Transmission of malaria to mosquitoes blocked by bumped kinase inhibitors.

Effective control and eradication of malaria will require new tools to prevent transmission. Current antimalarial therapies targeting the asexual stage of Plasmodium do not prevent transmission of circulating gametocytes from infected humans to mosquitoes. Here, we describe a new class of transmission-blocking compounds, bumped kinase inhibitors (BKIs), which inhibit microgametocyte exflagellation. Oocyst formation and sporozoite production, necessary for transmission to mammals, were inhibited in mosquitoes fed on either BKI-1-treated human blood or mice treated with BKI-1. BKIs are hypothesized to act via inhibition of Plasmodium calcium-dependent protein kinase 4 and predicted to have little activity against mammalian kinases. Our data show that BKIs do not inhibit proliferation of mammalian cell lines and are well tolerated in mice. Used in combination with drugs active against asexual stages of Plasmodium, BKIs could prove an important tool for malaria control and eradication.

Development of Toxoplasma gondii calcium-dependent protein kinase 1 (TgCDPK1) inhibitors with potent anti-toxoplasma activity.

Toxoplasmosis is a disease of prominent health concern that is caused by the protozoan parasite Toxoplasma gondii. Proliferation of T. gondii is dependent on its ability to invade host cells, which is mediated in part by calcium-dependent protein kinase 1 (CDPK1). We have developed ATP competitive inhibitors of TgCDPK1 that block invasion of parasites into host cells, preventing their proliferation. The presence of a unique glycine gatekeeper residue in TgCDPK1 permits selective inhibition of the parasite enzyme over human kinases. These potent TgCDPK1 inhibitors do not inhibit the growth of human cell lines and represent promising candidates as toxoplasmosis therapeutics.

Structure of Leishmania major methionyl-tRNA synthetase in complex with intermediate products methionyladenylate and pyrophosphate.

Leishmania parasites cause two million new cases of leishmaniasis each year with several hundreds of millions of people at risk. Due to the paucity and shortcomings of available drugs, we have undertaken the crystal structure determination of a key enzyme from Leishmania major in hopes of creating a platform for the rational design of new therapeutics. Crystals of the catalytic core of methionyl-tRNA synthetase from L. major (LmMetRS) were obtained with the substrates MgATP and methionine present in the crystallization medium. These crystals yielded the 2.0 Å resolution structure of LmMetRS in complex with two products, methionyladenylate and pyrophosphate, along with a Mg(2+) ion that bridges them. This is the first class I aminoacyl-tRNA synthetase (aaRS) structure with pyrophosphate bound. The residues of the class I aaRS signature sequence motifs, KISKS and HIGH, make numerous contacts with the pyrophosphate. Substantial differences between the LmMetRS structure and previously reported complexes of Escherichia coli MetRS (EcMetRS) with analogs of the methionyladenylate intermediate product are observed, even though one of these analogs only differs by one atom from the intermediate. The source of these structural differences is attributed to the presence of the product pyrophosphate in LmMetRS. Analysis of the LmMetRS structure in light of the Aquifex aeolicus MetRS-tRNA(Met) complex shows that major rearrangements of multiple structural elements of enzyme and/or tRNA are required to allow the CCA acceptor triplet to reach the methionyladenylate intermediate in the active site. Comparison with sequences of human cytosolic and mitochondrial MetRS reveals interesting differences near the ATP- and methionine-binding regions of LmMetRS, suggesting that it should be possible to obtain compounds that selectively inhibit the parasite enzyme.

Prediction of protein crystallization outcome using a hybrid method.

The great power of protein crystallography to reveal biological structure is often limited by the tremendous effort required to produce suitable crystals. A hybrid crystal growth predictive model is presented that combines both experimental and sequence-derived data from target proteins, including novel variables derived from physico-chemical characterization such as R(30), the ratio between a protein's DSF intensity at 30°C and at T(m). This hybrid model is shown to be more powerful than sequence-based prediction alone - and more likely to be useful for prioritizing and directing the efforts of structural genomics and individual structural biology laboratories.

Crystal structures of trypanosomal histidyl-tRNA synthetase illuminate differences between eukaryotic and prokaryotic homologs.

Crystal structures of histidyl-tRNA synthetase (HisRS) from the eukaryotic parasites Trypanosoma brucei and Trypanosoma cruzi provide a first structural view of a eukaryotic form of this enzyme and reveal differences from bacterial homologs. HisRSs in general contain an extra domain inserted between conserved motifs 2 and 3 of the Class II aminoacyl-tRNA synthetase catalytic core. The current structures show that the three-dimensional topology of this domain is very different in bacterial and archaeal/eukaryotic forms of the enzyme. Comparison of apo and histidine-bound trypanosomal structures indicates substantial active-site rearrangement upon histidine binding but relatively little subsequent rearrangement after reaction of histidine with ATP to form the enzyme's first reaction product, histidyladenylate. The specific residues involved in forming the binding pocket for the adenine moiety differ substantially both from the previously characterized binding site in bacterial structures and from the homologous residues in human HisRSs. The essentiality of the single HisRS gene in T. brucei is shown by a severe depression of parasite growth rate that results from even partial suppression of expression by RNA interference.

The crystal structure and activity of a putative trypanosomal nucleoside phosphorylase reveal it to be a homodimeric uridine phosphorylase.

Purine nucleoside phosphorylases (PNPs) and uridine phosphorylases (UPs) are closely related enzymes involved in purine and pyrimidine salvage, respectively, which catalyze the removal of the ribosyl moiety from nucleosides so that the nucleotide base may be recycled. Parasitic protozoa generally are incapable of de novo purine biosynthesis; hence, the purine salvage pathway is of potential therapeutic interest. Information about pyrimidine biosynthesis in these organisms is much more limited. Though all seem to carry at least a subset of enzymes from each pathway, the dependency on de novo pyrimidine synthesis versus salvage varies from organism to organism and even from one growth stage to another. We have structurally and biochemically characterized a putative nucleoside phosphorylase (NP) from the pathogenic protozoan Trypanosoma brucei and find that it is a homodimeric UP. This is the first characterization of a UP from a trypanosomal source despite this activity being observed decades ago. Although this gene was broadly annotated as a putative NP, it was widely inferred to be a purine nucleoside phosphorylase. Our characterization of this trypanosomal enzyme shows that it is possible to distinguish between PNP and UP activity at the sequence level based on the absence or presence of a characteristic UP-specificity insert. We suggest that this recognizable feature may aid in proper annotation of the substrate specificity of enzymes in the NP family.

Crystal structure of the aspartyl-tRNA synthetase from Entamoeba histolytica.

The crystal structure of the aspartyl-tRNA synthetase from the eukaryotic parasite Entamoeba histolytica has been determined at 2.8Aresolution. Relative to homologous sequences, the E. histolytica protein contains a 43-residue insertion between the N-terminal anticodon binding domain and the C-terminal catalytic domain. The present structure reveals that this insertion extends an arm of the hinge region that has previously been shown to mediate interaction of aspartyl-tRNA synthetase with the cognate tRNA D-stem. Modeling indicates that this Entamoeba-specific insertion is likely to increase the interaction surface with the cognate tRNA(Asp). In doing so it may substitute functionally for an RNA-binding motif located in N-terminal extensions found in AspRS sequences from lower eukaryotes but absent in Entamoeba. The E. histolytica AspRS structure shows a well-ordered N-terminus that contributes to the AspRS dimer interface.

Structures of substrate- and inhibitor-bound adenosine deaminase from a human malaria parasite show a dramatic conformational change and shed light on drug selectivity.

Plasmodium and other apicomplexan parasites are deficient in purine biosynthesis, relying instead on the salvage of purines from their host environment. Therefore, interference with the purine salvage pathway is an attractive therapeutic target. The plasmodial enzyme adenosine deaminase (ADA) plays a central role in purine salvage and, unlike mammalian ADA homologs, has a further secondary role in methylthiopurine recycling. For this reason, plasmodial ADA accepts a wider range of substrates, as it is responsible for deamination of both adenosine and 5'-methylthioadenosine. The latter substrate is not accepted by mammalian ADA homologs. The structural basis for this natural difference in specificity between plasmodial and mammalian ADA has not been well understood. We now report crystal structures of Plasmodium vivax ADA in complex with adenosine, guanosine, and the picomolar inhibitor 2'-deoxycoformycin. These structures highlight a drastic conformational change in plasmodial ADA upon substrate binding that has not been observed for mammalian ADA enzymes. Further, these complexes illuminate the structural basis for the differential substrate specificity and potential drug selectivity between mammalian and parasite enzymes.

Heterologous expression of proteins from Plasmodium falciparum: results from 1000 genes.

As part of a structural genomics initiative, 1000 open reading frames from Plasmodium falciparum, the causative agent of the most deadly form of malaria, were tested in an E. coli protein expression system. Three hundred and thirty-seven of these targets were observed to express, although typically the protein was insoluble. Sixty-three of the targets provided soluble protein in yields ranging from 0.9 to 406.6 mg from one liter of rich media. Higher molecular weight, greater protein disorder (segmental analysis, SEG), more basic isoelectric point (pI), and a lack of homology to E. coli proteins were all highly and independently correlated with difficulties in expression. Surprisingly, codon usage and the percentage of adenosines and thymidines (%AT) did not appear to play a significant role. Of those proteins which expressed, high pI and a hypothetical annotation were both strongly and independently correlated with insolubility. The overwhelmingly important role of pI in both expression and solubility appears to be a surprising and fundamental issue in the heterologous expression of P. falciparum proteins in E. coli. Twelve targets which did not express in E. coli from the native gene sequence were codon-optimized through whole gene synthesis, resulting in the (insoluble) expression of three of these proteins. Seventeen targets which were expressed insolubly in E. coli were moved into a baculovirus/Sf-21 system, resulting in the soluble expression of one protein at a high level and six others at a low level. A variety of factors conspire to make the heterologous expression of P. falciparum proteins challenging, and these observations lay the groundwork for a rational approach to prioritizing and, ultimately, eliminating these impediments.