Molecular Tricks of the Motile Malaria Pathogen

ADF Protein

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. Researchers from the Helmholtz Centre for Infection Research (Helmholtz-Zentrum für Infektionsforschung, HZI) have 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 at the Centre for Structural Systems Biology (CSSB) at the German Electron Synchrotron (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 HZI 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, P. 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 at the German Electron Synchrotron (Deutsches Elektronensynchrotron, DESY) and University of Oulu, Finland, using highly sophisticated synchrotron radiation 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.  

Publication: Crystal structures explain functional differences in the two actin depolymerization factors of the malaria parasite. Singh BK, Sattler JM, Chatterjee M, Huttu J, Schüler H, Kursula I. J Biol Chem. 2011 Aug 12;286(32):28256-64.

Helmholtz Centre for Infection Research (Helmholtz-Zentrum für Infektionsforschung, HZI):

The Helmholtz Centre for Infection Research contributes to the achievement of the goals of the Helmholtz Association of German Research Centres and to the successful implementation of the research strategy of the German Federal Government. The HZI focuses on the programme “Infection and Immunity”. The goal of the programme is to solve the challenges in infection research and make a contribution to public health with new strategies for the prevention and therapy of infectious diseases.

Deutsches Elektronen-Synchrotron (DESY):

DESY is one of the world’s leading accelerator centres and a member of the Helmholtz Association. DESY develops, builds and operates large particle accelerators used to investigate the structure of matter. DESY offers a broad research spectrum of international standing focusing on three main areas: accelerator development, construction and operation; photon science; particle and astroparticle physics.

Centre for Structural Systems Biology (CSSB):

Located at the DESY campus in Hamburg, the inter-disciplinary Centre for Structural Systems Biology (CSSB) unites partners from various universities and research facilities from Lower-Saxony, Hamburg and Schleswig-Holstein under scientific co-ordination of the Helmholtz Centre for Infection Research (HZI). The CSSB is creating a bridge between structural biology and systems biology; this is the place where biologists, chemists, physicians, physicists and engineers are jointly evaluating the interactions between pathogens and their hosts. In this regard, DESY is able to make available to them, unique in Germany, the following: PETRA III, the world’s most brilliant storage-ring-based X-ray source, and FLASH, the world’s first-ever X-ray free-electron laser in the vacuum ultraviolet and in the soft X-ray region. On this basis, researchers can examine biological samples in various styles – from the structural analysis of individual molecules to real-time representation of the functions within living cells.