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1.  The macromolecular complex of ICP and falcipain-2 from Plasmodium: preparation, crystallization and preliminary X-ray diffraction analysis 
The macromolecular complex of ICP (inhibitor of cysteine proteases) from P. berghei and falcipain-2 from P. falciparum has been prepared and crystallized, and a diffraction data set has been collected to a resolution of 2.6 Å.
The malaria parasite Plasmodium depends on the tight control of cysteine-protease activity throughout its life cycle. Recently, the characterization of a new class of potent inhibitors of cysteine proteases (ICPs) secreted by Plasmodium has been reported. Here, the recombinant production, purification and crystallization of the inhibitory C-terminal domain of ICP from P. berghei in complex with the P. falciparum haemoglobinase falcipain-2 is described. The 1:1 complex was crystallized in space group P43, with unit-cell parameters a = b = 71.15, c = 120.09 Å. A complete diffraction data set was collected to a resolution of 2.6 Å.
PMCID: PMC3212462  PMID: 22102243
malaria; inhibitor of cysteine proteases; ICP; falcipain; chagasin
2.  Fungal effector Ecp6 outcompetes host immune receptor for chitin binding through intrachain LysM dimerization 
eLife  2013;2:e00790.
While host immune receptors detect pathogen-associated molecular patterns to activate immunity, pathogens attempt to deregulate host immunity through secreted effectors. Fungi employ LysM effectors to prevent recognition of cell wall-derived chitin by host immune receptors, although the mechanism to compete for chitin binding remained unclear. Structural analysis of the LysM effector Ecp6 of the fungal tomato pathogen Cladosporium fulvum reveals a novel mechanism for chitin binding, mediated by intrachain LysM dimerization, leading to a chitin-binding groove that is deeply buried in the effector protein. This composite binding site involves two of the three LysMs of Ecp6 and mediates chitin binding with ultra-high (pM) affinity. Intriguingly, the remaining singular LysM domain of Ecp6 binds chitin with low micromolar affinity but can nevertheless still perturb chitin-triggered immunity. Conceivably, the perturbation by this LysM domain is not established through chitin sequestration but possibly through interference with the host immune receptor complex.
eLife digest
The ability to launch an immune response is not unique to animals. Plants have also evolved the ability to detect molecules present on the surface of pathogens such as fungi. These molecular signatures are known as pathogen-associated molecular patterns (PAMPs), and they are detected by specialized receptors on the surface of plant cells.
Chitin, the main structural component of the cell wall in fungi, is one example of a PAMP. Many species of plants are able to detect chitin using receptors that contain sequences of amino acids called lysin motifs. Previous work in the model plant Arabidopsis has shown that chitin binds to a single lysin motif within each plant receptor.
However, just as plants have evolved the ability to recognize PAMPs, so fungi have evolved ways to outwit plants. They have developed small molecules called effector proteins that bind to PAMPs, in effect hiding them from the plant receptors. The tomato fungus Cladosporium fulvum, for example, secretes an effector protein called Ecp6, which contains lysin motifs just like those in the plant receptors. By binding chitin fragments, Ecp6 helps the fungus to avoid detection by its host plant.
Now, Sánchez-Vallet et al. present the high resolution crystal structure of Ecp6 and reveal the mechanism by which it outcompetes the plant’s own chitin receptors. In the presence of chitin, two lysin binding motifs within the Ecp6 protein combine to produce a binding site with ultrahigh affinity for chitin. This can outcompete the plant receptors, which use only a single lysin domain to bind the fungal protein.
As well as providing a molecular explanation for how certain fungi manage to evade the immune response in plants, the work of Sánchez-Vallet et al. offers an unusual example of convergent evolution, in which two evolutionarily distant organisms have evolved the ability to recognize the same molecule through structurally diverse proteins.
PMCID: PMC3700227  PMID: 23840930
Cladosporium; tomato; effector; immunity; receptor; Other
3.  Crystallization and preliminary X-ray analysis of glutathione transferases from cyanobacteria 
Glutathione S-transferases (GSTs) are a group of detoxifying enzymes that are found in animals, plants and microorganisms. Here, the crystallizations of two cyanobacterial GSTs are reported with the aim of determining their atomic structures.
Glutathione S-transferases (GSTs) are a group of multifunctional enzymes that are found in animals, plants and microorganisms. Their primary function is to remove toxins derived from exogenous sources or the products of metabolism from the cell. Mammalian GSTs have been extensively studied, in contrast to bacterial GSTs which have received relatively scant attention. A new class of GSTs called Chi has recently been identified in cyanobacteria. Chi GSTs exhibit a high glutathionylation activity towards isothiocyanates, compounds that are normally found in plants. Here, the crystallization of two GSTs are presented: TeGST produced by Thermosynechococcus elongates BP-1 and SeGST from Synechococcus elongates PCC 6301. Both enzymes formed crystals that diffracted to high resolution and appeared to be suitable for further X-ray diffraction studies. The structures of these GSTs may shed further light on the evolution of GST catalytic activity and in particular why these enzymes possess catalytic activity towards plant antimicrobial compounds.
PMCID: PMC2675588  PMID: 19407380
cyanobacteria; detoxification; glutathione transferases; isothiocyanates; toxins
4.  Exoerythrocytic Plasmodium Parasites Secrete a Cysteine Protease Inhibitor Involved in Sporozoite Invasion and Capable of Blocking Cell Death of Host Hepatocytes 
PLoS Pathogens  2010;6(3):e1000825.
Plasmodium parasites must control cysteine protease activity that is critical for hepatocyte invasion by sporozoites, liver stage development, host cell survival and merozoite liberation. Here we show that exoerythrocytic P. berghei parasites express a potent cysteine protease inhibitor (PbICP, P. berghei inhibitor of cysteine proteases). We provide evidence that it has an important function in sporozoite invasion and is capable of blocking hepatocyte cell death. Pre-incubation with specific anti-PbICP antiserum significantly decreased the ability of sporozoites to infect hepatocytes and expression of PbICP in mammalian cells protects them against peroxide- and camptothecin-induced cell death. PbICP is secreted by sporozoites prior to and after hepatocyte invasion, localizes to the parasitophorous vacuole as well as to the parasite cytoplasm in the schizont stage and is released into the host cell cytoplasm at the end of the liver stage. Like its homolog falstatin/PfICP in P. falciparum, PbICP consists of a classical N-terminal signal peptide, a long N-terminal extension region and a chagasin-like C-terminal domain. In exoerythrocytic parasites, PbICP is posttranslationally processed, leading to liberation of the C-terminal chagasin-like domain. Biochemical analysis has revealed that both full-length PbICP and the truncated C-terminal domain are very potent inhibitors of cathepsin L-like host and parasite cysteine proteases. The results presented in this study suggest that the inhibitor plays an important role in sporozoite invasion of host cells and in parasite survival during liver stage development by inhibiting host cell proteases involved in programmed cell death.
Author Summary
Plasmodium sporozoites are transmitted by Anopheles mosquitoes to the vertebrate host. They migrate through the skin before entering blood vessels and being transported with the bloodstream to liver sinusoids. There the sporozoites transmigrate through Kupffer cells and several hepatocytes before they invade a final hepatocyte and develop into thousands of merozoites. These daughter parasites are transported inside host cell-derived vesicles (merosomes) back to the bloodstream where they are finally released and infect red blood cells. Most of these processes depend on the activity of proteases, which must be tightly controlled to avoid proteolytic destruction of the parasite. We have identified a potent cysteine protease inhibitor of the rodent parasite Plasmodium berghei, which is expressed throughout the life cycle of the parasite. The inhibitor appears to play a role in sporozoite invasion of host cells and in parasite survival during liver stage development by inhibiting host cell proteases involved in programmed cell death.
PMCID: PMC2845656  PMID: 20361051
5.  Expression, purification, crystallization and preliminary X-ray diffraction analysis of chloride intracellular channel 2 (CLIC2) 
Chloride intracellular channel 2 (CLIC2) belongs to a family of intracellular chloride-channel proteins that can exist in a soluble form. The expression, purification and crystallization in two different crystal forms of human CLIC2 is reported.
The chloride intracellular channel (CLIC) family of proteins are unusual in that they can exist in either an integral membrane-channel form or a soluble form. Here, the expression, purification, crystallization and preliminary diffraction analysis of CLIC2, one of the least-studied members of this family, are reported. Human CLIC2 was crystallized in two different forms, both in the presence of reduced glutathione and both of which diffracted to better than 1.9 Å resolution. Crystal form A displayed P212121 symmetry, with unit-cell parameters a = 44.0, b = 74.7, c = 79.8 Å. Crystal form B displayed P21 symmetry, with unit-cell parameters a = 36.0, b = 66.9, c = 44.1 Å. Structure determination will shed more light on the structure and function of this enigmatic family of proteins.
PMCID: PMC2339751  PMID: 18007051
chloride intracellular channel 2; chloride-channel proteins
6.  Crystallization and preliminary X-ray diffraction analysis of the ternary human GM-CSF receptor complex 
Crystals of the soluble ternary GM-CSF receptor complex were obtained which diffracted to a resolution of 3.3 Å.
Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a haemopoietic growth factor that acts though a ternary receptor signalling complex containing specific α (GMRα) and common β (βc) receptor subunits. Human GM-CSF is encoded by the gene csf2, while the genes for GMRα and βc are csf2ra and csf2rb, respectively. Crystals of the ternary ectodomain complex comprising GM-CSF and the soluble extracellular regions of both the GMRα subunit and either βc or its glutamine-substitution mutant N346Q were obtained using the hanging-drop vapour-diffusion method. The best diffracting crystals of the ternary complex were obtained using the N346Q mutation of the βc subunit. These crystals grew using polyethylene glycol 3350 with a high concentration of proline, belonged to space group P6322 and diffracted to 3.3 Å resolution.
PMCID: PMC2494959  PMID: 18678938
arthritis; asthma; cytokines; cytokine receptor complex; GM-CSF; interleukins; leukaemia; signalling
7.  The SARS-Unique Domain (SUD) of SARS Coronavirus Contains Two Macrodomains That Bind G-Quadruplexes 
PLoS Pathogens  2009;5(5):e1000428.
Since the outbreak of severe acute respiratory syndrome (SARS) in 2003, the three-dimensional structures of several of the replicase/transcriptase components of SARS coronavirus (SARS-CoV), the non-structural proteins (Nsps), have been determined. However, within the large Nsp3 (1922 amino-acid residues), the structure and function of the so-called SARS-unique domain (SUD) have remained elusive. SUD occurs only in SARS-CoV and the highly related viruses found in certain bats, but is absent from all other coronaviruses. Therefore, it has been speculated that it may be involved in the extreme pathogenicity of SARS-CoV, compared to other coronaviruses, most of which cause only mild infections in humans. In order to help elucidate the function of the SUD, we have determined crystal structures of fragment 389–652 (“SUDcore”) of Nsp3, which comprises 264 of the 338 residues of the domain. Both the monoclinic and triclinic crystal forms (2.2 and 2.8 Å resolution, respectively) revealed that SUDcore forms a homodimer. Each monomer consists of two subdomains, SUD-N and SUD-M, with a macrodomain fold similar to the SARS-CoV X-domain. However, in contrast to the latter, SUD fails to bind ADP-ribose, as determined by zone-interference gel electrophoresis. Instead, the entire SUDcore as well as its individual subdomains interact with oligonucleotides known to form G-quadruplexes. This includes oligodeoxy- as well as oligoribonucleotides. Mutations of selected lysine residues on the surface of the SUD-N subdomain lead to reduction of G-quadruplex binding, whereas mutations in the SUD-M subdomain abolish it. As there is no evidence for Nsp3 entering the nucleus of the host cell, the SARS-CoV genomic RNA or host-cell mRNA containing long G-stretches may be targets of SUD. The SARS-CoV genome is devoid of G-stretches longer than 5–6 nucleotides, but more extended G-stretches are found in the 3′-nontranslated regions of mRNAs coding for certain host-cell proteins involved in apoptosis or signal transduction, and have been shown to bind to SUD in vitro. Therefore, SUD may be involved in controlling the host cell's response to the viral infection. Possible interference with poly(ADP-ribose) polymerase-like domains is also discussed.
Author Summary
The genome of the SARS coronavirus codes for 16 non-structural proteins that are involved in replicating this huge RNA (approximately 29 kilobases). The roles of many of these in replication (and/or transcription) are unknown. We attempt to derive conclusions concerning the possible functions of these proteins from their three-dimensional structures, which we determine by X-ray crystallography. Non-structural protein 3 contains at least seven different functional modules within its 1922-amino-acid polypeptide chain. One of these is the so-called SARS-unique domain, a stretch of about 338 residues that is completely absent from any other coronavirus. It may thus be responsible for the extraordinarily high pathogenicity of the SARS coronavirus, compared to other viruses of this family. We describe here the three-dimensional structure of the SARS-unique domain and show that it consists of two modules with a known fold, the so-called macrodomain. Furthermore, we demonstrate that these domains bind unusual nucleic-acid structures formed by consecutive guanosine nucleotides, where four strands of nucleic acid are forming a superhelix (so-called G-quadruplexes). SUD may be involved in binding to viral or host-cell RNA bearing this peculiar structure and thereby regulate viral replication or fight the immune response of the infected host cell.
PMCID: PMC2674928  PMID: 19436709

Results 1-7 (7)