Exit of Plasmodium gametes from red blood cells

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Malaria-causing parasites are transmitted to mosquitoes in the form of Plasmodium gametocytes. Within the mosquito these sexual cells exit the infected red blood cell and differentiate rapidly into gametes, which fuse to form a zygote. Particularly striking is the differentiation of the male gametocyte into eight gametes, which constitute very simple sperm cells containing only an axoneme and a nucleus surrounded by a plasma membrane. Formation of male gametes and their egress is tightly coupled but not well understood. In this project we intend to elucidate molecular mechanisms of this coupled process by genetic modification, biochemical experiments and 3D electron tomography. After intracellular genome multiplication gametes need to pull out part of a nucleus containing a set of chromosomes, a process that appears error prone as only about 50% of male gametes contain a nucleus. Nuclear uptake of forming gametes is likely facilitated by a bipartite microtubule organizing center (MTOC) that links cytoplasmic axoneme to nuclear spindle formation. Through analogy to daughter cell formation in Toxoplasma gondii and Plasmodium sporozoite formation in the oocysts, we hypothesize that the striated rootlet fiber provides a link between the two MTOCs. For exit the membranes surrounding the parasite, the parasitophorous vacuole and the red cell plasma membrane, need to be lysed. Several proteins have been identified as being essential for gamete formation and membrane disruption. Yet, we still have only limited mechanistic understanding of the function of these proteins as few protein-protein interactions have been established. We have previously shown that a membrane spanning adhesion protein called MTRAP is important for gamete egress from red blood cells in the rodent infecting parasite Plasmodium berghei. To further understand MTRAP function we will take two main approaches: For functional studies of MTRAP we will genetically delete key structural domains of MTRAP for functional studies, and investigate interaction partners using MTRAP-fusions with biotinylating enzymes. We generated transgenic parasite lines for both approaches during the first funding period and now aim to thoroughly analyse these lines. Lastly, we will investigate the function of striated fiber assemblins, the likely building blocks of the rootlet fiber, with gene deletions and use 3D electron tomography to shed light on these coupled events of gamete egress.