Biochemical Characterization of IpaH E3 Ubiquitin Ligase Effector Proteins and Their Host Substrates
Shigella flexneri is a gram negative pathogen that utilizes its type 3 secretion system (T3SS) to inject effector proteins in the cytoplasm of host cells to manipulate host cells processes. T3SS effectors are able to post translationally modify host proteins to reprogram intracellular signaling pathways, actin dynamics, membrane trafficking, and innate immune pathways. This allows Shigella to modify the intracellular environment to be conducive to bacterial replication and dissemination to neighboring cells. Shigella flexneri and other bacteria including Salmonella and Yersinia secreted E3 ubiquitin ligases into the host cell cytoplasm via the Type III secretion system (T3SS) apparatus. The invasion plasmid antigen Hs (IpaHs) are a novel family of bacterial E3 ubiquitin ligases that are secreted by Shigella, Salmonella, and Yersinia. These bacterial enzymes highjack the host ubiquitin conjugation machinery by binding to ubiquitin-charged E2 conjugating enzymes and facilitating direct transfer of ubiquitin onto host substrates. IpaH effectors induce polyubiquitination and subsequent proteasomal degradation of their substrates during bacterial infection. The effector substrate interaction of IpaH1.4/2/5 and HOIP was previously characterized. I went on to identify that IpaH2.5 is able to inhibit the in vitro catalytic activity of HOIP via mono-ubiquitination of catalytic lysine residues in the HOIP ring-between-ring domain (RBR-C). Subsequent to this Ubiquitin activated interactive trapping (UBAIT) screening was then utilized to identify the host substrate of IpaH7.8, Gasdermin B (GSDMB). GSDMB belongs to a large family of pore forming cytolysins that execute inflammatory cell death programs. While genetic studies have linked GSDMB polymorphisms to inflammatory disease, its function in human physiology remains poorly understood. I investigated a previously unrecognized host-pathogen conflict between GSDMB and the IpaH7.8 effector protein encoded by Shigella flexneri. Through extensive biochemical and cellular characterization, I show that IpaH7.8 ubiquitinates and targets GSDMB for proteasome destruction. This virulence strategy protects Shigella from the bacteriocidic activity of Natural Killer cells by suppressing Granzyme-A mediated activation of GSDMB. In contrast to the canonical function of most Gasdermin-family members, GSDMB does not inhibit Shigella by lysing infected cells. Rather, GSDMB exhibits direct microbiocidal activity through recognition of phospholipids found on Gram-negative bacterial membranes. These findings place GSDMB as a central executioner of intracellular bacterial killing and reveals a mechanism employed by pathogens to counteract this host defense system.