The protozoan parasite Toxoplasma gondii is an important human pathogen. As many as 4000 children are born each year in the USA with congenital toxoplasmosis, the symptoms of which range from impaired vision to hydrocephalus and death. T. gondii has also emerged as a major opportunistic pathogen of immunocompromised persons. Toxoplasmic encephalitis is one of the most common causes of central nervous system pathology in AIDS patients: it has been estimated that 20-47 percent of AIDS patients in the USA develop toxoplasmic encephalitis, which is uniformly fatal if left untreated. There is an urgent need for the development of new therapeutic approaches to treating toxoplasmosis, particularly for AIDS patients. The pathology of toxoplasmosis is the direct result of host cell invasion by the parasite, followed by parasite replication and host cell lysis. Little is known about proteins on the surface of the parasite which mediate host cell invasion. The goal of the proposed work is to fill in this gap in our knowledge, by identifying novel parasite surface proteins and determining their biological function. The identification of surface proteins which play a role in invasion may be an important first step in the development of new chemotherapeutic or vaccine-based approaches to preventing or controlling acute infection. Specific Aims. All of the parasite surface proteins identified to date are anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. None of these proteins appears to play an essential role in invasion. They are abundant and immunogenic, making the detection of other surface proteins difficult. We will take three independent approaches to identifying previously undescribed T. gondii surface proteins: (1) Non-GPI-linked surface proteins will be identified by surface labeling parasites after enzymatic removal of the GPI-linked proteins; (2) Photoaffinity methods will be used to specifically identify transmembrane surface proteins; and (3) A novel hybridoma screening strategy will be used to identify externally exposed parasite proteins which play a role in invasion. Immunochemical, cell biological, and molecular genetic approaches will then be used to (4) Determine the biological function of new parasite surface proteins identified in the course of accomplishing Specific Aims 1-3.