A comprehensive survey of protein-protein interactions between Plasmodium falciparum merozoites and human receptors

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Malaria is one of the most common infectious diseases in the world, with more than 300 million cases of malaria each year, leading to more than 1 million deaths, primarily in children under the age of five in Africa. Malaria is caused by single-celled Plasmodium parasites and one species is particularly deadly - Plasmodium falciparum - which is responsible for almost all malaria deaths. Plasmodium parasites have a complex life cycle, but all the symptoms of malaria are caused when they invade human red blood cells, also known as erythrocytes. The parasite uses the erythrocyte as a source of food, multiplies inside it, and, after 48 hours, breaks open the erythrocyte to release multiple new parasites. This cycle of invasion, multiplication and re-invasion results in high numbers of parasites in the blood stream.Because Plasmodium parasites spend the majority of their life cycle inside human cells, they are difficult to target with vaccines, and there is currently no licensed vaccine for malaria. However, the process of erythrocyte invasion is one of the few stages of the parasite's life cycle when they are exposed to the host immune system, and is therefore a potential target for vaccine-induced invasion-blocking antibodies or drug development. For vaccines or drugs to be developed, it is critical that we understand which proteins on the parasite surface are binding directly to erythrocytes and the identity of the erythrocyte proteins that they are binding to. By first identifying these parasite-erythrocyte interactions, it may be possible to develop novel therapeutics that prevent invasion and hence cure or prevent malaria. Research over the last two decades has led to the identification of numerous P. falciparum proteins that may be involved in erythrocyte invasion, but only in very cases do we know which erythrocyte proteins they bind to. This is in large part because of the technical challenges involved: protein-protein interactions between the surface of cells are often very short-lived (lasting only a few seconds) and so are very difficult to detect using standard experimental approaches.At the Wellcome Trust Sanger Institute we have recently developed a new approach to detect short-lived protein-protein interactions. Called AVEXIS (Avidity based Extracellular Interaction Screen), it uses libraries of proteins, expressed and purified in the lab, to detect novel interactions. Over the last two years we have started to apply this to the process of P. falciparum erythrocyte invasion and have created small pilot libraries of erythrocyte and P. falciparum proteins to screen for interactions. This preliminary work has identified two new parasite-erythrocyte interactions, one of which appears to play a very important role in erythrocyte invasion. However, this initial screen was limited in scope and we still do not have binding partners for the majority of P. falciparum proteins. In this research we will expand our libraries of parasite and erythrocyte proteins, and also include other components of the host blood which P. falciparum parasites may be interacting with, such as sugar structures that are present on cell surfaces. After screening the parasite and blood component libraries for interactions in an all vs. all manner, we will test whether any newly discovered interactions have roles in erythrocyte invasion using P. falciparum parasites that can be cultured in the lab. This research will build up the first systematic map of interactions between P. falciparum proteins and human blood cell components, and will identify protein-protein interactions that are of particular interest for vaccine or drug development. We will also deposit the DNA constructs that are used to create our recombinant protein libraries in not-for-profit reagent resource collections, where they can be freely accessed by other researchers and thereby empower malaria research in many labs across the globe.