins, which may lead to asthma. An aerobiological spore monitoring program can inform patients about the current spore concentration in air and help their physicians determine the spore dose that is harmful for a given individual. This makes it possible to develop optimized responses and propose personalized therapy for a particular sensitive patient. The aim of this study was to assess the extent of the human health hazard posed by the fungal genus Cladosporium. For the first time, we have determined the number of days on which air samples in Poland exceeded the concentrations linked to allergic responses of sensitive patients, according to thresholds established by three different groups (2800/3000/4000 spores per 1m3 of the air). The survey was conducted over three consecutive growing seasons (April-September, 2010-2012) in three cities located in different climate zones of Poland (Poznan, Lublin and Rzeszow). The average number of days exceeding 2800 spores per cubic meter (the lowest threshold) ranged from 61 (2010) through 76 (2011) to 93 (2012), though there was significant variation between cities. In each year the highest concentration of spores in the air was detected in either Poznan or Lublin, both located on large plains with intensive agriculture. We have proposed that an effective, science-based software platform to support policy-making on air quality should incorporate biological air pollutant data, such as allergenic fungal spores and pollen grains.

Mechanochemical Encapsulation of Fullerenes in Peptidic Containers Prepared by Dynamic Chiral Self-Sorting and Self-Assembly.

Molecular capsules composed of amino acid or peptide derivatives connected to resorcin[4]arene scaffolds through acylhydrazone linkers have been synthesized using dynamic covalent chemistry (DCC) and hydrogen-bond-based self-assembly. The dynamic character of the linkers and the preference of the peptides towards self-assembly into ß-barrel-type motifs lead to the spontaneous amplification of formation of homochiral capsules from mixtures of different substrates. The capsules have cavities of around 800 Å(3) and exhibit good kinetic stability. Although they retain their dynamic character, which allows processes such as chiral self-sorting and chiral self-assembly to operate with high fidelity, guest complexation is hindered in solution. However, the quantitative complexation of even very large guests, such as fullerene C60 or C70 , is possible through the utilization of reversible covalent bonds or the application of mechanochemical methods. The NMR spectra show the influence of the chiral environment on the symmetry of the fullerene molecules, which results in the differentiation of diastereotopic carbon atoms for C70 , and the X-ray structures provide unique information on the modes of peptide-fullerene interactions.

Contribution of Leptosphaeria species ascospores to autumn asthma in areas of oilseed rape production.

BACKGROUND: An increase in the number of hospital admissions from September to November in the northern hemisphere has been frequently reported. At this time, some species of fungal genus Leptosphaeria produce numerous ascospores, which are easily airborne. However, we lack knowledge about whether Leptosphaeria produces allergenic proteins. OBJECTIVE: To evaluate the potential of Leptosphaeria ascospores to contribute to autumn asthma. METHODS: Detailed bioinformatic analysis of proteins produced by Leptosphaeria maculans available in databases was performed and the data compared with allergens found in other airborne fungi. The concentrations of Leptosphaeria ascospores detected at 2 sites were compared to these obtained in other environments worldwide. RESULTS: We found that Leptosphaeria species produce proteins with a high identity to commonly known aeroallergens of several well-characterized molds. The level of amino acid identity significantly exceeded the allergen identity thresholds recommended by the Food and Agricultural Organization/World Health Organization (35%), which indicates allergenic properties of L maculans and ensures the same properties in the other Leptosphaeria species. CONCLUSION: High concentrations of Leptosphaeria species ascospores in the autumn and postulated allergenicity of their proteins strongly suggest that this genus contributes to worldwide reported autumn asthma. The finding opens the question of allergenicity of the other never studied fungal species present in aeroplankton.

Structure of γ-conglutin: insight into the quaternary structure of 7S basic globulins from legumes.

γ-Conglutin from lupin seeds is an unusual 7S basic globulin protein. It is capable of reducing glycaemia in mammals, but the structural basis of this activity is not known. γ-Conglutin shares a high level of structural homology with glycoside hydrolase inhibitor proteins, although it lacks any kind of inhibitory activity against plant cell-wall degradation enzymes. In addition, γ-conglutin displays a less pronounced structural similarity to pepsin-like aspartic proteases, but it is proteolytically dysfunctional. Only one structural study of a legume 7S basic globulin, that isolated from soybean, has been reported to date. The quaternary assembly of soybean 7S basic globulin (Bg7S) is arranged as a cruciform-shaped tetramer comprised of two superposed dimers. Here, the crystal structure of γ-conglutin isolated from Lupinus angustifolius seeds (LangC) is presented. The polypeptide chain of LangC is post-translationally cleaved into α and ß subunits but retains its covalent integrity owing to a disulfide bridge. The protomers of LangC undergo an intricate quaternary assembly, resulting in a ring-like hexamer with noncrystallographic D3 symmetry. The twofold-related dimers are similar to those in Bg7S but their assembly is different as a consequence of mutations in a ß-strand that is involved in intermolecular ß-sheet formation in γ-conglutin. Structural elucidation of γ-conglutin will help to explain its physiological role, especially in the evolutionary context, and will guide further research into the hypoglycaemic activity of this protein in humans, with potential consequences for novel antidiabetic therapies.

High-resolution crystal structure of Z-DNA in complex with Cr(3+) cations.

This work is part of our project aimed at characterizing metal-binding properties of left-handed Z-DNA helices. The three Cr(3+) cations found in the asymmetric unit of the d(CGCGCG)2-Cr(3+) crystal structure do not form direct coordination bonds with atoms of the Z-DNA molecule. Instead, the hydrated Cr(3+) ions are engaged in outer-sphere interactions with phosphate groups and O6 and N7 guanine atoms of the DNA. The Cr(3+)(1) and Cr(3+)(2) ions have disordered coordination spheres occupied by six water molecules each. These partial-occupancy chromium cations are 2.354(15) Å apart and are bridged by three water molecules from their hydration spheres. The Cr(3+)(3) cation has distorted square pyramidal geometry. In addition to the high degree of disorder of the DNA backbone, alternate conformations are also observed for the deoxyribose and base moieties of the G2 nucleotide. Our work illuminates the question of conformational flexibility of Z-DNA and its interaction mode with transition-metal cations.

Na⁺/K⁺ exchange switches the catalytic apparatus of potassium-dependent plant L-asparaginase.

Plant-type L-asparaginases, which are a subclass of the Ntn-hydrolase family, are divided into potassium-dependent and potassium-independent enzymes with different substrate preferences. While the potassium-independent enzymes have already been well characterized, there are no structural data for any of the members of the potassium-dependent group to illuminate the intriguing dependence of their catalytic mechanism on alkali-metal cations. Here, three crystal structures of a potassium-dependent plant-type L-asparaginase from Phaseolus vulgaris (PvAspG1) differing in the type of associated alkali metal ions (K(+), Na(+) or both) are presented and the structural consequences of the different ions are correlated with the enzyme activity. As in all plant-type L-asparaginases, immature PvAspG1 is a homodimer of two protein chains, which both undergo autocatalytic cleavage to α and ß subunits, thus creating the mature heterotetramer or dimer of heterodimers (αß)2. The αß subunits of PvAspG1 are folded similarly to the potassium-independent enzymes, with a sandwich of two ß-sheets flanked on each side by a layer of helices. In addition to the `sodium loop' (here referred to as the `stabilization loop') known from potassium-independent plant-type asparaginases, the potassium-dependent PvAspG1 enzyme contains another alkali metal-binding loop (the `activation loop') in subunit α (residues Val111-Ser118). The active site of PvAspG1 is located between these two metal-binding loops and in the immediate neighbourhood of three residues, His117, Arg224 and Glu250, acting as a catalytic switch, which is a novel feature that is identified in plant-type L-asparaginases for the first time. A comparison of the three PvAspG1 structures demonstrates how the metal ion bound in the activation loop influences its conformation, setting the catalytic switch to ON (when K(+) is coordinated) or OFF (when Na(+) is coordinated) to respectively allow or prevent anchoring of the reaction substrate/product in the active site. Moreover, it is proposed that Ser118, the last residue of the activation loop, is involved in the potassium-dependence mechanism. The PvAspG1 structures are discussed in comparison with those of potassium-independent L-asparaginases (LlA, EcAIII and hASNase3) and those of other Ntn-hydrolases (AGA and Tas1), as well as in the light of noncrystallographic studies.

Probing the active site of Class 3 L-asparaginase by mutagenesis. I. Tinkering with the zinc coordination site of ReAV.

ReAV, the inducible Class-3 L-asparaginase from the nitrogen-fixing symbiotic bacterium Rhizobium etli, is an interesting candidate for optimizing its enzymatic potential for antileukemic applications. Since it has no structural similarity to known enzymes with this activity, it may offer completely new ways of approach. Also, as an unrelated protein, it would evade the immunological response elicited by other asparaginases. The crystal structure of ReAV revealed a uniquely assembled protein homodimer with a highly specific C135/K138/C189 zinc binding site in each subunit. It was also shown before that the Zn2+ cation at low and optimal concentration boosts the ReAV activity and improves substrate specificity, which indicates its role in substrate recognition. However, the detailed catalytic mechanism of ReAV is still unknown. In this work, we have applied site-directed mutagenesis coupled with enzymatic assays and X-ray structural analysis to elucidate the role of the residues in the zinc coordination sphere in catalysis. Almost all of the seven ReAV muteins created in this campaign lost the ability to hydrolyze L-asparagine, confirming our predictions about the significance of the selected residues in substrate hydrolysis. We were able to crystallize five of the ReAV mutants and solve their crystal structures, revealing some intriguing changes in the active site area as a result of the mutations. With alanine substitutions of Cys135 or Cys189, the zinc coordination site fell apart and the mutants were unable to bind the Zn2+ cation. Moreover, the absence of Lys138 induced atomic shifts and conformational changes of the neighboring residues from two active-site Ser-Lys tandems. Ser48 from one of the tandems, which is hypothesized to be the catalytic nucleophile, usually changes its hydration pattern in response to the mutations. Taken together, the results provide many useful clues about the catalytic mechanism of the enzyme, allowing one to cautiously postulate a possible enzymatic scenario.

Towards a dependable data set of structures for L-asparaginase research.

The Protein Data Bank (PDB) includes a carefully curated treasury of experimentally derived structural data on biological macromolecules and their various complexes. Such information is fundamental for a multitude of projects that involve large-scale data mining and/or detailed evaluation of individual structures of importance to chemistry, biology and, most of all, to medicine, where it provides the foundation for structure-based drug discovery. However, despite extensive validation mechanisms, it is almost inevitable that among the ∼215 000 entries there will occasionally be suboptimal or incorrect structure models. It is thus vital to apply careful verification procedures to those segments of the PDB that are of direct medicinal interest. Here, such an analysis was carried out for crystallographic models of L-asparaginases, enzymes that include approved drugs for the treatment of certain types of leukemia. The focus was on the adherence of the atomic coordinates to the rules of stereochemistry and their agreement with the experimental electron-density maps. Whereas the current clinical application of L-asparaginases is limited to two bacterial proteins and their chemical modifications, the field of investigations of such enzymes has expanded tremendously in recent years with the discovery of three entirely different structural classes and with numerous reports, not always quite reliable, of the anticancer properties of L-asparaginases of different origins.

Waterless structures in the Protein Data Bank.

The absence of solvent molecules in high-resolution protein crystal structure models deposited in the Protein Data Bank (PDB) contradicts the fact that, for proteins crystallized from aqueous media, water molecules are always expected to bind to the protein surface, as well as to some sites in the protein interior. An analysis of the contents of the PDB indicated that the expected ratio of the number of water molecules to the number of amino-acid residues exceeds 1.5 in atomic resolution structures, decreasing to 0.25 at around 2.5 Šresolution. Nevertheless, almost 800 protein crystal structures determined at a resolution of 2.5 Šor higher are found in the current release of the PDB without any water molecules, whereas some other depositions have unusually low or high occupancies of modeled solvent. Detailed analysis of these depositions revealed that the lack of solvent molecules might be an indication of problems with either the diffraction data, the refinement protocol, the deposition process or a combination of these factors. It is postulated that problems with solvent structure should be flagged by the PDB and addressed by the depositors.

Solvent organization in the ultrahigh-resolution crystal structure of crambin at room temperature.

Ultrahigh-resolution structures provide unprecedented details about protein dynamics, hydrogen bonding and solvent networks. The reported 0.70 Å, room-temperature crystal structure of crambin is the highest-resolution ambient-temperature structure of a protein achieved to date. Sufficient data were collected to enable unrestrained refinement of the protein and associated solvent networks using SHELXL. Dynamic solvent networks resulting from alternative side-chain conformations and shifts in water positions are revealed, demonstrating that polypeptide flexibility and formation of clathrate-type structures at hydrophobic surfaces are the key features endowing crambin crystals with extraordinary diffraction power.

Ultrahigh-resolution crystal structures of Z-DNA in complex with Mn(2+) and Zn(2+) ions.

X-ray crystal structures of the spermine(4+) form of the Z-DNA duplex with the self-complementary d(CG)3 sequence in complexes with Mn(2+) and Zn(2+) cations have been determined at the ultrahigh resolutions of 0.75 and 0.85 Å, respectively. Stereochemical restraints were only used for the sperminium cation (in both structures) and for nucleotides with dual conformation in the Zn(2+) complex. The Mn(2+) and Zn(2+) cations at the major site, designated M(2+)(1), bind at the N7 position of G6 by direct coordination. The coordination geometry of this site was octahedral, with complete hydration shells. An additional Zn(2+)(2) cation was bis-coordinated in a tetrahedral fashion by the N7 atoms of G10 and G12 from a symmetry-related molecule. The coordination distances of Zn(2+)(1) and Zn(2+)(2) to the O6 atom of the guanine residues were 3.613 (6) and 3.258 (5) Å, respectively. Moreover, a chloride ion was also identified in the coordination sphere of Zn(2+)(2). Alternate conformations were observed in the Z-DNA-Zn(2+) structure not only at internucleotide linkages but also at the terminal C3'-OH group of G12. The conformation of the sperminium chain in the Z-DNA-Mn(2+) complex is similar to the spermine(4+) conformation in analogous Z-DNA-Mg(2+) structures. In the Z-DNA-Zn(2+) complex the sperminium cation is disordered and partially invisible in electron-density maps. In the Z-DNA-Zn(2+) complex the sperminium cation only interacts with the phosphate groups of the Z-DNA molecules, while in the Z-DNA-Mn(2+) structure it forms hydrogen bonds to both the phosphate groups and DNA bases.

Right-handed Z-DNA at ultrahigh resolution: a tale of two hands and the power of the crystallographic method.

The self-complementary L-d(CGCGCG)2 purine/pyrimidine hexanucleotide was crystallized in complex with the polyamine cadaverine and potassium cations. Since the oligonucleotide contained the enantiomeric 2'-deoxy-L-ribose, the Z-DNA duplex is right-handed, as confirmed by the ultrahigh-resolution crystal structure determined at 0.69 Šresolution. Although the X-ray diffraction data were collected at a very short wavelength (0.7085 Å), where the anomalous signal of the P and K atoms is very weak, the signal was sufficiently outstanding to clearly indicate the wrong hand when the structure was mistakenly solved assuming the presence of 2'-deoxy-D-ribose. The electron density clearly shows the entire cadaverinium dication, which has an occupancy of 0.53 and interacts with one Z-DNA duplex. The K+ cation, with an occupancy of 0.32, has an irregular coordination sphere that is formed by three OP atoms of two symmetry-related Z-DNA duplexes and one O5' hydroxyl O atom, and is completed by three water sites, one of which is twofold disordered. The K+ site is complemented by a partial water molecule, the hydrogen bonds of which have the same lengths as the K-O bonds. The sugar-phosphate backbone assumes two conformations, but the base pairs do not show any sign of disorder.

Rhizobium etli has two L-asparaginases with low sequence identity but similar structure and catalytic center.

The genome of Rhizobium etli, a nitrogen-fixing bacterial symbiont of legume plants, encodes two L-asparaginases, ReAIV and ReAV, that have no similarity to the well characterized enzymes of class 1 (bacterial type) and class 2 (plant type). It has been hypothesized that ReAIV and ReAV might belong to the same structural class 3 despite their low level of sequence identity. When the crystal structure of the inducible and thermolabile protein ReAV was solved, this hypothesis gained a stronger footing because the key residues of ReAV are also present in the sequence of the constitutive and thermostable ReAIV protein. High-resolution crystal structures of ReAIV now confirm that it is a class 3 L-asparaginase that is structurally similar to ReAV but with important differences. The most striking differences concern the peculiar hydration patterns of the two proteins, the presence of three internal cavities in ReAIV and the behavior of the zinc-binding site. ReAIV has a high pH optimum (9-11) and a substrate affinity of ∼1.3 mM at pH 9.0. These parameters are not suitable for the direct application of ReAIV as an antileukemic drug, although its thermal stability and lack of glutaminase activity would be of considerable advantage. The five crystal structures of ReAIV presented in this work allow a possible enzymatic scenario to be postulated in which the zinc ion coordinated in the active site is a dispensable element. The catalytic nucleophile seems to be Ser47, which is part of two Ser-Lys tandems in the active site. The structures of ReAIV presented here may provide a basis for future enzyme-engineering experiments to improve the kinetic parameters for medicinal applications.

Structural and biophysical studies of new L-asparaginase variants: lessons from random mutagenesis of the prototypic Escherichia coli Ntn-amidohydrolase.

This work reports the results of random mutagenesis of the Escherichia coli class 2 L-asparaginase EcAIII belonging to the Ntn-hydrolase family. New variants of EcAIII were studied using structural, biophysical and bioinformatic methods. Activity tests revealed that the L-asparaginase activity is abolished in all analyzed mutants with the absence of Arg207, but some of them retained the ability to undergo the autoproteolytic maturation process. The results of spectroscopic studies and the determined crystal structures showed that the EcAIII fold is flexible enough to accept different types of mutations; however, these mutations may have a diverse impact on the thermal stability of the protein. The conclusions from the experiments are grouped into six lessons focused on (i) the adaptation of the EcAIII fold to new substitutions, (ii) the role of Arg207 in EcAIII activity, (iii) a network of residues necessary for autoprocessing, (iv) the complexity of the autoprocessing reaction, (v) the conformational changes observed in enzymatically inactive variants and (vi) the cooperativity of the EcAIII dimer subunits. Additionally, the structural requirements (pre-maturation checkpoints) that are necessary for the initiation of the autocleavage of Ntn-hydrolases have been classified. The findings reported in this work provide useful hints that should be considered before planning enzyme-engineering experiments aimed at the design of proteins for therapeutic applications. This is especially important for L-asparaginases that can be utilized in leukemia therapy, as alternative therapeutics are urgently needed to circumvent the severe side effects associated with the currently used enzymes.

SVM model for quality assessment of medium resolution mass spectra from 18O-water labeling experiments.

We describe a method for assessing the quality of mass spectra and improving reliability of relative ratio estimations from (18)O-water labeling experiments acquired from low resolution mass spectrometers. The mass profiles of heavy and light peptide pairs are often affected by artifacts, including coeluting contaminant species, noise signal, instrumental fluctuations in measuring ion position and abundance levels. Such artifacts distort the profiles, leading to erroneous ratio estimations, thus reducing the reliability of ratio estimations in high throughput quantification experiments. We used support vector machines (SVMs) to filter out mass spectra that deviated significantly from expected theoretical isotope distributions. We built an SVM classifier with a decision function that assigns a score to every mass profile based on such spectral features as mass accuracy, signal-to-noise ratio, and differences between experimental and theoretical isotopic distributions. The classifier was trained using a data set obtained from samples of mouse renal cortex. We then tested it on protein samples (bovine serum albumin) mixed in five different ratios of labeled and unlabeled species. We demonstrated that filtering the data using our SVM classifier results in as much as a 9-fold reduction in the coefficient of variance of peptide ratios, thus significantly improving the reliability of ratio estimations.

High-resolution structure of a retroviral protease folded as a monomer.

Mason-Pfizer monkey virus (M-PMV), a D-type retrovirus assembling in the cytoplasm, causes simian acquired immunodeficiency syndrome (SAIDS) in rhesus monkeys. Its pepsin-like aspartic protease (retropepsin) is an integral part of the expressed retroviral polyproteins. As in all retroviral life cycles, release and dimerization of the protease (PR) is strictly required for polyprotein processing and virion maturation. Biophysical and NMR studies have indicated that in the absence of substrates or inhibitors M-PMV PR should fold into a stable monomer, but the crystal structure of this protein could not be solved by molecular replacement despite countless attempts. Ultimately, a solution was obtained in mr-rosetta using a model constructed by players of the online protein-folding game Foldit. The structure indeed shows a monomeric protein, with the N- and C-termini completely disordered. On the other hand, the flap loop, which normally gates access to the active site of homodimeric retropepsins, is clearly traceable in the electron density. The flap has an unusual curled shape and a different orientation from both the open and closed states known from dimeric retropepsins. The overall fold of the protein follows the retropepsin canon, but the C(α) deviations are large and the active-site 'DTG' loop (here NTG) deviates up to 2.7 Å from the standard conformation. This structure of a monomeric retropepsin determined at high resolution (1.6 Å) provides important extra information for the design of dimerization inhibitors that might be developed as drugs for the treatment of retroviral infections, including AIDS.