Molecular and functional characterization of type I signal peptidase from Legionella pneumophila

Abstract

Legionella pneumophila is a facultative intracellular Gram-negative rod-shaped bacterium that has become an important cause of both community-acquired and nosocomial pneumonia. Numerous studies concerning the unravelling of the virulence mechanism of this important pathogen have been initiated. As evidence is now accumulating for the involvement of protein secretion systems in bacterial virulence in general, the type I signal peptidase (LepB) of L. pneumophila was of particular interest. This endopeptidase plays an essential role in the processing of preproteins carrying a typical amino-terminal signal peptide, upon translocation across the cytoplasmic membrane. This paper reports the cloning and the transcriptional analysis of the L. pneumophila lepB gene encoding the type I signal peptidase (SPase). Reverse transcription PCR experiments showed clear lepB expression when L. pneumophila was grown both in culture medium, and also intracellularly in Acanthamoeba castellanii, a natural eukaryotic host of L. pneumophila. In addition, LepB was shown to be encoded by a polycistronic mRNA transcript together with two other proteins, i.e. a LepA homologue and a ribonuclease III homologue. SPase activity of the LepB protein was demonstrated by in vivo complementation analysis in a temperature-sensitive Escherichia coli lepB mutant. Protein sequence and predicted membrane topology were compared to those of leader peptidases of other Gram-negative human pathogens. Most strikingly, a strictly conserved methionine residue in the substrate binding pocket was replaced by a leucine residue, which might influence substrate recognition. Finally it was shown by in vivo experiments that L. pneumophila LepB is a target for (5S,6S)-6-[(R)-acetoxyethyl]-penem-3-carboxylate, a specific inhibitor of type I SPases. INTRODUCTIONLegionella pneumophila, the causative agent of Legionnaires' disease in man, is a Gram-negative facultative intracellular parasite of monocytes and alveolar macrophages. In the environment, L. pneumophila inhabits fresh water, where it can reside and proliferate within various free-living protozoa, or is present in a biofilm-associated state. Human infection can occur following inhalation of small aerosolized droplets containing Legionella cells. Since the initial isolation of L. pneumophila in 1976, significant progress has been made in identifying virulence determinants for entry and intracellular multiplication in eukaryotic hosts. As in other Gram-negative pathogens, these virulence factors are usually proteins and are generally either secreted into the extracellular environment or attached to the cell surface (Finlay & Falkow, 1997). Although the secreted proteins are numerous and diverse, only a few pathways exist by which these proteins are transported from the bacterial cytoplasm to the extracellular environment (reviewed by Lee & Schneewind, 2001). For L. pneumophila, the presence of type II and type IV secretion systems has been described. The first type IV apparatus, encoded by the icm/dot genes , was sh...

Pseudomonas aeruginosa Possesses Two Putative Type I Signal Peptidases, LepB and PA1303, Each with Distinct Roles in Physiology and Virulence

“…The sequences were aligned using CLUSTAL X (1.83). Conserved boxes A to E (shaded) were identified using sequence alignments and the following consensus motifs, in which uppercase underlined letters denote absolutely conserved, uppercase letters denote conserved, and X denotes not conserved (box B, IPSGSMXPTLX; box C, RGDIVVFXXP; box D, YIKRXXGXPGDXV; and box E, VPXGXYFXMGDN RDNSXDSR) (12,34). Box A is labeled as previously identified (34).…”

“…Conserved boxes A to E (shaded) were identified using sequence alignments and the following consensus motifs, in which uppercase underlined letters denote absolutely conserved, uppercase letters denote conserved, and X denotes not conserved (box B, IPSGSMXPTLX; box C, RGDIVVFXXP; box D, YIKRXXGXPGDXV; and box E, VPXGXYFXMGDN RDNSXDSR) (12,34). Box A is labeled as previously identified (34). The first transmembrane helix identified for LepB using two prediction methods (aa 7 to 28, HMMTOP; aa 5 to 28, DAS) and the single short transmembrane region identified for PA1303 (aa 6 to 14, DAS) are underlined.…”

“…This has been observed for Escherichia coli, whose SPase (LepB) has been extensively characterized, and documented for many organisms, including Haemophilus influenzae, Legionella pneumophila, Pseudomonas fluorescens, Rhodobacter sphaeroides, Salmonella enterica serovar Typhimurium, and Yersinia pestis (34,69). However, the possession of multiple SPases is not exclusively a trait of Gram-positive organisms; two SPases have been demonstrated in Synechocystis sp.…”

“…In addition, its natural resistance to many frontline antibiotics is compounded by its genetic capacity to express a wide repertoire of resistance mechanisms and acquisition of additional resistance genes and beneficial mutations (33). The need for a functional SPase for viability has been demonstrated in other organisms, such as E. coli, S. aureus, and Streptococcus pneumoniae (10,13,79), and several inhibitors, including arylomycin and lipoglycopeptide natural products and ␤-lactam analogs (penem inhibitors), have been shown to have activity against SPases (3,5,31,34,37,43,51). In this study, we performed a molecular characterization of the two SPase homologues present within the P. aeruginosa genome, LepB and PA1303, in order to determine their physiological roles and their suitability as drug targets.…”

Legionella pneumophila LbtU Acts as a Novel, TonB-Independent Receptor for the Legiobactin Siderophore

“…3), a known OM protein of L. pneumophila (51,63). As further validation, the localization of LbtU and MOMP was in marked contrast to that of LepB, a signal peptidase from the IM, and ICDH, an enzyme from the cytoplasm (78,131) (Fig. 3).…”