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Summary

Legionella pneumophila infects alveolar macrophages and protozoa through establishment of an intracellular replication niche. This process is mediated by bacterial effectors translocated into the host cell via the Icm/Dot type IV secretion system. Most of the effectors identified so far are unique to L. pneumophila; however, some of the effectors are homologous to eukaryotic proteins. We performed a distribution analysis of many known L. pneumophila effectors and found that several of them, mostly eukaryotic homologous proteins, are present in different Legionella species. In-depth analysis of LegS2, a L. pneumophila homologue of the highly conserved eukaryotic enzyme sphingosine-1-phosphate lyase (SPL), revealed that it was most likely acquired from a protozoan organism early during Legionella evolution. The LegS2 protein was found to translocate into host cells using a C-terminal translocation domain absent in its eukaryotic homologues. LegS2 was found to complement the sphingosine-sensitive phenotype of a Saccharomyces serevisia SPL-null mutant and this complementation depended on evolutionary conserved residues in the LegS2 catalytic domain. Interestingly, unlike the eukaryotic SPL that localizes to the endoplasmic reticulum, LegS2 was found to be targeted mainly to host cell mitochondria. Collectively, our results demonstrate the remarkable adaptations of a eukaryotic protein to the L. pneumophila pathogenesis system.