SUMMARY: The obligate intracellular bacterium Chlamydia trachomatis causes substantial morbidity in the US and worldwide. A C. trachomatis effector called Tarp for translocated actin recruiting protein is a candidate virulence factor. Tarp is tyrosine phosphorylated by a host cell kinase and is associated with actin recruitment during C. trachomatis entry. All reference and clinical isolates of Chlamydiae species examined to date harbor the tarP gene. We have identified and biochemically characterized four Tarp protein domains including: i) a phosphorylation domain ii) a proline rich oligomerization domain iii) a G-actin binding and nucleating domain, and iv) two F-actin binding and bundling domains. We hypothesize that specific Tarp domains are required for bacterial entry and/or chlamydial development. We will test this hypothesis by: 1) examining the mechanics of Tarp mediated actin bundles, 2) analyzing C. trachomatis tarp mutants and complements that express Tarp domain deletions for chlamydial invasion of host cells and development and 3). Investigate the requirement for tarP in a mouse infection model. Elucidation of the molecular mechanisms employed by C. trachomatis to initiate a successful infection may provide clues that can be applied to novel therapeutic interventions for this prolific pathogen.