Bicyclic azetidines kill the diarrheal pathogen Cryptosporidium in mice by inhibiting parasite phenylalanyl-tRNA synthetase.

Cryptosporidium is a protozoan parasite and a leading cause of diarrheal disease and mortality in young children. Currently, there are no fully effective treatments available to cure infection with this diarrheal pathogen. In this study, we report a broad drug repositioning effort that led to the identification of bicyclic azetidines as a new anticryptosporidial series. Members of this series blocked growth in in vitro culture of three Cryptosporidium parvum isolates with EC50 's in 1% serum of <0.4 to 96 nM, had comparable potencies against Cryptosporidium hominis and C. parvum, and was effective in three of four highly susceptible immunosuppressed mice with once-daily dosing administered for 4 days beginning 2 weeks after infection. Comprehensive genetic, biochemical, and chemical studies demonstrated inhibition of C. parvum phenylalanyl-tRNA synthetase (CpPheRS) as the mode of action of this new lead series. Introduction of mutations directly into the C. parvum pheRS gene by CRISPR-Cas9 genome editing resulted in parasites showing high degrees of compound resistance. In vitro, bicyclic azetidines potently inhibited the aminoacylation activity of recombinant ChPheRS. Medicinal chemistry optimization led to the identification of an optimal pharmacokinetic/pharmacodynamic profile for this series. Collectively, these data demonstrate that bicyclic azetidines are a promising series for anticryptosporidial drug development and establish a broad framework to enable target-based drug discovery for this infectious disease.

Structures of the cGMP-dependent protein kinase in malaria parasites reveal a unique structural relay mechanism for activation.

The cyclic guanosine-3',5'-monophosphate (cGMP)-dependent protein kinase (PKG) was identified >25 y ago; however, efforts to obtain a structure of the entire PKG enzyme or catalytic domain from any species have failed. In malaria parasites, cooperative activation of PKG triggers crucial developmental transitions throughout the complex life cycle. We have determined the cGMP-free crystallographic structures of PKG from Plasmodium falciparum and Plasmodium vivax, revealing how key structural components, including an N-terminal autoinhibitory segment (AIS), four predicted cyclic nucleotide-binding domains (CNBs), and a kinase domain (KD), are arranged when the enzyme is inactive. The four CNBs and the KD are in a pentagonal configuration, with the AIS docked in the substrate site of the KD in a swapped-domain dimeric arrangement. We show that although the protein is predominantly a monomer (the dimer is unlikely to be representative of the physiological form), the binding of the AIS is necessary to keep Plasmodium PKG inactive. A major feature is a helix serving the dual role of the N-terminal helix of the KD as well as the capping helix of the neighboring CNB. A network of connecting helices between neighboring CNBs contributes to maintaining the kinase in its inactive conformation. We propose a scheme in which cooperative binding of cGMP, beginning at the CNB closest to the KD, transmits conformational changes around the pentagonal molecule in a structural relay mechanism, enabling PKG to orchestrate rapid, highly regulated developmental switches in response to dynamic modulation of cGMP levels in the parasite.

Biochemical and Structural Characterization of Selective Allosteric Inhibitors of the Plasmodium falciparum Drug Target, Prolyl-tRNA-synthetase.

Plasmodium falciparum (Pf) prolyl-tRNA synthetase (ProRS) is one of the few chemical-genetically validated drug targets for malaria, yet highly selective inhibitors have not been described. In this paper, approximately 40,000 compounds were screened to identify compounds that selectively inhibit PfProRS enzyme activity versus Homo sapiens (Hs) ProRS. X-ray crystallography structures were solved for apo, as well as substrate- and inhibitor-bound forms of PfProRS. We identified two new inhibitors of PfProRS that bind outside the active site. These two allosteric inhibitors showed >100 times specificity for PfProRS compared to HsProRS, demonstrating this class of compounds could overcome the toxicity related to HsProRS inhibition by halofuginone and its analogues. Initial medicinal chemistry was performed on one of the two compounds, guided by the cocrystallography of the compound with PfProRS, and the results can instruct future medicinal chemistry work to optimize these promising new leads for drug development against malaria.

Crystal structure of human nuclear pore complex component NUP43.

Nuclear pore complexes (NPC) form nuclear pores that cross the nuclear envelope and allow molecules to transport between the nucleus and the cytoplasm. We solved the crystal structure of human Nup43 (hNUP43), an important component in the Nup107 subcomplex of NPC. hNup43 adopts a seven-bladed ß-propeller fold. We confirmed by ITC that neither human Nup37 (hNup37) nor human Nup133 (hNup133) interacts with hNup43. We demonstrated by analytical gel filtration that the human Nup85-Seh1L binary complex recruits hNup43 to form a ternary complex. Based on amino acid sequence analysis, we predicted the hNup85-hSeh1L binding surface of hNup43.

Salvage or recovery of failed targets by in situ proteolysis.

In situ proteolysis is the method of proactively adding tiny amounts of nonspecific proteases to aid in the crystallization of proteins and protein macromolecular complexes. The simplicity of the procedure and high recovery rate make it a method of first choice for recalcitrant targets. An improved and updated in situ proteolysis protocol used in high-throughput structural biology platforms is described.

Exploring the Trypanosoma brucei Hsp83 potential as a target for structure guided drug design.

Human African trypanosomiasis is a neglected parasitic disease that is fatal if untreated. The current drugs available to eliminate the causative agent Trypanosoma brucei have multiple liabilities, including toxicity, increasing problems due to treatment failure and limited efficacy. There are two approaches to discover novel antimicrobial drugs--whole-cell screening and target-based discovery. In the latter case, there is a need to identify and validate novel drug targets in Trypanosoma parasites. The heat shock proteins (Hsp), while best known as cancer targets with a number of drug candidates in clinical development, are a family of emerging targets for infectious diseases. In this paper, we report the exploration of T. brucei Hsp83--a homolog of human Hsp90--as a drug target using multiple biophysical and biochemical techniques. Our approach included the characterization of the chemical sensitivity of the parasitic chaperone against a library of known Hsp90 inhibitors by means of differential scanning fluorimetry (DSF). Several compounds identified by this screening procedure were further studied using isothermal titration calorimetry (ITC) and X-ray crystallography, as well as tested in parasite growth inhibitions assays. These experiments led us to the identification of a benzamide derivative compound capable of interacting with TbHsp83 more strongly than with its human homologs and structural rationalization of this selectivity. The results highlight the opportunities created by subtle structural differences to develop new series of compounds to selectively target the Trypanosoma brucei chaperone and effectively kill the sleeping sickness parasite.

CDR-H3 diversity is not required for antigen recognition by synthetic antibodies.

A synthetic phage-displayed antibody repertoire was constructed with equivalent chemical diversity in the third complementarity-determining regions of the heavy (CDR-H3) and light (CDR-L3) chains, which contrasts with natural antibodies in which CDR-H3 is much more diverse than CDR-L3 due to the genetic mechanisms that generate antibody encoding genes. Surprisingly, the synthetic repertoire yielded numerous functional antibodies that contained mutated CDR-L3 sequences but a fixed CDR-H3 sequence. Alanine-scanning analysis of antibodies that recognized 10 different antigens but contained a common CDR-H3 loop showed that, in most cases, the fixed CDR-H3 sequence was able to contribute favorably to antigen recognition, but in some cases, the loop was functionally inert. Structural analysis of one such antibody in complex with antigen showed that the inert CDR-H3 loop was nonetheless highly buried at the antibody-antigen interface. Taken together, these results show that CDR-H3 diversity is not necessarily required for the generation of antibodies that recognize diverse protein antigens with high affinity and specificity, and if given the chance, CDR-L3 readily assumes the dominant role for antigen recognition. These results contrast with the commonly accepted view of antigen recognition derived from the analysis of natural antibodies, in which CDR-H3 is presumed to be dominant and CDR-L3 is presumed to play an auxiliary role. Furthermore, the results show that natural antibody function is genetically constrained, and it should be possible to develop more functional synthetic antibody libraries by expanding the diversity of CDR-L3 beyond what is observed in nature.

Structural insights into the inactive subunit of the apicoplast-localized caseinolytic protease complex of Plasmodium falciparum.

The ATP-dependent caseinolytic protease, ClpP, is highly conserved in bacteria and in the organelles of different organisms. In cyanobacteria, plant plastids, and the apicoplast of the genus Plasmodium, a noncatalytic paralog of ClpP, termed ClpR, has been identified. ClpRs are found to form heterocomplexes with ClpP resulting in a ClpRP tetradecameric cylinder having less than 14 catalytic triads. The exact role of ClpR in such a complex remains enigmatic. Here we describe the x-ray crystal structure of ClpR protein heptamer from Plasmodium falciparum (PfClpR). This is the first structure of a ClpR protein. The structure shows that the PfClpR monomer adopts a fold similar to that of ClpP, but has a unique motif, which we named the R-motif, forming a ß turn located near the inactive catalytic triad in a three-dimensional space. The PfClpR heptamer exhibits a more open and flat ring than a ClpP heptamer. PfClpR was localized in the P. falciparum apicoplast as is the case of PfClpP. However, biochemical and structural data suggest that, contrary to what has been observed in other organisms, PfClpP and PfClpR do not form a stable heterocomplex in the apicoplast of P. falciparum.

Catalytic site remodelling of the DOT1L methyltransferase by selective inhibitors.

Selective inhibition of protein methyltransferases is a promising new approach to drug discovery. An attractive strategy towards this goal is the development of compounds that selectively inhibit binding of the cofactor, S-adenosylmethionine, within specific protein methyltransferases. Here we report the three-dimensional structure of the protein methyltransferase DOT1L bound to EPZ004777, the first S-adenosylmethionine-competitive inhibitor of a protein methyltransferase with in vivo efficacy. This structure and those of four new analogues reveal remodelling of the catalytic site. EPZ004777 and a brominated analogue, SGC0946, inhibit DOT1L in vitro and selectively kill mixed lineage leukaemia cells, in which DOT1L is aberrantly localized via interaction with an oncogenic MLL fusion protein. These data provide important new insight into mechanisms of cell-active S-adenosylmethionine-competitive protein methyltransferase inhibitors, and establish a foundation for the further development of drug-like inhibitors of DOT1L for cancer therapy.

Phosphoglucomutase is absent in Trypanosoma brucei and redundantly substituted by phosphomannomutase and phospho-N-acetylglucosamine mutase.

The enzymes phosphomannomutase (PMM), phospho-N-acetylglucosamine mutase (PAGM) and phosphoglucomutase (PGM) reversibly catalyse the transfer of phosphate between the C6 and C1 hydroxyl groups of mannose, N-acetylglucosamine and glucose respectively. Although genes for a candidate PMM and a PAGM enzymes have been found in the Trypanosoma brucei genome, there is, surprisingly, no candidate gene for PGM. The TbPMM and TbPAGM genes were cloned and expressed in Escherichia coli and the TbPMM enzyme was crystallized and its structure solved at 1.85 Å resolution. Antibodies to the recombinant proteins localized endogenous TbPMM to glycosomes in the bloodstream form of the parasite, while TbPAGM localized to both the cytosol and glycosomes. Both recombinant enzymes were able to interconvert glucose-phosphates, as well as acting on their own definitive substrates. Analysis of sugar nucleotide levels in parasites with TbPMM or TbPAGM knocked down by RNA interference (RNAi) suggests that, in vivo, PGM activity is catalysed by both enzymes. This is the first example in any organism of PGM activity being completely replaced in this way and it explains why, uniquely, T. brucei has been able to lose its PGM gene. The RNAi data for TbPMM also showed that this is an essential gene for parasite growth.

Crystal structures from the Plasmodium peroxiredoxins: new insights into oligomerization and product binding.

BACKGROUND: Plasmodium falciparum is the protozoan parasite primarily responsible for more than one million malarial deaths, annually, and is developing resistance to current therapies. Throughout its lifespan, the parasite is subjected to oxidative attack, so Plasmodium antioxidant defences are essential for its survival and are targets for disease control. RESULTS: To further understand the molecular aspects of the Plasmodium redox system, we solved 4 structures of Plasmodium peroxiredoxins (Prx). Our study has confirmed PvTrx-Px1 to be a hydrogen peroxide (H2O2)-sensitive peroxiredoxin. We have identified and characterized the novel toroid octameric oligomer of PyTrx-Px1, which may be attributed to the interplay of several factors including: (1) the orientation of the conserved surface/buried arginine of the NNLA(I/L)GRS-loop; and (2) the C-terminal tail positioning (also associated with the aforementioned conserved loop) which facilitates the intermolecular hydrogen bond between dimers (in an A-C fashion). In addition, a notable feature of the disulfide bonds in some of the Prx crystal structures is discussed. Finally, insight into the latter stages of the peroxiredoxin reaction coordinate is gained. Our structure of PyPrx6 is not only in the sulfinic acid (RSO2H) form, but it is also with glycerol bound in a way (not previously observed) indicative of product binding. CONCLUSIONS: The structural characterization of Plasmodium peroxiredoxins provided herein provides insight into their oligomerization and product binding which may facilitate the targeting of these antioxidant defences. Although the structural basis for the octameric oligomerization is further understood, the results yield more questions about the biological implications of the peroxiredoxin oligomerization, as multiple toroid configurations are now known. The crystal structure depicting the product bound active site gives insight into the overoxidation of the active site and allows further characterization of the leaving group chemistry.

Structural insights into the allosteric effects of 4EBP1 on the eukaryotic translation initiation factor eIF4E.

The eukaryotic translation initiation factor eIF4E plays key roles in cap-dependent translation and mRNA export. These functions rely on binding the 7-methyl-guanosine moiety (5'cap) on the 5'-end of all mRNAs. eIF4E is regulated by proteins such as eIF4G and eIF4E binding proteins (4EBPs) that bind the dorsal surface of eIF4E, distal to the cap binding site, and modulate cap binding activity. Both proteins increase the affinity of eIF4E for 5'cap. Our understanding of the allosteric effects and structural underpinnings of 4EBP1 or eIF4G binding can be advanced by obtaining structural data on cap-free eIF4E bound to one of these proteins. Here, we report the crystal structure of apo-eIF4E and cap-free eIF4E in complex with a 4EBP1 peptide. We also monitored 4EBP1 binding to cap-free eIF4E in solution using NMR. Together, these studies suggest that 4EBP1 transforms eIF4E into a cap-receptive state. NMR methods were also used to compare the allosteric routes activated by 4EBP1, eIF4G, and the arenavirus Z protein, a negative regulator of cap binding. We observed chemical shift perturbation at the dorsal binding site leading to alterations in the core of the protein, which were ultimately communicated to the unoccupied cap binding site of eIF4E. There were notable similarities between the routes taken by 4EBP1 and eIF4G and differences from the negative regulator Z. Thus, binding of 4EBP1 or eIF4G allosterically drives alterations throughout the protein that increase the affinity of eIF4E for the 5'cap.

The Cryptosporidium parvum kinome.

BACKGROUND: Hundreds of millions of people are infected with cryptosporidiosis annually, with immunocompromised individuals suffering debilitating symptoms and children in socioeconomically challenged regions at risk of repeated infections. There is currently no effective drug available. In order to facilitate the pursuit of anti-cryptosporidiosis targets and compounds, our study spans the classification of the Cryptosporidium parvum kinome and the structural and biochemical characterization of representatives from the CDPK family and a MAP kinase. RESULTS: The C. parvum kinome comprises over 70 members, some of which may be promising drug targets. These C. parvum protein kinases include members in the AGC, Atypical, CaMK, CK1, CMGC, and TKL groups; however, almost 35% could only be classified as OPK (other protein kinases). In addition, about 25% of the kinases identified did not have any known orthologues outside of Cryptosporidium spp. Comparison of specific kinases with their Plasmodium falciparum and Toxoplasma gondii orthologues revealed some distinct characteristics within the C. parvum kinome, including potential targets and opportunities for drug design. Structural and biochemical analysis of 4 representatives of the CaMK group and a MAP kinase confirms features that may be exploited in inhibitor design. Indeed, screening CpCDPK1 against a library of kinase inhibitors yielded a set of the pyrazolopyrimidine derivatives (PP1-derivatives) with IC50 values of < 10 nM. The binding of a PP1-derivative is further described by an inhibitor-bound crystal structure of CpCDPK1. In addition, structural analysis of CpCDPK4 identified an unprecedented Zn-finger within the CDPK kinase domain that may have implications for its regulation. CONCLUSIONS: Identification and comparison of the C. parvum protein kinases against other parasitic kinases shows how orthologue- and family-based research can be used to facilitate characterization of promising drug targets and the search for new drugs.

Target highlights in CASP9: Experimental target structures for the critical assessment of techniques for protein structure prediction.

One goal of the CASP community wide experiment on the critical assessment of techniques for protein structure prediction is to identify the current state of the art in protein structure prediction and modeling. A fundamental principle of CASP is blind prediction on a set of relevant protein targets, that is, the participating computational methods are tested on a common set of experimental target proteins, for which the experimental structures are not known at the time of modeling. Therefore, the CASP experiment would not have been possible without broad support of the experimental protein structural biology community. In this article, several experimental groups discuss the structures of the proteins which they provided as prediction targets for CASP9, highlighting structural and functional peculiarities of these structures: the long tail fiber protein gp37 from bacteriophage T4, the cyclic GMP-dependent protein kinase Iß dimerization/docking domain, the ectodomain of the JTB (jumping translocation breakpoint) transmembrane receptor, Autotaxin in complex with an inhibitor, the DNA-binding J-binding protein 1 domain essential for biosynthesis and maintenance of DNA base-J (ß-D-glucosyl-hydroxymethyluracil) in Trypanosoma and Leishmania, an so far uncharacterized 73 residue domain from Ruminococcus gnavus with a fold typical for PDZ-like domains, a domain from the phycobilisome core-membrane linker phycobiliprotein ApcE from Synechocystis, the heat shock protein 90 activators PFC0360w and PFC0270w from Plasmodium falciparum, and 2-oxo-3-deoxygalactonate kinase from Klebsiella pneumoniae.

Characterization of 14-3-3 proteins from Cryptosporidium parvum.

UNLABELLED: The parasite Cryptosporidium parvum has three 14-3-3 proteins: Cp14ε, Cp14a and Cp14b, with only Cp14ε similar to human 14-3-3 proteins in sequence, peptide-binding properties and structure. Structurally, Cp14a features the classical 14-3-3 dimer but with a uniquely wide pocket and a disoriented RRY triad potentially incapable of binding phosphopeptides. The Cp14b protein deviates from the norm significantly: (i) In one subunit, the phosphorylated C-terminal tail is bound in the binding groove like a phosphopeptide. This supports our binding study indicating this protein was stabilized by a peptide mimicking its last six residues. (ii) The other subunit has eight helices instead of nine, with αA and αB forming a single helix and occluding the peptide-binding cleft. (iii) The protein forms a degenerate dimer with the two binding grooves divided and facing opposite directions. These features conspire to block and disrupt the bicameral substrate-binding pocket, suggesting a possible tripartite auto-regulation mechanism that has not been observed previously. ENHANCED VERSION: This article can also be viewed as an enhanced version in which the text of the article is integrated with interactive 3D representations and animated transitions. Please note that a web plugin is required to access this enhanced functionality. Instructions for the installation and use of the web plugin are available in Text S1.

Characterization of a new phosphatase from Plasmodium.

Plasmodium falciparum malaria is the most important parasitic disease worldwide, responsible for an estimated 1 million deaths annually. Two P. falciparum genes code for putative phosphoglycerate mutases (PGMases), a widespread protein group characterized by the involvement of histidine residues in their catalytic mechanism. PGMases are responsible for the interconversion between 2 and 3-phosphoglycerate, an intermediate step in the glycolysis pathway. We have determined the crystal structures of one of the P. falciparum's PGMases (PfPGM2) and a functionally distinct phosphoglycerate mutase from Cryptosporidium parvum, a related apicomplexan parasite. We performed sequence and structural comparisons between the two structures, another P. falciparum enzyme (PfPGM1) and several other PGM family members from other organisms. The comparisons revealed a distinct conformation of the catalytically active residues not seen in previously determined phosphoglycerate mutase structures. Furthermore, characterization of their enzymatic activities revealed contrasting behaviors between the PfPGM2 and the classical cofactor-dependent PGMase from C. parvum, clearly establishing PfPGM2 as a phosphatase with a residual level of mutase activity. Further support for this function attribution was provided by our structural comparison with previously characterized PGM family members. Genetic characterization of PGM2 in the rodent parasite Plasmodium berghei indicated that the protein might be essential to blood stage asexual growth, and a GFP tagged allele is expressed in both blood and zygote ookinete development and located in the cytoplasm. The P. falciparum PGM2 is either an enzyme implicated in the phosphate metabolism of the parasite or a regulator of its life cycle.

Characterization, localization, essentiality, and high-resolution crystal structure of glucosamine 6-phosphate N-acetyltransferase from Trypanosoma brucei.

A gene predicted to encode Trypanosoma brucei glucosamine 6-phosphate N-acetyltransferase (TbGNA1; EC 2.3.1.4) was cloned and expressed in Escherichia coli. The recombinant protein was enzymatically active, and its high-resolution crystal structure was obtained at 1.86 Å. Endogenous TbGNA1 protein was localized to the peroxisome-like microbody, the glycosome. A bloodstream-form T. brucei GNA1 conditional null mutant was constructed and shown to be unable to sustain growth in vitro under nonpermissive conditions, demonstrating that there are no metabolic or nutritional routes to UDP-GlcNAc other than via GlcNAc-6-phosphate. Analysis of the protein glycosylation phenotype of the TbGNA1 mutant under nonpermissive conditions revealed that poly-N-acetyllactosamine structures were greatly reduced in the parasite and that the glycosylation profile of the principal parasite surface coat component, the variant surface glycoprotein (VSG), was modified. The significance of results and the potential of TbGNA1 as a novel drug target for African sleeping sickness are discussed.

Structures of parasitic CDPK domains point to a common mechanism of activation.

We recently determined the first structures of inactivated and calcium-activated calcium-dependent protein kinases (CDPKs) from Apicomplexa. Calcium binding triggered a large conformational change that constituted a new mechanism in calcium signaling and a novel EF-hand fold (CAD, for CDPK activation domain). Thus we set out to determine if this mechanism was universal to all CDPKs. We solved additional CDPK structures, including one from the species Plasmodium. We highlight the similarities in sequence and structure across apicomplexan and plant CDPKs, and strengthen our observations that this novel mechanism could be universal to canonical CDPKs. Our new structures demonstrate more detailed steps in the mechanism of calcium activation and possible key players in regulation. Residues involved in making the largest conformational change are the most conserved across Apicomplexa, leading us to propose that the mechanism is indeed conserved. CpCDPK3_CAD and PfCDPK_CAD were captured at a possible intermediate conformation, lending insight into the order of activation steps. PfCDPK3_CAD adopts an activated fold, despite having an inactive EF-hand sequence in the N-terminal lobe. We propose that for most apicomplexan CDPKs, the mode of activation will be similar to that seen in our structures, while specific regulation of the inactive and active forms will require further investigation.

Molecular characterization of a novel geranylgeranyl pyrophosphate synthase from Plasmodium parasites.

We present here a study of a eukaryotic trans-prenylsynthase from the malaria pathogen Plasmodium vivax. Based on the results of biochemical assays and contrary to previous indications, this enzyme catalyzes the production of geranylgeranyl pyrophosphate (GGPP) rather than farnesyl pyrophosphate (FPP). Structural analysis shows that the product length is constrained by a hydrophobic cavity formed primarily by a set of residues from the same subunit as the product as well as at least one other from the dimeric partner. Furthermore, Plasmodium GGPP synthase (GGPPS) can bind nitrogen-containing bisphosphonates (N-BPs) strongly with the energetically favorable cooperation of three Mg(2+), resulting in inhibition by this class of compounds at IC(50) concentrations below 100 nM. In contrast, human and yeast GGPPSs do not accommodate a third magnesium atom in the same manner, resulting in their insusceptibility to N-BPs. This differentiation is in part attributable to a deviation in a conserved motif known as the second aspartate-rich motif: whereas the aspartates at the start and end of the five-residue motif in FFPP synthases and P. vivax GGPPSs both participate in the coordination of the third Mg(2+), an asparagine is featured as the last residue in human and yeast GGPPSs, resulting in a different manner of interaction with nitrogen-containing ligands.

The crystal structure of Toxoplasma gondii pyruvate kinase 1.

BACKGROUND: Pyruvate kinase (PK), which catalyzes the final step in glycolysis converting phosphoenolpyruvate to pyruvate, is a central metabolic regulator in most organisms. Consequently PK represents an attractive therapeutic target in cancer and human pathogens, like Apicomplexans. The phylum Aplicomplexa, a group of exclusively parasitic organisms, includes the genera Plasmodium, Cryptosporidium and Toxoplasma, the etiological agents of malaria, cryptosporidiosis and toxoplasmosis respectively. Toxoplasma gondii infection causes a mild illness and is a very common infection affecting nearly one third of the world's population. METHODOLOGY/PRINCIPAL FINDINGS: We have determined the crystal structure of the PK1 enzyme from T. gondii, with the B domain in the open and closed conformations. We have also characterized its enzymatic activity and confirmed glucose-6-phosphate as its allosteric activator. This is the first description of a PK enzyme in a closed inactive conformation without any bound substrate. Comparison of the two tetrameric TgPK1 structures indicates a reorientation of the monomers with a concomitant change in the buried surface among adjacent monomers. The change in the buried surface was associated with significant B domain movements in one of the interacting monomers. CONCLUSIONS: We hypothesize that a loop in the interface between the A and B domains plays an important role linking the position of the B domain to the buried surface among monomers through two α-helices. The proposed model links the catalytic cycle of the enzyme with its domain movements and highlights the contribution of the interface between adjacent subunits. In addition, an unusual ordered conformation was observed in one of the allosteric binding domains and it is related to a specific apicomplexan insertion. The sequence and structural particularity would explain the atypical activation by a mono-phosphorylated sugar. The sum of peculiarities raises this enzyme as an emerging target for drug discovery.