Inhalation of the intracellular bacterium, Legionella pneumophila from contaminated aerosols has resulted in large-scale outbreaks of Legionnaire’s disease, a potentially fatal form of pneumonia. L. pneumophila is typically found in aquatic environments where it replicates within freshwater protozoa. Following phagocytosis by human alveolar macrophages, L. pneumophila evade endosome fusion by establishing a replicative vacuole termed the L. pneumophila containing vacuole (LCV). LCV biogenesis depends on the translocation of over 300 effector proteins into the host cell via the Dot /Icm type IV secretion system. The LCV membrane is composed of ER membrane and is rich in poly-ubiquitinated proteins. In order to maintain and enhance L. pneumophila proliferation within the LCV, effector proteins hijack and often mimic host machinery, including proteins involved in ubiquitination pathways.
Using yeast two-hybrid screening and co-immunoprecipitation techniques, we discovered that the L. pneumophila Dot/Icm effector protein, MavL, interacted with the mammalian E2 ubiquitin-conjugating enzyme, UBE2Q1. MavL and UBE2Q1 co-localised during L. pneumophila infection of HeLa cells and ubiquitination assays suggested that the host ubiquitination profile is altered in the presence of MavL. In particular, MavL binding to UBE2Q1 resulted in increased ubiquitination of downstream E3 ubiquitin ligase, CHIP (C terminus of HSC70-Interacting Protein). CHIP is traditionally activated by ER-stress and functions as a co-chaperone of eukaryotic heat shock proteins - critical for regulating protein folding in the cell cytosol. As an E3 ligase, CHIP ubiquitinates misfolded chaperone substrates leading to proteosomal degradation. This study highlights host E2 ligases as a target for Dot/Icm effector function and underscores the exploitation of host ubiquitination pathways for L. pneumophila replication. Future work will focus on elucidating the mechanism by which MavL interacts with UBE2Q1 and the consequences of MavL-induced increased CHIP ubiquitination of known substrates and the role of MavL in LCV ubiquitination and L. pneumophila infection.