Short communication

Uptake and hydrolysis of sphingomyelin analogues in Plasmodium falciparum-infected red cells

Sphingomyelinase (SMase) activity was measured in Entamoeba histolytica particulate and soluble subcellular fractions. The effects on SMase of incubation time, total protein concentration, pH, and several divalent cations were determined. SMase-C and other unidentified esterase activity were detected in soluble and particulate fractions. SMase-C was 94.5–96.0% higher than the unidentified esterase activity. Soluble and insoluble SMase-C specific activities increased with protein dose and incubation time. Soluble and insoluble SMase-C activities were maximum at pH 7.5 and were dependent on Mg²⁺, Mn²⁺, or Co²⁺, and inhibited by Zn²⁺, Hg²⁺, Ca²⁺, and EDTA. SMase-C was active in the pH range of 3–10 and its maximum activity was at pH 7.5. The soluble and insoluble SMases have remarkably similar physicochemical properties, strongly suggesting that E. histolytica has just one isoform of neutral SMase-C that had not been described before and might be essential for E. histolytica metabolism or virulence.

The cell biology of intracellular pathogens (viruses, bacteria, eukaryotic parasites) has provided us with molecular information of host–pathogen interactions. As a result it is becoming increasingly evident that lipids play important roles at various stages of host–pathogen interactions. They act in first line recognition and host cell signaling during pathogen docking, invasion and intracellular trafficking. Lipid metabolism is a housekeeping function in energy homeostasis and biomembrane synthesis during pathogen replication and persistence. Lipids of enormous chemical diversity play roles as immunomodulatory factors. Thus, novel biochemical analytics in combination with cell and molecular biology are a promising recipe for dissecting the roles of lipids in host–pathogen interactions.

A novel dynamin-like GTPase gene, Pfdyn1, was cloned from an asexual stage cDNA library of Plasmodium falciparum Dd2 strain. Pfdyn1 contains a highly conserved N-terminal tripartite GTPase domain, a coiled-coil region, and a C-terminal 129 aa unknown function domain. Like yeast Vps1p, it lacks pleckstrin homology domain and proline-rich region. Western blot analysis showed that Pfdyn1 is a Triton X-100 insoluble protein expressed only in the mature sub-stage. Morphological studies indicated that Pfdyn1 is partly co-localized with PfGRP, a known ER-resident protein, and localizes diffusely with several membrane structures and a 60–100 nm vesicle both inside and on surface of the parasites and also in the cytoplasm of infected erythrocytes. The dsRNA originated by C-terminus fragment of Pfdyn1 inhibits markedly the growth of P. falciparum parasite at the erythrocyte stage. Those data showed that Pfdyn1 is a conservative, membrane related protein and plays an essential role for the survival of Plasmodium parasite.

The emergence and spread of drug-resistant parasites coupled with the absence of an effective vaccine makes malaria treatment more complicated, and thus the development of new antimalarial drugs is one of the urgent tasks in malaria research. This review highlights lipid metabolism in Plasmodium parasite cells, the study of which would lead to providing new targets for therapeutic intervention.

The malarial “apicoplast” derived from an algal plastid, has stimulated interest for its novel evolutionary biology and potential as a drug target. An endoplasmic reticulum-type signal sequence followed by a plastid targeting sequence are required to target proteins to the apicoplast but the pathway by which proteins are transported to the organelle is unknown. By stage regulating the expression of transgenes we show that early (0–12 h) in the parasite’s development in red cells, newly synthesized green fluorescent protein that contains the plastid targeting sequence (plastid targeting sequence-green fluorescent protein (PTS-GFP)) is recruited into the parasite’s secretory pathway. PTS-GFP in 0–12-h parasites is found released into the parasitophorous vacuole (PV) and in apposition with the Golgi. However, import into the apicoplast and processing to GFP does not occur until 18–36 h in development. In intermediate, 18-h parasites PTS-GFP resides in the PV. Quantitative exit of PTS-GFP from the PV and its conversion to GFP is seen at 36 h. The data suggest that: (i) import into the apicoplast is stage regulated and (ii) the PTS can signal endomembrane targeting from the PV to the apicoplast.