A unique feature of parasites of the phylum Apicomplexa, such as Toxoplasma gondii, is the presence of a single tubular mitochondrion, which is essential for parasite survival and a validated drug target. Most studies of the apicomplexan mitochondrion have focused on its biochemistry and physiology. By contrast little is known about the machinery that controls mitochondrial division and that regulate its structure, information that would be critical for a thorough exploration of the mitochondrion as a drug target. Toxoplasma's singular mitochondrion is very dynamic and undergoes morphological changes throughout the parasite's life cycle including during the transition from the intracellular to the extracellular environment. While inside a host cell the mitochondrion is maintained in a lasso shape that stretches around the parasite periphery where it has regions of coupling with the parasite pellicle, suggesting the presence of membrane contact sites. Promptly after exit from the host cell, these contact sites disappear, and the mitochondrion collapses indicating that dynamic membrane contact sites regulate the positioning of the mitochondrion. Neither the functional significance nor the proteins needed for the contact between Toxoplasma's mitochondrion and pellicle are known. We have discovered a novel protein, Fip1, that associates with the mitochondrion and that when knocked out the normal morphology of the mitochondrion is severely affected. In intracellular fip1 knockout parasites the mitochondrion is not in a lasso shape as seen in wildtype parasites, but instead it is collapsed. Additionally, proper mitochondrial segregation is disrupted in the knockout parasites, resulting in parasites with no mitochondrion and mitochondrial material outside of the parasites. These gross morphological changes are associated with a significant reduction of parasite propagation and can be rescued by reintroduction of a wildtype copy of Fip1. Accordingly, we hypothesize that Fip1 mediates contact between the mitochondrion and the parasite pellicle in a regulatable fashion, and that the Fip1 dependent mitochondrial morphology and dynamics are critical for parasite propagation. Through a combination of molecular genetics, microscopy and proteomics we will address the functional relevance and the mechanics of the mitochondrial morphology. In aim one we will conduct a thorough in vivo and in vitro phenotypic characterization of Fip1 mutant strains to determine the role of Fip1 and mitochondrial shape in parasite viability. Aim two focuses on identifying and characterizing components of the Fip1 complex that mediates the association of the mitochondrion with the periphery of the parasites. Finally, in aim three we will determine the regulatory mechanisms that drive the mitochondrial morphological changes as the parasite exits its host cell. In conjunction, these experiments will shed light onto the molecular mechanisms driving and regulating the morphodynamics of ...