Apicomplexan parasites employ common mechanisms for protein secretion, translocation, and proteolytic processing of adhesins. Efficient invasion is mediated by coupling these events to promote apical attachment and cell entry. Toxoplasma gondii provides an excellent experimental system for delineating the molecular mechanisms of invasion by this important group of parasites. The proposed studies will focus on three key steps in parasite invasion using the Toxoplasma adhesin MIC2 as a model. MIC2 is a member of the TRAP family of proteins, which are important for invasion in Toxoplasma and malaria. In AIM1 we will explore the role of the extracellular domains of MIC2 in multimerization and receptor binding. Following translocation to the posterior end of the cell, MIC2 and related adhesins are processed via an intramembranous cleavage event, before being shed into the supernatant. Disruption of MIC2 processing prevents efficient cell entry, highlighting the importance of this step. In AIM2 we will characterize putative rhomboid-like proteases that process cell-surface adhesins such as MIC2 during invasion. MIC2 is a type I transmembrane protein that links adhesion to the cytoskeleton by interacting with aldolase. The specific interactions that mediate bridging between the C-terminal domain of MIC2, aldolase, and the cytoskeleton will be explored in detail in AIMS. Understanding these key steps will provide insights into the entry mechanisms utilized by T. gondii and related apicomplexans.