Molecular mechanism of gamete fusion in malaria parasite

Project Summary/Abstract Malaria remains a major global health burden with 608,000 deaths and 249 million cases annually. Sexual reproduction of Plasmodium parasites in the mosquito vector is essential for successful transmission and continuation of the malaria life cycle and constitutes a bottleneck in the parasite life cycle. Male and female gamete fertilization and its central cell fusion event are an essential part of sexual reproduction but the molecular mechanisms underlying fertilization are poorly understood. Yet, this process constitutes a valuable target for the development of vaccines that can block transmission. We have recently discovered that Plasmodium has a hybrid fertilization machinery with components from both plant and animal sexual reproduction and gamete fusion machinery suggesting presence of gamete fusion complexes like those present in animals and plants. We have identified three proteins, PfHAP2, PfHAP2p and PfHAP2p2 in the human malaria parasite Plasmodium falciparum (Pf) that are similar to HAP2/GCS1 proteins, which in plants and other eukaryotes, are involved in gamete fusion. We have demonstrated that both PfHAP2 and PfHAP2p are essential for transmission. Discovery of a third HAP2-like protein (HAP2p2) in very interesting as no other organism has three gamete fusion proteins in their genome. PfHAP2p2 is expressed in both male and female gametocytes, gametes and continues to be expressed during fertilization which makes HAP2p2 a target of potent transmission- blocking antibodies. In this application, we will further elucidate the role of PfHAP2p2 in Pf fertilization. In aim 1, we will delineate the functional importance and cell biology of PfHAP2p2. We will also demonstrate its cell fusogenic function. In aim 2, we will establish whether PfHAP2, PfHAP2p and PfHAP2p2 together function in a complex and identify potential additional components of the male and female gamete fusion complexes. We will also establish the utility of PfHAP2p2 as straightforward transmission blocking targets. We will validate these gamete fusion complex proteins for their role in fertilization and establish their relationship to the gamete fusion complex. Together, this work constitutes a critical step towards a comprehensive molecular understanding of Plasmodium gamete fertilization and will inform new ways by which transmission of the parasite can be blocked with vaccines.