Chapter 56 : Legionella pneumophila Mip: New Function for an Old Protein?
The macrophage infectivity potentiator (Mip) protein of Legionella pneumophila is one of the most studied Legionella proteins and has long been known to promote virulence. Mip protein has been purified and shown to possess peptidyl-prolyl cis/trans isomerase (PPI-ase) activity. Comparison of primary structures and crystallographic studies shows that Mip proteins have an N-terminal and a C-terminal domain. Type II secretion is one of five protein secretion systems that can mediate the export of proteins across the gram-negative outer membrane into the extracellular milieu and/or into target cells, and the authors have shown that the type II protein secretion system of L. pneumophila is required for optimal replication in macrophages, amoebae, and mice. Interestingly, one of transposon mutants (previously designated NU247) proved to contain a single transposon insertion in the mip gene, suggesting, for the first time, that Mip might influence protein secretion. This chapter talks about the NU247 reproducibly that showed a 40 to 70% reduction in p-NPPC hydrolase activity in its culture supernatants in comparison to the wild-type strain 130b. Since Mip is highly conserved in the Legionella genus, and surface and secreted Mip-like proteins are present in other pathogenic microorganisms, Mip and Mip-like proteins might promote the secretion of other important effectors.
Legionella pneumophila Mip: New Function for an Old Protein?
Models to explain the impact of Mip on a secreted p-NPPC hydrolase activity. Initially, like most type II-secreted proteins, the unfolded p-NPPC hydrolase interacts with components of the Sec system ( 2 ). After the signal peptide is cleaved, the protein is translocated across the inner membrane into the periplasm, where it is folded, possibly with the help of periplasmic chaperones. The protein then interacts with the type II secretion apparatus (Lsp), in order to transit across the outer membrane. At this point, surface-localized Mip may be involved in the final release of active enzyme into the extracellular milieu (A). Alternately, Mip might act on newly released protein and, through additional folding (e.g., PPIase) reactions, render it an active enzyme (B). Although not depicted here, it is possible that the p-NPPC hydrolase substrate is translocated across the inner membrane via the twinarginine translocation (Tat) system ( 2 ).
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Levels of p-NPPC hydrolase activity in culture supernatants of wild type 130b and the mip mutant NU247, carrying the vector pMMB2002 or full-length mip cloned into pMMB2002 (pmip)a .