Salmonella enterica serovar typhimurium (Salmonella typhimurium) is a significant pathogen that causes gastrointestinal disease in humans and other animals and a systemic infection that resembles typhoid fever in mice. As part of its infectious cycle, S. typhimurium enters host epithelial and phagocytic cells and takes up residence in a late endocytic compartment (the Salmonella-containing vacuole, or SCV) that becomes biochemically and structurally modified to support bacterial replication. These modifications involve the action of a set of bacterially produced effector proteins that are delivered into cells via a type three secretion system. A hallmark of infected epithelial cells is the formation of elongated, membranous tubules, known as Salmonella-induced filaments (SIFs), that emanate from the SCV and are aligned with microtubules. SIF formation is required for both systemic disease and localized infection in the intestinal epithelium, highlighting the importance of these unique structures in pathogenesis. A number of bacterial effector proteins have been identified that contribute to SIF formation, but the molecular details of how these proteins impact the architecture of endosome membranes, particularly the microtubules and motors that contribute to endosome movement, are poorly characterized. ? ? The experiments described in this proposal are intended to further understanding of the molecular basis of SIF formation. Late endosomes, the host cell compartment that becomes subverted to form the SCV and SIFs, are ordinarily highly motile, so we will begin by visualizing SIF formation in living cells infected with wild type and mutant Salmonella strains using a vital fluorescent probe of the SIF membrane. We will then explore the roles of two different microtubule-based motors, kinesin 2 and kinesin 1, in SIF formation, using dominant negative inhibitors. The ability of different bacterial effector proteins to bind microtubules will be tested biochemically using co-precipitation assays, and their impact on microtubule-based motility will be determined in vitro. How different effectors alter microtubule organization and dynamics in host cells will be explored by evaluating the behavior of cells infected with mutant strains. Together, this analysis will provide a clear picture of how Salmonella modifies the activities of the microtubule cytoskeleton during the course of intracellular infection. ? ? Relevance: Salmonella infections cause serious human diseases such as food poisoning and typhoid fever. Salmonella invade and take up residence in host cells, exploiting a number of normal cell functions in the process. A comprehensive understanding of the molecular mechanisms that underlie this process is necessary to identify novel targets for therapy and disease prevention. ? ? ?