Toxoplasma gondii is a widespread apicomplexan parasite that is a common cause of serious food borne illness in the USA and which causes opportunistic disease in immunocompromised individuals. Toxoplasma gondii offers a number of advantages as a model system for studying the biology of related apicomplexan parasites including excellent animals models, robust in vitro systems, and forward and reverse genetics. Consequently, advances made in this system may also be relevant to other important human pathogens such as Cryptosporidium and Plasmodium (the causative agent of malaria). <P> Apicomplexan parasites rely on active invasion to enter host cells and this obligatory step is controlled in part by the regulated secretion of adhesins from micronemes. Microneme secretion is a calcium-dependent process and agents that disrupt calcium signaling block secretion, motility, and invasion. <P> Our previous studies have shown that calcium levels in the parasite undergo oscillations that correlate with motility. Disruption of these calcium oscillations prevents motility and cell invasion by the parasite. Recent evidence in malaria suggests that the Chinese herbal medicine Qinghaosu (Artemisinin) acts directly on one component of the calcium regulatory system, the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA). To expand further on this discovery, we have recently cloned the SERCA gene from T. gondii. <P> In AIM 1, we will characterize the expression and localization of SERCA in T. gondii and test the hypothesis that artemisinin binds to and inhibits the calcium pumping action of SERCA, thus disrupting parasite survival.<P> In AIM2, we plan to use to robust genetic systems available in T. gondii to confirm the molecular target of artemisinin and to map the active site of the compound. <P> This project addresses the need for greater understanding of fundamental processes in pathogens through the examination of regulatory pathways in calcium homeostasis. Knowledge gained through these studies may improve the ability to pharmacologically inhibit key calcium regulatory enzymes and hence block motility and invasion by this important group of parasites.