17.08.2011

Molecular Tricks of the Motile Malaria Pathogen

How Plasmodium falciparum is able to quickly alter its cytoskeleton

It infects men and mosquitoes, changes its form multiple times, and moves very elegantly and rapidly through the body of its host: Plasmodium falciparum, the causative agent of malaria, shows an astonishing versatility and motility. An international team of researchers has now investigated the molecular basis behind this behaviour. They discovered that the pathogen is able to regulate its cytoskeleton very flexibly in an unusual way – with biochemical tools that, to this extent, are not known in unicellular organisms. The results have now been published in the current issue of the scientific magazine "Journal of Biological Chemistry". The work was done in a German-Finnish-Swedish collaboration, led by the Helmholtz Centre for Infection Research (Helmholtz-Zentrum für Infektionsforschung, HZI), at the Centre for Structural Systems Biology (CSSB) at DESY in Hamburg.

The cytoskeleton is a molecular scaffolding structure in highly developed cells. In the malaria parasite, it plays a pivotal role during motility within the infected host and during infection of liver or red blood cells. The researchers have now deciphered how certain factors regulate and affect the cytoskeleton of the malaria pathogen – utilizing proteins that no other Protozoa possess.

More than 300 million people worldwide are infected with the malaria pathogen Plasmodium falciparum, causing one million fatalities every year – half of them children. 90 percent of the patients live on the African continent. At the moment, no protective vaccine against malaria is available. Chloroquine- or artemisinin-based drugs help; resistance against the existing drugs is increasing, however.

Plasmodium falciparum displays a very sophisticated and complex life cycle that includes different developmental stages within mosquitoes and human beings. The pathogen enters our body by a sting from an infected Anopheles mosquito. Soon after, the parasites infest liver cells, where they multiply to subsequently get back into the blood stream. The next targets are the red blood cells, in which they mature – leading to rupture of the red blood cells and severe anaemia. Furthermore, the pathogens produce toxins that affect the host cells and cause the well-known fever attacks.

To move in a targeted manner within its host, Plasmodium falciparum is dependent on a fine-tuned regulation of its cytoskeleton, which is composed of actin building blocks. Actin can further congregate to fibres, so-called microfilaments, and regulatory proteins adjust the length of these filaments. Dr. Inari Kursula, a scientist at the HZI Structural Biology Division, took a deeper look at two of the regulatory proteins, the actin depolymerizing factors ADF1 and ADF2, which control the assembly and dismantling of the actin filaments. Both factors show very different behaviour, despite being closely related to each other. While ADF1 only binds the single actin components of the cytoskeleton and prepares them for the integration into the microfilament, ADF2 is doing the exact opposite: It breaks up actin filaments.

"It is very unusual that Plasmodium possesses two ADF proteins," says Dr. Inari Kursula. These parasites belong to the family of unicellular Protozoa, which usually only have a single ADF. Additionally, according to Dr. Inari Kursula, the actin filaments of these parasites are very short compared to actin filaments of its host.

To investigate the structural and functional differences of both Plasmodium ADFs, Dr. Inari Kursula deciphered the structure of both proteins with her colleagues from DESY and University of Oulu, Finland, using highly sophisticated synchrotron radiation facilities at both DESY and MAX-Lab, Lund, Sweden. The team discovered that the filament-severing ADF2 protein possesses a kind of molecular shovel that slots itself between the building blocks of the filament, resulting in the cleavage of the filament. ADF1 lacks the structures required for filament binding and rather functions as an efficient monomer sequesterer, providing a constant pool of new building blocks to be rapidly inserted into the growing filament.

This unusual regulation of the cytoskeleton may be an adaptation of the malaria pathogen to its two very different hosts, man and mosquito, says Dr. Inari Kursula. As pathogens, Plasmodia have to quickly alter and rearrange their cytoskeleton, in order to be motile and to infect cells. Both the short microfilaments and the two different ADFs play an important role during those processes.

"New knowledge about these unique mechanisms in the malaria pathogen may help to develop alternative drugs or new therapies," concludes Dr. Inari Kursula.