Barbara Clough

A Subtilisin-like Protein in Secretory Organelles of Plasmodium falciparum Merozoites

In the vertebrate host, the malaria parasite invades and replicates asexually within circulating erythrocytes. Parasite proteolytic enzymes play an essential but poorly understood role in erythrocyte invasion. We have identified a Plasmodium falciparum gene, denoted pfsub-1, encoding a member of the subtilisin-like serine protease family (subtilases). Thepfsub-1 gene is expressed in asexual blood stages ofP. falciparum, and the primary gene product (PfSUB-1) undergoes post-translational processing during secretory transport in a manner consistent with its being converted to a mature, enzymatically active form, as documented for other subtilases. In the invasive merozoite, the putative mature protease (p47) is concentrated in dense granules, which are secretory organelles located toward the apical end of the merozoite. At some point following merozoite release and completion of erythrocyte invasion, p47 is secreted from the parasite in a truncated, soluble form. The subcellular location and timing of secretion of p47 suggest that it is likely to play a role in erythrocyte invasion. PfSUB-1 is a new potential target for antimalarial drug development.

Enzymes for heme biosynthesis are found in both the mitochondrion and plastid of the malaria parasite Plasmodium falciparum.

All eight enzymes required for de novo heme biosynthesis have been predicted from the nuclear genome of the human malaria parasite Plasmodium falciparum. We have studied the subcellular localization of three of these using a GFP reporter in live transfected parasites. The first enzyme in the pathway delta-aminolevulinic acid synthase (ALAS) is targeted to the mitochondrion, but the next two enzymes porphobilinogen synthase (PBGS) and hydroxymethylbilane synthase (HMBS) are targeted to the plastid. An enzymatically active recombinant version of PBGS from P. falciparum was over-expressed and its activity found to be stimulated by Mg2+ (and enhanced by Mn2+) but not by Zn2+. A hypothetical scheme for the exchange of intermediates in heme biosynthesis between the mitochondrion and plastid organelle, as well as organelle attachment is discussed.

Validation of N -myristoyltransferase as an antimalarial drug target using an integrated chemical biology approach

Malaria is an infectious disease caused by parasites of the genus Plasmodium, which leads to approximately one million deaths per annum worldwide. Chemical validation of new antimalarial targets is urgently required in view of rising resistance to current drugs. One such putative target is the enzyme N-myristoyltransferase, which catalyses the attachment of the fatty acid myristate to protein substrates (N-myristoylation). Here, we report an integrated chemical biology approach to explore protein myristoylation in the major human parasite P. falciparum, combining chemical proteomic tools for identification of the myristoylated and glycosylphosphatidylinositol-anchored proteome with selective small-molecule N-myristoyltransferase inhibitors. We demonstrate that N-myristoyltransferase is an essential and chemically tractable target in malaria parasites both in vitro and in vivo, and show that selective inhibition of N-myristoylation leads to catastrophic and irreversible failure to assemble the inner membrane complex, a critical subcellular organelle in the parasite life cycle. Our studies provide the basis for the development of new antimalarials targeting N-myristoyltransferase.

Basis of unique red cell membrane properties in hereditary ovalocytosis

Hereditary ovalocytes from a Mauritian subject are extremely rigid, with a shear elastic modulus about three times that of normal cells, and have increased resistance to invasion by the malaria parasite Plasmodium falciparum in vitro. The genetic anomaly resides in band 3; the protein gives rise to chymotryptic fragments with reduced mobility in SDS/polyacrylamide gel electrophoresis, but this is a result of anomalous binding of SDS and not a higher molecular weight. Analysis of the band 3 gene reveals (1) a point mutation (Lys56----Glu), which also occurs in a common asymptomatic band 3 (Memphis) variant and governs the electrophoretic properties, and (2) a deletion of nine amino acid residues, including a proline residue, encompassing the interface between the membrane-associated and the N-terminal cytoplasmic domains. The interaction of the mutant band 3 with ankyrin appears unperturbed. The fraction of band 3 capable of undergoing translation diffusion in the membrane is greatly reduced in the ovalocytes. Cells containing the asymptomatic band 3 variant were normal with respect to all the properties that we have studied. Possible mechanisms by which a structural change in band 3 at the membrane interface could regulate rigidity are examined.

Thiostrepton binds to malarial plastid rRNA

Binding of the thiazolyl peptide antibiotic thiostrepton to the GTPase domain of 23S rRNA involves a few crucial nucleotides, notably A1067 (E. coli). Small RNA transcripts were prepared corresponding to the GTPase domain of the plastid 23S rRNA and the two forms of cytosolic 28S rRNAs found in the human malaria parasite Plasmodium falciparum, as well as the plastid form of rRNA of the AIDS‐related pathogen Toxoplasma gondii. Binding affinities of the wild type and mutated RNA sequences were as predicted; the malarial plastid sequence had by far the highest affinity, whereas that from toxoplasma did not bind thiostrepton.

Antibiotic Inhibitors of Organellar Protein Synthesis in Plasmodium falciparum

Elongation factor Tu (EF-Tu) is encoded by the tuf gene of the plastid organelle of the malaria parasite Plasmodium falciparum. A range of structurally unrelated inhibitors of this GTP-dependent translation factor was shown to have antimalarial activity in blood cultures. The most active was the cyclic thiazolyl peptide amythiamicin A with an IC50 = 0.01 microM. Demonstrable complexes were formed in vitro between a recombinant version of P. falciparum EF-Tu(pl) and inhibitors that bind to different sites on EF-Tu; these included the antibiotics kirromycin, GE2270A and enacyloxin IIa.

Cell Death of Gamma Interferon-Stimulated Human Fibroblasts upon Toxoplasma gondii Infection Induces Early Parasite Egress and Limits Parasite Replication

ABSTRACT The intracellular protozoan parasite Toxoplasma gondii is a major food-borne illness and opportunistic infection for the immunosuppressed. Resistance to Toxoplasma is dependent on gamma interferon (IFN-γ) activation of both hematopoietic and nonhematopoietic cells. Although IFN-γ-induced innate immunity in nonhematopoietic cells has been extensively studied in mice, it remains unclear what resistance mechanisms are relied on in nonhematopoietic human cells. Here, we report an IFN-γ-induced mechanism of resistance to Toxoplasma in primary human foreskin fibroblasts (HFFs) that does not depend on the deprivation of tryptophan or iron. In addition, infection is still controlled in HFFs deficient in the p65 guanylate binding proteins GBP1 or GBP2 and the autophagic protein ATG5. Resistance is coincident with host cell death that is not dependent on the necroptosis mediator RIPK3 or caspases and is correlated with early egress of the parasite before replication. This IFN-γ-induced cell death and early egress limits replication in HFFs and could promote clearance of the parasite by immune cells.

A study of red cell membrane properties in relation to malarial invasion.

The shape and mechanical properties of human red cells were modified in several ways and the consequences for the efficiency of invasion by Plasmodium falciparum in culture were investigated. Inhibition of invasion by depletion of ATP was shown to be unrelated to cell shape or deformability changes. Treatment of cells with N-ethylmaleimide (NEM), which dissociates some 70% of the native spectrin tetramers into the dimer, grossly reduced deformation of the cells under shear and increased by a factor of two or more the shear elastic modulus, as measured by the micropipette aspiration technique. Cells thus treated were efficiently invaded by P. falciparum (ca. 75% of control). In a population of cells pretreated with chlorpromazine, parasites were found in stomatocytic cells which were highly undeformable under shear. There was also considerable invasion into cells from subjects with hereditary pyropoikilocytosis, and two types of elliptocytosis. Cells treated with wheat germ agglutinin showed a dose-dependent increase in rigidity; a fivefold increase in elastic modulus (with total loss of deformation under shear in our conditions) still permitted invasion at a level of 50% of the control. The results suggest that gross mechanical properties of the membrane per se, at least within any physiologically relevant range, are unlikely to be the primary determinant of malarial invasion; this may instead be linked to the freedom of membrane proteins to migrate in the course of entry of the parasite.

Hydrodynamic characterization of the SufBC and SufCD complexes and their interaction with fluorescent adenosine nucleotides

Bacteria, as well as the plastid organelles of algae and higher plants, utilize proteins of the suf operon. These are involved in Fe‐S cluster assembly, particularly under conditions of iron limitation or oxidative stress. Genetic experiments in some organisms found that the ATPase SufC is essential, though its role in Fe‐S biogenesis remains unclear. To ascertain how interactions with other individual Suf proteins affect the activity of SufC we coexpressed it with either SufB or SufD from Thermotoga maritima and purified the resulting SufBC and SufCD complexes. Analytical ultracentrifuge and multiangle light‐scattering measurements showed that the SufBC complex exists in solution as the tetrameric SufB2C2 species, whereas SufCD exists as an equilibrium mixture of SufCD and SufC2D2. Transient kinetic studies of the complexes were made using fluorescent 2′(3′)‐O‐(N‐methylanthraniloyl‐(mant) analogues of ATP and ADP. Both SufBC and SufCD bound mantATP and mantADP much more tightly than does SufC alone. Compared to the cleavage step of the mantATPase of SufC alone, that of SufBC was accelerated 180‐fold and that of SufCD only fivefold. Given that SufB and SufD have 20% sequence identity and similar predicted secondary structures, the different hydrodynamic properties and kinetic mechanisms of the two complexes are discussed.

Substituted imidazopyridazines are potent and selective inhibitors of Plasmodium falciparum calcium-dependent protein kinase 1 (PfCDPK1)

Graphical abstract