The Legionella pneumophila Dot/Icm Type IV Secretion System is Structurally and Functionally Resilient in Absence of Species-specific Proteins Dis2 and Dis3.

Legionella pneumophila is a pathogenic Gram-negative bacterium that causes Legionnaires' disease. The main virulence factor of L. pneumophila is the Dot/Icm Type IV Secretion System (T4SS), which translocates effector proteins into the cytoplasm of the host cell, allowing the bacterium to establish a replicative niche. The outer membrane core complex (OMCC), the T4SS machinery localized between the inner and outer membranes, is composed of at least nine proteins organized into various sub-complexes that include the dome, outer membrane cap (OMC), periplasmic ring (PR), and stalk. In this study we describe how two uncharacterized Dot/Icm T4SS components, Dis2 and Dis3, contribute to the structure of the T4SS, the ability of the T4SS to translocate effectors, and the pathogenicity of L. pneumophila. Using cryo-electron microscopy we show that OMCCs purified from a Δdis2 strain are only missing the density for Dis2, while OMCCs purified from the Δdis3 strain lack densities for Dis3 and DotF in the OMC. Despite missing these proteins, the OMC and PR of both mutant OMCCs remain structurally stable. Strains lacking dis2 and or dis3 efficiently replicate in human macrophages; however, they have subtle differences in translocation efficiency for four tested substrates. Combined these data indicate that Dis2 or Dis3 are not required for the stability or global organization of the OMCC, but each protein may contribute to the efficient translocation of specific effectors.