In order to successfully invade and survive within a host cell, pathogens such as the bacteria Shigella flexneri have to suppress the host cell’s immune response. To do this, Shigella flexneri inject bacterial effector proteins into the host cell that hamper the host cell’s ability to detect and respond to invasion. One such protein is IpaH4.5, a bacterial E3 ubiquitin ligase, which hijacks the host cell’s ubiquitylation machinery to transfer ubiquitin, a small signaling protein, to a substrate. Ubiquitin signaling impacts all aspects of eukaryotic cell biology. Often modification of a protein by ubiquitin serves to mark protein for destruction. IpaH4.5 targets proteins involved in the host cell’s immune response, dampening the host cell’s ability to respond to invasion by Shigella flexneri. I am investigating IpaH4.5 and its interactions with both host substrates and the host ubiquitin machinery. After sub cloning out the various domains of TbK1, a known substrate of IpaH4.5, I will perform ubiquitilation assays in order to determine which domain of TBK1 is polyubiquitilated, indicating that IpaH4.5 interacts with this domain. Then, this domain can be used for structural and mechanical analysis of IpaH4.5, such as determining how mutations in IpaH4.5 affect ubiquitilation through biochemical assays. Understanding how IpaH4.5 binds to and interacts with its substrates will increase our understanding of how Shigella flexneri dampens host immune response as well as help us better understand the mechanisms of bacterial E3 ubiquitin ligases. Since IpaH4.5 functions differently than eukaryotic E3 ubiquitin ligases, increasing our understanding of its mechanisms could allow us to use it as a target for future drug development. Finding new targets for drug development is becoming increasingly important with the spread of antibiotic resistant bacteria.