Legionella micdadei protein kinase catalyzes phosphorylation of tubulin and phosphatidylinositol

Phosphoinositides and cellular pathogens

Phosphoinositides are considered as highly dynamic players in the spatiotemporal organization of key signaling pathways, actin cytoskeleton rearrangements, establishment of cell polarity and intracellular vesicle trafficking. Their metabolism is accurately controlled and mutations in several phosphoinositide metabolizing enzymes take part in the development of human pathologies. Interestingly, evidence is accumulating that modulation of the phosphoinositide metabolism is critical for pathogenicity and virulence of many human pathogens. Given the importance of phosphoinositides, which link membrane and cytoskeleton dynamics to cell responses, it is not surprising that many invasive pathogens hijack their metabolism as part of their strategies to establish infection. In fact, according to their lifestyle, cellular pathogens use the phosphoinositide metabolism in order to trigger their uptake in nonphagocytic cells and/or modulate the maturation of the pathogen-containing vacuole to establish their replicative niche or escape in the cytosol and promote host cell survival. The last two decades have been marked by the discovery of different tactics used by cellular pathogens to modulate the phosphoinositide metabolism as part of their strategies to survive, proliferate and disseminate in a hostile environment.

Stress-induced synthesis of phosphatidylinositol 3-phosphate in mycobacteria

Phosphoinositides play key roles in regulating membrane dynamics and intracellular signaling in eukaryotic cells. However, comparable lipid-based signaling pathways have not been identified in bacteria. Here we show that Mycobacterium smegmatis and other Actinomycetes bacteria can synthesize the phosphoinositide, phosphatidylinositol 3-phosphate (PI3P). This lipid was transiently labeled with [(3)H]inositol. Sensitivity of the purified lipid to alkaline phosphatase, headgroup analysis by high-pressure liquid chromatography, and mass spectrometry demonstrated that it had the structure 1,2-[tuberculostearoyl, octadecenoyl]-sn-glycero 3-phosphoinositol 3-phosphate. Synthesis of PI3P was elevated by salt stress but not by exposure to high concentrations of non-ionic solutes. Synthesis of PI3P in a cell-free system was stimulated by the synthesis of CDP-diacylglycerol, a lipid substrate for phosphatidylinositol (PI) biosynthesis, suggesting that efficient cell-free PI3P synthesis is dependent on de novo PI synthesis. In vitro experiments further indicated that the rapid turnover of this lipid was mediated, at least in part, by a vanadate-sensitive phosphatase. This is the first example of de novo synthesis of PI3P in bacteria, and the transient synthesis in response to environmental stimuli suggests that some bacteria may have evolved similar lipid-mediated signaling pathways to those observed in eukaryotic cells.

Signal transduction during Legionella pneumophila entry into human monocytes

Legionella pneumophila causes Legionnaires' disease by replication in alveolar macrophages and monocytes. The bacteria are internalized most efficiently by opsonin-dependent, CR3-mediated phagocytosis. This investigation focused on determining the role of actin polymerization and phosphorylation signals in this uptake mechanism. Uptake inhibition assays and confocal microscopic analysis indicated that entry of L. pneumophila activated tyrosine kinase (TK) and protein kinase C (PKC) and induced actin polymerization at the site of bacterial entry. Upon L. pneumophila entry, six major cellular proteins (75, 71, 59, 56, 53, and 52 kDa) were TK phosphorylated in soluble fractions of monocytes, and three of these proteins (52, 53, and 56 kDa) were consistently found in insoluble (i.e., cytoskeletal) fractions of monocytes as well. Tyrosine phosphorylation was suppressed when cells were pretreated with the kinase inhibitor genistein, tyrphostin, or staurosporine. A similar tyrosine-phosphorylated protein pattern was observed with CR3-mediated entry of avirulent L. pneumophila, Escherichia coli, or zymosan into monocytes. This study has shown that PKC and TK signals which activate actin polymerization during the process of phagocytosis are induced upon L. pneumophila entry. In addition, CR3 receptor-mediated phagocytosis into monocytes may involve tyrosine phosphorylation of similar proteins, regardless of the particle being phagocytosed. Therefore, the tyrosine-induced phosphorylation observed during opsonized L. pneumophila entry is not a virulence-associated event.

Infection of macrophages with Legionella pneumophila induces phosphorylation of a 76-kilodalton protein

Infection of peritoneal macrophages from susceptible A/J mice with Legionella pneumophila induced phosphorylation of a 76-kDa protein. The phosphorylation occurred when macrophages were infected with a virulent strain of L. pneumophila but did not occur when they were infected with an avirulent strain or with other bacteria such as either Pseudomonas aeruginosa or Salmonella typhimurium. Also, no phosphorylation of this protein was observed when macrophages were stimulated with either lipopolysaccharide or phorbol myristate acetate. However, phosphorylation did occur in macrophages infected with a virulent strain of L. pneumophila and treated with either erythromycin to inhibit growth or with cytochalasin D to inhibit uptake of L. pneumophila by macrophages. These results support the view that phosphorylation of this protein occurs during the early phases of interaction between L. pneumophila and macrophages. The role of this specific protein in the recognition, intracellular uptake, and growth of L. pneumophila in permissive macrophages remains to be clarified.

Virulence factors of the family Legionellaceae

Whereas bacteria in the genus Legionella have emerged as relatively frequent causes of pneumonia, the mechanisms underlying their pathogenicity are obscure. The legionellae are facultative intracellular pathogens which multiply within the phagosome of mononuclear phagocytes and are not killed efficiently by polymorphonuclear leukocytes. The functional defects that might permit the intracellular survival of the legionellae have remained an enigma until recently. Phagosome-lysosome fusion is inhibited by a single strain (Philadelphia 1) of Legionella pneumophila serogroup 1, but not by other strains of L. pneumophila or other species. It has been found that following the ingestion of Legionella organisms, the subsequent activation of neutrophils and monocytes in response to both soluble and particulate stimuli is profoundly impaired and the bactericidal activity of these cells is attenuated, suggesting that Legionella bacterial cell-associated factors have an inhibitory effect on phagocyte activation. Two factors elaborated by the legionellae which inhibit phagocyte activation have been described. First, the Legionella (cyto)toxin blocks neutrophil oxidative metabolism in response to various agonists by an unknown mechanism. Second, L. micdadei bacterial cells contain a phosphatase which blocks superoxide anion production by stimulated neutrophils. The Legionella phosphatase disrupts the formation of critical intracellular second messengers in neutrophils. In addition to the toxin and phosphatase, several other moieties that may serve as virulence factors by promoting cell invasion or intracellular survival and multiplication are elaborated by the legionellae. Molecular biological studies show that a cell surface protein named Mip is necessary for the efficient invasion of monocytes. A possible role for a Legionella phospholipase C as a virulence factor is still largely theoretical. L. micdadei contains an unusual protein kinase which catalyzes the phosphorylation of eukaryotic substrates, including phosphatidylinositol and tubulin. Since the phosphorylation of either phosphatidylinositol or tubulin might compromise phagocyte activation and bactericidal functions, this enzyme may well be a virulence factor. Administration of the L. pneumophila exoprotease induces lesions resembling those of Legionella pneumonia and kills guinea pigs, suggesting that this protein plays a role in the pathogenesis of legionellosis. However, recent work with a genetically engineered strain has convincingly shown that the protease is not necessary for intracellular survival or virulence. As might be expected with a complex process like intracellular parasitism, it appears that the capability of Legionella strains to invade and multiply in host phagocytes is multifactorial and that no single moiety which is responsible for the virulence phenotype will be found.

Nucleotide sequence analysis of the Legionella micdadei mip gene, encoding a 30-kilodalton analog of the Legionella pneumophila Mip protein

After the demonstration of analogs of the Legionella pneumophila macrophage infectivity potentiator (Mip) protein in other Legionella species, the Legionella micdadei mip gene was cloned and expressed in Escherichia coli. DNA sequence analysis of the L. micdadei mip gene contained in the plasmid pBA6004 revealed a high degree of homology (71%) to the L. pneumophila mip gene, with the predicted secondary structures of the two Mip proteins following the same pattern. Southern hybridization experiments, with the plasmid pBA6004 as the probe, suggested that the mip gene of L. micdadei has extensive homology with the mip-like genes of several Legionella species. Furthermore, amino acid sequence comparisons revealed significant homology to two eukaryotic proteins with isomerase activity (FK506-binding proteins).