Legionella pneumophila, the agent of
Legionnaires' disease, is an intracellular parasite of aquatic amoebae and human macrophages. A key factor for L. pneumophila in intracellular
infection is its type II
protein secretion system (Lsp). In order to more completely define Lsp output, we recently performed a proteomic analysis of culture supernatants. Based upon the predictions of that analysis, we found that L. pneumophila secretes two distinct
aminopeptidase activities encoded by the genes lapA and lapB. Whereas lapA conferred activity against
leucine,
phenylalanine, and
tyrosine aminopeptides, lapB was linked to the cleavage of
lysine- and
arginine-containing substrates. To assess the role of secreted
aminopeptidases in intracellular
infection, we examined the relative abilities of lapA and lapB mutants to infect human U937 cell macrophages as well as Hartmannella vermiformis and Acanthamoeba castellanii amoebae. Although these experiments identified a dispensable role for LapA and LapB, they uncovered a previously unrecognized role for the type II-dependent ProA (MspA)
metalloprotease. Whereas proA mutants were not defective for macrophage or A. castellanii
infection, they (but not their complemented derivatives) were impaired for growth upon coculture with H. vermiformis. Thus, ProA represents the first type II effector implicated in an intracellular
infection event. Furthermore, proA represents an L. pneumophila gene that shows differential importance among
protozoan infection models, suggesting that the legionellae might have evolved some of its factors to especially target certain of their protozoan hosts.