Project Summary:Approximately 20% of the people on Earth are permanently infected with the unicellular parasite,Toxoplasma gondii. T. gondii is one of the leading causes of congenital neurological defects inhumans, and an agent for devastating opportunistic infections in immunocompromised individuals.Ocular toxoplasmosis is one of the most common retinal infections and can lead to blindness. As anobligate intracellular parasite, T. gondii must first invade cells of its host in order to replicate. In thepreliminary study, we found that the disruption of an iconic structure in T. gondii, the conoid, is linked todrastically impaired invasion. This is an aspect of invasion that has not been explored, as previouswork on invasion has mainly centered on the regulation and activity of the actomyosin motor complexand protein secretion from specialized membrane-bound organelles. To explore this new direction, wepropose two complementary aims to (a) determine the function of the conoid in host cell invasion (Aim1), and (b) elucidate how this novel cytoskeletal complex is constructed (Aim 2). Using the mutants wehave generated, Aim 1 will determine whether the conoid facilitates invasion by secretion, motility,directly assisting the physical interaction between the parasite and the host cell during attachment, orovercoming the cortical tension of the host cell. Aim 2 will identify and characterize structuraldeterminants of the conoid by integrating gene discovery with structural and functional analyses. Wewill combine state of the art imaging, biophysical, biochemical, and evolutionary approaches. Thiscomprehensive strategy will greatly enhance our ability to identify novel components in this uniquestructure as drug targets.