Inter-kingdom signaling by the Legionella autoinducer LAI-1 involves the antimicrobial guanylate binding protein GBP.

The causative agent of Legionnaires' disease, Legionella pneumophila, is an amoebae-resistant environmental bacterium, which replicates intracellularly in a distinct compartment, the "Legionella-containing vacuole" (LCV). L. pneumophila employs the α-hydroxyketone compound LAI-1 (Legionella autoinducer-1) for intra-species and inter-kingdom signaling. LAI-1 promotes intracellular replication and inhibits the migration of mammalian cells and Dictyostelium discoideum. In this study, we revealed that LAI-1 and "clickable" azido-LAI-1 derivatives inhibit the migration of D. discoideum and localize to LCVs. Azido-LAI-1 colocalizes with the LCV markers calnexin, P4C, and AmtA, but not with mitochondrial or lipid droplet markers. Intriguingly, LAI-1-dependent inhibition of D. discoideum migration involves the single guanylate-binding protein (GBP), a member of the GBP family of large GTPases, which in metazoan organisms promote cell autonomous immunity. D. discoideum lacking GBP (Δgnbp) allows more efficient intracellular replication of L. pneumophila, without apparently compromising LCV formation or integrity, and GBP-GFP localizes to the ER at LCV-ER membrane contact sites (MCS). However, the peri-LCV localization of LAI-1 and GBP is not mutually dependent. Synthetic LAI-1 inhibits the expansion/remodeling of LCVs (but not vacuoles harboring avirulent L. pneumophila) in a GBP-dependent manner. Taken together, the work shows that LAI-1 localizes to LCVs, and LAI-1-dependent inter-kingdom signaling involves D. discoideum GBP, which localizes to LCV-ER MCS and acts as an antimicrobial factor by restricting the intracellular growth of L. pneumophila.