The effector EmcB is a deubiquitinase that inhibits RIG-I signaling.

Eukaryotes have cytosolic surveillance systems to detect invading microorganisms and initiate protective immune responses. In turn, host-adapted pathogens have evolved strategies to modulate these surveillance systems, which can promote dissemination and persistence in the host. The obligate intracellular pathogen infects mammalian hosts without activating many innate immune sensors. The Defect in Organelle Trafficking/Intracellular Multiplication (Dot/Icm) protein secretion system is necessary for to establish a vacuolar niche inside of host cells, which sequesters these bacteria in a specialized organelle that could evade host surveillance systems. However, bacterial secretion systems often introduce agonists of immune sensors into the host cytosol during infection. For instance, nucleic acids are introduced to the host cytosol by the Dot/Icm system of , which results in type I interferon production. Despite host infection requiring a homologous Dot/Icm system, does not induce type I interferon production during infection. Here, it was found that type I interferons are detrimental to infection and that blocks type I interferon production mediated by retionic acid inducible gene I (RIG-I) signaling. Two Dot/Icm effector proteins, EmcA and EmcB, are required for inhibition of RIG-I signaling. EmcB is sufficient to block RIG-I signaling and is a ubiquitin-specific cysteine protease capable of deconjugating ubiquitin chains from RIG-I that are necessary for signaling. EmcB preferentially cleaves K63-linked ubiquitin chains of three or more monomers, which represent ubiquitin chains that potently activate RIG-I signaling. Identification of a deubiquitinase encoded by provides insights into how a host-adapted pathogen antagonizes immune surveillance.