The type II signal peptidase of Legionella pneumophila

Bacterial Signal Peptidases

Signal peptidases are the membrane bound enzymes that cleave off the amino-terminal signal peptide from secretory preproteins . There are two types of bacterial signal peptidases . Type I signal peptidase utilizes a serine/lysine catalytic dyad mechanism and is the major signal peptidase in most bacteria. Type II signal peptidase is an aspartic protease specific for prolipoproteins. This chapter will review what is known about the structure, function and mechanism of these unique enzymes.

Type II secretion and Legionella virulence

Type II secretion (T2S) is one of six systems that can occur in Gram-negative bacteria for the purpose of secreting proteins into the extracellular milieu and/or into host cells. This chapter will describe the T2S system of Legionella pneumophila. Topics to be covered include the genetic basis of T2S in L. pneumophila, the numbers (>25), types, and novelties of Legionella proteins that are secreted via T2S, and the many ways in which T2S and its substrates promote L. pneumophila physiology, ecology, and virulence. Within the aquatic environment, T2S plays a major role in L. pneumophila intracellular infection of multiple types of (Acanthamoeba, Hartmannella, and Naegleria) amoebae. Within the mammalian host, T2S promotes bacterial persistence in lungs, intracellular infection of both macrophages and epithelial cells, and a dampening of the host innate immune response. In this context, T2S may represent a potential target for both industrial and biomedical application.

A unique Coxiella burnetii lipoprotein involved in metal binding (LimB)

Coxiella burnetii is the bacterial agent of Q fever in humans. Here, we describe a unique, ~7.2 kDa, surface-exposed lipoprotein involved in metal binding which we have termed LimB. LimB was initially identified as a potential metal-binding protein on far-Western (FW) blots containing whole-cell lysate proteins when probed with nickel-coated horseradish peroxidase (Ni-HRP) and developed with a chemiluminescent HRP substrate. The corresponding identity of LimB as CBU1224a was established by matrix-assisted laser desorption ionization-tandem time-of-flight mass spectrometry. blast analyses with CBU1224a showed no significant similarity to sequences outside strains of C. burnetii. Additional in silico analyses revealed a putative 20 residue signal sequence with the carboxyl end demarcated by a potential lipobox (LSGC) whose Cys residue is predicted to serve as the N-terminal, lipidated Cys of mature LimB. The second residue of mature LimB is predicted to be Ala, an uncharged envelope localization residue. These features suggest that CBU1224a is synthesized as a prolipoprotein which is subsequently lipidated, secreted and anchored in the outer membrane. Mature LimB is predicted to contain 45 aa, of which there are 10 His and 5 Cys; both amino acids are frequently involved in binding transition metal cations. Recombinant LimB (rLimB) was generated and its Ni-HRP-binding activity demonstrated on FW blots. Ni-HRP binding by rLimB was inhibited by >95 % on FW blots done in the presence of EDTA, imidazole, Ni(2+) or Zn(2+), and roughly halved in the presence of Co(2+) or Fe(3+). The limB gene was maximally expressed at 3-7 days post-infection in Coxiella-infected Vero cells, coinciding with exponential phase growth. Two isoforms of LimB were detected on FW and Western blots, including a smaller (~7.2 kDa) species that was the predominant form in small cell variants and a larger isoform (~8.7 kDa) in large cell variants. LimB is Sarkosyl-insoluble, like many omps. The predicted surface location of LimB was verified by immunoelectron and immunofluorescence microscopy using anti-rLimB antibodies. Overall, the results suggest that LimB is a unique Coxiella lipoprotein that serves as a surface receptor for divalent metal cations and may play a role in acquiring at least one of these metals during intracellular growth.

Comparative in vivo gene expression of the closely related bacteria Photorhabdus temperata and Xenorhabdus koppenhoeferi upon infection of the same insect host, Rhizotrogus majalis

BACKGROUND

Photorhabdus and Xenorhabdus are Gram-negative, phylogenetically related, enterobacteria, forming mutualism with the entomopathogenic nematodes Heterorhabditis and Steinernema, respectively. The mutualistic bacteria living in the intestines of the nematode infective juveniles are pathogenic to the insect upon release by the nematodes into the insect hemolymph. Such a switch needs activation of genes that promote bacterial virulence. We studied in vivo gene expression in Photorhabdus temperata and Xenorhabdus koppenhoeferi upon infection of the white grub Rhizotrogus majalis using selective capture of transcribed sequences technique.

RESULTS

A total of 40 genes in P. temperata and 39 in X. koppenhoeferi were found to be upregulated in R. majalis hemolymph at 24 h post infection. Genomic presence or upregulation of these genes specific in either one of the bacterium was confirmed by the assay of comparative hybridization, and the changes of randomly selected genes were further validated by quantitative real-time PCR. The identified genes could be broadly divided into seven functional groups including cell surface structure, regulation, virulence and secretion, stress response, intracellular metabolism, nutrient scavenging, and unknown. The two bacteria shared more genes in stress response category than any other functional group. More than 60% of the identified genes were uniquely induced in either bacterium suggesting vastly different molecular mechanisms of pathogenicity to the same insect host. In P. temperata lysR gene encoding transcriptional activator was induced, while genes yijC and rseA encoding transcriptional repressors were induced in X. koppenhoeferi. Lipopolysaccharide synthesis gene lpsE was induced in X. koppenhoeferi but not in P. temperata. Except tcaC and hemolysin related genes, other virulence genes were different between the two bacteria. Genes involved in TCA cycle were induced in P. temperata whereas those involved in glyoxylate pathway were induced in X. koppenhoeferi, suggesting differences in metabolism between the two bacteria in the same insect host. Upregulation of genes encoding different types of nutrient uptake systems further emphasized the differences in nutritional requirements of the two bacteria in the same insect host. Photorhabdus temperata displayed upregulation of genes encoding siderophore-dependent iron uptake system, but X. koppenhoeferi upregulated genes encoding siderophore-independent ion uptake system. Photorhabdus temperata induced genes for amino acid acquisition but X. koppenhoeferi upregulated malF gene, encoding a maltose uptake system. Further analyses identified possible mechanistic associations between the identified gene products in metabolic pathways, providing an interactive model of pathogenesis for each bacterium species.

CONCLUSION

This study identifies set of genes induced in P. temperata and X. koppenhoeferi upon infection of R. majalis, and highlights differences in molecular features used by these two closely related bacteria to promote their pathogenicity in the same insect host.

The role of protein secretion systems in the virulence of the intracellular pathogen Legionella pneumophila

Legionella pneumophila is a Gram-negative facultative intracellular pathogen, which multiplies in protozoa in its natural environment and can cause Legionnaires' disease in man, following infection of alveolar macrophages. In each of the different stages of infection of host cells, virulence proteins need to be delivered to their specific place of action and therefore must cross two barriers: the inner and the outer membrane. To date, several specialized secretion machineries for transport of proteins across the inner and outer membrane have been identified in L. pneumophila. Most of these secretion pathways have been shown to affect the virulence of this pathogen. An overview will be given of all the secretion pathways and the proteins transported by these secretion systems identified so far, with special attention paid to those that play a role in the pathogenicity of L. pneumophila.