Structure of a Shigella effector reveals a new class of ubiquitin ligases.

Bacterial pathogens have evolved effector proteins with ubiquitin E3 ligase activities through structural mimicking. Here we report the crystal structure of the Shigella flexneri type III effector IpaH3, a member of the leucine-rich repeat (LRR)-containing bacterial E3 family. The LRR domain is structurally similar to Yersinia pestis YopM and potentially binds to substrates. The structure of the C-terminal E3 domain differs from the typical RING- and HECT-type E3s. IpaH3 synthesizes a Lys48-linked ubiquitin chain, and the reaction requires noncovalent binding between ubiquitin and a specific E2, UbcH5. Free ubiquitin serves as an acceptor for IpaH3-catalyzed ubiquitin transfer. Cys363 within a conserved CXD motif acts as a nucleophile to catalyze ubiquitin transfer through a transthiolation reaction. The D365N mutant is devoid of E3 activities but turns into a potent ubiquitin-E2 thioesterase. Our analysis establishes a structurally and mechanistically distinct class of ubiquitin ligases found exclusively in pathogenic or symbiotic bacteria.