Toxoplasma gondii is the leading cause of local central nervous system infections in patients with AIDS. T. gondii is a member of the apicomplexan family of parasites, which includes crytosporidium and plasmodium species, all of which are obligate intracellular parasites. For Toxoplasma and Plasmodia, the vacuole is surrounded by a specialized membrane, termed the parasitophorous vacuole membrane (PVM), which is the interface between the parasite and the host cell. Although selected characteristics of the T. gondii PVM are understood, other features are more well defined and/or easier to study in plasmodium-infected erythrocytes. In particular, the biosynthesis, transport, and subsequent topology of the transmembrane protein EXP-1 within the P. falciparum PVM is established, and is the transport of a subset of soluble proteins (including glycophorin-binding protein, or GBP) across the PVM and into the erythrocyte cytosol. To facilitate our understanding of the T. gondii PVM, using tools developed to study P. falciparum-infected erythrocytes, the Investigator proposes to: (1) identify the topology of EXP-1 expressed in Toxoplasma and determine the subcellular localization and transport of the protein in T. gondii- infected cells, and (2) identify T. gondii proteins which are transported across the PVM and into the host cell cytosol and explore the mechanism for this process. These experiment will simultaneously provide insight into the structure and function of the T. gondii PVM and will delineate both the similarities and differences between the PVM in the two parasites. This collaboration between a U.S. laboratory with expertise in T. gondii biology and a german laboratory with expertise in Plasmodia biology will foster our understanding of the PVM of both parasites and have implication for drug delivery into the intracellular organisms.