Aggregatibacter actinomycetemcomitans is a gram negative pathogen that is the etiologic agent of localized aggressive periodontitis (LAP) and other systemic infections, including infective endocarditis. LAP, a disease which affects medically-underserved children in both the United States and throughout the world, is characterized by a breakdown of the periodontal ligament and alveolar bone structure that holds the teeth in place. Without intervention, loss of teeth occurs, causing both a cosmetic deformity and a functional defect. The manner by which A. actinomycetemcomitans causes disease has not been determined; however, it is known that the bacterium produces several putative virulence factors, including a leukotoxin (LtxA) that is a member of the repeats-in-toxin (RTX) family. This toxin kills human white blood cells and likely plays a key role in the ability of the organism to evade the immune response during its establishment of an ecological niche in the host. Thus, preventing or interfering with LtxA activity may be one option for successful treatment of the disease. The mechanism by which LtxA exerts its lethal effect on the host cell involves the formation of a physical disruption, such as a pore. Specifically, the interaction of the toxin and membrane results in a bending of the bilayer that leads to the formation of a toroidal, or lipid-lined, pore. A logical extension of this argument would suggest that LtxA will therefore interact most strongly with lipids that are able to bend or curve more easily. The proposed study is designed to explore the ability of LtxA to damage cells by altering the structure of their membranes. In the first aim, the effect of lipid curvature on LtxA-induced membrane changes, such as phase behavior and pore formation, will be characterized. In the second aim, the effect of lipid curvature on LtxA structure will be determined and the relation between those LtxA structural changes and the membrane changes observed in Aim 1 will be examined. The final aim will elucidate the biophysical properties of LtxA- induced pores. This study will allow determination of the role of the physical properties of the membrane, specifically curvature, on LtxA-induced membrane disruption, and will establish the mechanism of this disruption.