The Legionella genus core effectors display functional conservation among orthologs by themselves or combined with an accessory protein

The intracellular pathogen Legionella pneumophila, as well as other Legionella species, utilize the Icm/Dot type-IV secretion system to translocate an exceptionally large and diverse repertoire of effectors into their host cells. However, only nine core effectors were found to be present in all analyzed Legionella species. In this study, we investigated the core effectors, and used intracellular growth complementation to determine whether orthologs of core effectors perform the same function in different Legionella species. We found that three out of the nine L. pneumophila core effectors are required for maximal intracellular growth. Examination of orthologous core effectors from four Legionella species spread over the Legionella phylogenetic tree revealed that most of them perform the same function. Nevertheless, some of the orthologs of the core effector LegA3 did not complement the L. pneumophila legA3 deletion mutant for intracellular growth. LegA3 is encoded as part of an operon together with another gene, which we named legA3C, encoding a non-translocated protein. We found that LegA3 and LegA3C physically interact with each other, are both required for maximal intracellular growth, and the LegA3-LegA3C orthologous pairs from all the Legionella species examined fully complement the L. pneumophila legA3 deletion mutant for intracellular growth. Our results indicate that the Legionella core effectors orthologs generally perform the same function and establish that LegA3 requires LegA3C to fulfill its conserved function.