Actomyosin motor in the merozoite of the malaria parasite, Plasmodium falciparum: implications for red cell invasion

A potentially exhaustive screening strategy reveals two novel divergent myosins in Dictyostelium

In recent years, the myosin superfamily has kept expanding at an explosive rate, but the understanding of their complex functions has been lagging. Therefore, Dictyostelium discoideum, a genetically and biochemically tractable eukaryotic amoeba, appears as a powerful model organism to investigate the involvement of the actomyosin cytoskeleton in a variety of cellular tasks. Because of the relatively high degree of functional redundancy, such studies would be greatly facilitated by the prior knowledge of the whole myosin repertoire in this organism. Here, we present a strategy based on PCR amplification using degenerate primers and followed by negative hybridization screening which led to the potentially exhaustive identification of members of the myosin family in D. discoideum. Two novel myosins were identified and their genetic loci mapped by hybridization to an ordered YAC library. Preliminary inspection of myoK and myoM sequences revealed that, despite carrying most of the hallmarks of myosin motors, both molecules harbor features surprisingly divergent from most known myosins.

Toxoplasma gondii motility and host cell invasiveness are drastically impaired by jasplakinolide, a cyclic peptide stabilizing F-actin

Actin polymerization and actin-myosin coupling activity most likely provide the driving force that the protozoan parasite Toxoplasma gondii has to exert to propulse itself during gliding and host cell entry. Nevertheless, little information is available on T. gondii tachyzoite actin dynamics, and in particular, the presence of actin filaments remains largely uncharacterized. Here, we report that the marine sponge peptide jasplakinolide, known to bind to filamentous actin, does indeed stabilize a pool of a parasite detergent-insoluble actin. This pool is likely to be formed by a dynamic assembled actin complex: first, it is competent for assembly/disassembly and secondly, it is sensitive to nucleotide phosphate concentration. In addition, T. gondii tachyzoites contain molecules which inhibit actin assembly and destabilize actin filaments. Thus, these activities could account for the remarkably low amount of the myosin-containing F-actin pool we describe here. Furthermore, when parasites are treated with cell-permeant jasplakinolide, they display a significant loss of both motility and host cell invasiveness. These data suggest that in vivo, the detergent-insoluble pool of actin is dynamic.

Theileria parva: sporozoite entry into bovine lymphocytes is not dependent on the parasite cytoskeleton

The main conclusion from the present study is that T. parva sporozoite entry is dependent on a functional host cell actin cytoskeleton and is not driven by the parasite. Treating lymphocytes with cytochalasin D resulted in a dose-dependent reduction in the levels of host cell infection. However, the primary effect was to block sporozoite binding and only at the highest concentration (20 microM) was sporozoite internalization significantly reduced. In fact at lower concentrations (1-10 microM) cytochalasin treatment lead to a relative increase in sporozoite internalization. The results are consistent with sporozoite entry being primarily a passive process and with a functional host cell actin cytoskeleton that is required only to maintain the molecular integrity of the surface membrane. Thus T. parva sporozoite entry differs from the process in other apicomplexans, although the results are consistent with a number of features of sporozoite biology. Treatment of lymphocytes with either the microtubule-destabilizing agent, nocodazole, or taxol, which induces microtubule polymerization, had no significant effect on sporozoite binding or entry. As both reagents had the expected effects on the lymphocyte microtubule system, it is unlikely that host cell microtubules are essential for successful sporozoite invasion or establishment.

Intraerythrocytic Plasmodium falciparum expresses a high affinity facilitative hexose transporter

Asexual stages of Plasmodium falciparum cause severe malaria and are dependent upon host glucose for energy. We have identified a glucose transporter of P. falciparum (PfHT1) and studied its function and expression during parasite development in vitro. PfHT1 is a saturable, sodium-independent, and stereospecific transporter, which is inhibited by cytochalasin B, and has a relatively high affinity for glucose (Km = 0.48 mM) when expressed in Xenopus laevis oocytes. Competition experiments with glucose analogues show that hydroxyl groups at positions C-3 and C-4 are important for ligand binding. mRNA levels for PfHT1, assessed by the quantitative technique of tandem competitive polymerase chain reaction, are highest during the small ring stages of infection and lowest in gametocytes. Confocal immunofluorescence microscopy localizes PfHT1 to the region of the parasite plasma membrane and not to host structures. These findings have implications for development of new drug targets in malaria as well as for understanding of the pathophysiology of severe infection. When hypoglycemia complicates malaria, modeling studies suggest that the high affinity of PfHT1 is likely to increase the relative proportion of glucose taken up by parasites and thereby worsen the clinical condition.