Neisseria meningitidis App, a new adhesin with autocatalytic serine protease activity

Autoprocessing of the Escherichia coli AIDA-I autotransporter: a new mechanism involving acidic residues in the junction region

The cleavage of the autotransporter adhesin involved in diffuse adherence (AIDA-I) of Escherichia coli yields a membrane-embedded fragment, AIDAc, and an extracellular fragment, the mature AIDA-I adhesin. The latter remains noncovalently associated with AIDAc but can be released by heat treatment. In this study we determined the mechanism of AIDA-I cleavage. We showed that AIDA-I processing is an autocatalytic event by monitoring the in vitro cleavage of an uncleaved mutant protein isolated from inclusion bodies. Furthermore, by following changes in circular dichroism spectra and protease resistance of the renaturated protein, we showed that the cleavage of the protein is correlated with folding. With site-directed deletions, we showed that the catalytic activity of the protein lies in a region encompassing amino acids between Ala-667 and Thr-953, which includes the conserved junction domain of some autotransporters. With site-directed point mutations, we also found that Asp-878 and Glu-897 are involved in the processing of AIDA-I and that a mutation preserving the acidic side chain of Asp-878 was tolerated, giving evidence that this carboxylic acid group is directly involved in catalysis. Last, we confirmed that cleavage of AIDA-I is intramolecular. Our results unveil a new mechanism of auto-processing in the autotransporter family.

Pathogenic neisseriae: surface modulation, pathogenesis and infection control

Although renowned as a lethal pathogen, Neisseria meningitidis has adapted to be a commensal of the human nasopharynx. It shares extensive genetic and antigenic similarities with the urogenital pathogen Neisseria gonorrhoeae but displays a distinct lifestyle and niche preference. Together, they pose a considerable challenge for vaccine development as they modulate their surface structures with remarkable speed. Nonetheless, their host-cell attachment and invasion capacity is maintained, a property that could be exploited to combat tissue infiltration. With the primary focus on N. meningitidis, this Review examines the known mechanisms used by these pathogens for niche establishment and the challenges such mechanisms pose for infection control.

Two centuries of meningococcal infection: from Vieusseux to the cellular and molecular basis of disease

Scientific knowledge of meningococcal infection has increased greatly since the epidemic nature of the illness was first described by Vieusseux at the dawn of the nineteenth century. In fact, revolutionary advances have been made in public-health measures, antimicrobial therapy, diagnostic procedures, anti-inflammatory drugs and supportive care facilities. Based on the knowledge accumulated to date, it is generally accepted that the pathogenesis of meningococcal infection involves multiple links that interconnect in a complex web of phenomena from Neisseria meningitidis attachment to meningococcal sepsis or meningitis. In fact, a myriad of strongly interacting inflammatory molecules and cells have been implicated in neisserial infection, illustrating the complexity of meningococcal pathogenesis. In addition, many of these signallers are critically involved in outcomes in the human host. Deciphering the pathogenesis of meningococcal infection could expand our knowledge and provide important clues to the host-pathogen interaction, as well as leading to the development of new therapeutic tools. Herein, we review the history of the discovery and characterization of meningococcal disease, epidemiological features of the disease with an emphasis on recent developments in Brazil, the cellular and molecular basis of disease, and discuss diagnosis and therapy.

Meningococcal outer membrane protein NhhA is essential for colonization and disease by preventing phagocytosis and complement attack

Neisseria meningitidis is a leading cause of meningitis and septicemia worldwide, with a rapid onset of disease and a high morbidity and mortality. NhhA is a meningococcal outer membrane protein included in the family of trimeric autotransporter adhesins. The protein binds to the extracellular matrix proteins heparan sulfate and laminin and facilitates attachment to host epithelial cells. In this study, we show that NhhA is essential for bacterial colonization of the nasopharyngeal mucosa in a murine model of meningococcal disease. Successful colonization depends on bacterial attachment but also to the capacity to overcome innate host immune responses. We found that NhhA protected bacteria from phagocytosis, which is important for the mucosal survival of bacteria. In addition, NhhA mediated extensive serum resistance that increased bacterial survival in blood and promoted lethal sepsis. The presence of NhhA protected bacteria from complement-mediated killing by preventing the deposition of the membrane attack complex. Taken together, the results of this work reveal that NhhA inhibits phagocytosis and protects bacteria against complement-mediated killing, which enhances both nasal colonization and the development of sepsis in vivo.

Cleavage of a bacterial autotransporter by an evolutionarily convergent autocatalytic mechanism

Bacterial autotransporters are comprised of an N-terminal 'passenger domain' and a C-terminal beta barrel ('beta domain') that facilitates transport of the passenger domain across the outer membrane. Following translocation, the passenger domains of some autotransporters are cleaved by an unknown mechanism. Here we show that the passenger domain of the Escherichia coli O157:H7 autotransporter EspP is released in a novel autoproteolytic reaction. After purification, the uncleaved EspP precursor underwent proteolytic processing in vitro. An analysis of protein topology together with mutational studies strongly suggested that the reaction occurs inside the beta barrel and revealed that two conserved residues, an aspartate within the beta domain (Asp(1120)) and an asparagine (Asn(1023)) at the P1 position of the cleavage junction, are essential for passenger domain cleavage. Interestingly, these residues were also essential for the proteolytic processing of two distantly related autotransporters. The data strongly suggest that Asp(1120) and Asn(1023) form an unusual catalytic dyad that mediates self-cleavage through the cyclization of the asparagine. Remarkably, a very similar mechanism has been proposed for the maturation of eukaryotic viral capsids.

T-cell stimulating protein A (TspA) of Neisseria meningitidis is required for optimal adhesion to human cells

T-cell stimulating protein A (TspA) is an immunogenic, T-cell and B-cell stimulating protein of Neisseria meningitidis. Sequence similarity between TspA and FimV, a Pseudomonas aeruginosa protein involved in twitching motility, suggested a link between TspA and type IV pili (Tfp). To determine the role of TspA an isogenic deletion mutant was created. Loss of TspA did not affect twitching motility or piliation indicating that there are functional differences between TspA and FimV. Mutation of tspA led to a significant reduction in adhesion of meningococci to meningothelial and HEp-2 cells, which was not due to a lack of transcription of adjacent genes or pilC1. Other Tfp-mediated phenotypes (i.e. auto-aggregation and transformation competence) were not altered. Our results indicate that the role of TspA in adhesion is unlikely to be directly linked to the function of Tfp. TspA was expressed by all N. meningitidis and Neisseria polysaccharea strains examined but not by Neisseria gonorrhoeae or Neisseria lactamica, although sequences with homology to tspA were present in their genomes. In summary, TspA is a highly conserved antigen that is required for optimal adhesion of meningococci to human cells.

Neisseria meningitidis NhhA is a multifunctional trimeric autotransporter adhesin

NhhA, Neisseriahia/hsf homologue, or GNA0992, is an oligomeric outer membrane protein of Neisseria meningitidis, recently included in the family of trimeric autotransporter adhesins. In this study we present the structural and functional characterization of this protein. By expressing in Escherichia coli the full-length gene, deletion mutants and chimeric proteins of NhhA, we demonstrated that the last 72 C-terminal residues are able to allow trimerization and localization of the N-terminal protein domain to the bacterial surface. In addition, we investigated on the possible role of NhhA in bacterial-host interaction events. We assessed in vitro the ability of recombinant purified NhhA to bind human epithelial cells as well as laminin and heparan sulphate. Furthermore, we shown that E. coli strain expressing NhhA was able to adhere to epithelial cells, and observed a reduced adherence in a meningococcal isogenic MC58DeltaNhhA mutant. We concluded that this protein is a multifunctional adhesin, able to promote the bacterial adhesion to host cells and extracellular matrix components. Collectively, our results underline a putative role of NhhA in meningococcal pathogenesis and ascertain its structural and functional belonging to the emerging group of bacterial autotransporter adhesins with trimeric architecture.

A novel phase-variable autotransporter serine protease, AusI, of Neisseria meningitidis

The sequenced genomes of pathogenic Neisseria meningitidis strains contain up to eight genes putatively encoding autotransporters, which are secreted proteins implicated in virulence. Here, we have characterized one of these genes, designated ausI, which encodes an autotransporter of the serine protease family. It was found to be specific for N. meningitidis and present in 14 out of 20 isolates, although only six of them expressed the gene. We show that expression of the gene is subject to phase variation as a result of a variable number of cytosines in a poly-C tract in the coding region. The open reading frame went out-of-phase at the poly-C tract in seven strains that did not express AusI. In the eighth strain, the open reading frame remained in frame at the poly-C tract, but it was disrupted by a premature stop codon further downstream. In accordance with its assignment as an autotransporter, a secreted AusI passenger domain was released into the extracellular milieu. This release was influenced by another autotransporter, NalP, as different forms of AusI were produced in the presence or absence of NalP. In silico sequence analysis suggested several putative functions for AusI, which, however, could not be confirmed experimentally.

Adhesion mediated by autotransporters of Gram-negative bacteria: Structural and functional features

The ability of bacterial proteins to promote adhesion to biological surfaces is a fundamental step in bacterial infections. Some bacterial adhesins belong to the family of autotransporters, which are secreted to the surface of Gram-negative bacteria by an elegantly simple mechanism. This review will summarize their functional and structural features.

Characterization of MspA, an immunogenic autotransporter protein that mediates adhesion to epithelial and endothelial cells in Neisseria meningitidis

A novel putative autotransporter protein (NMB1998) was identified in the available genomic sequence of meningococcal strain MC58 (ET-5; ST-32). The mspA gene is absent from the genomic sequences of meningococcal strain Z2491 (ET-IV; ST-4) and the gonococcal strain FA1090. An orthologue is present in the meningococcal strain FAM18 (ET-37; ST-11), but the sequence contains a premature stop codon, suggesting that the protein may not be expressed in this strain. MspA is predicted to be a 157-kDa protein with low cysteine content, and it exhibits 36 and 33% identity to the meningococcal autotransporter proteins immunoglobulin A1 (IgA1) protease and App, respectively. Search of the Pfam database predicts the presence of IgA1 protease and autotransporter beta-barrel domains. MspA was cloned, and a recombinant protein of the expected size was expressed and after being affinity purified was used to raise rabbit polyclonal monospecific antiserum. Immunoblot studies showed that ca. 125- and 95-kDa fragments of MspA are secreted in meningococcal strain MC58, which are absent from the isogenic mutant. Secretion of MspA was shown to be modified in an AspA isogenic mutant. A strain survey showed that MspA is expressed by all ST-32 and ST-41/44 (lineage 3) strains, but none of the ST-8 (A4) strains examined. Sera from patients convalescing from meningococcal disease were shown to contain MspA-specific antibodies. In bactericidal assays, anti-MspA serum was shown to kill the homologous strain (MC58) and another ST-32 strain. Escherichia coli-expressing recombinant MspA was shown to adhere to both human bronchial epithelial cells and brain microvascular endothelial cells.

Reverse vaccinology — In search of a genome-derived meningococcal vaccine

Although significant advances have been made toward the control of bacterial meningitis in children with the development of capsular polysaccharide protein conjugate vaccines, this approach has proven problematic for the serogroup B meningococcus. Non-capsular vaccines based upon outer membrane vesicles of Neisseria meningitidis have been useful in control of clonal serogroup B outbreaks, although due to variability of PorA, these vaccines may be less useful in control of endemic disease. Genome-based vaccine discovery was evaluated in an attempt to produce a candidate capable of conferring a broadly protective vaccine against a diversity of meningococcal B strains.

Proteomic analysis of Neisseria lactamica and N eisseria meningitidis outer membrane vesicle vaccine antigens

Vaccines to prevent meningococcal disease have been developed from the outer membrane vesicles (OMVs) of Neisseria meningitidis and the related commensal organism Neisseria lactamica. In addition to lipopolysaccharide and the major porins, these vaccines contain a large number of proteins that are incompletely characterised. Here we describe comparative proteomic analyses of the N. lactamica OMV vaccine and OMVs from a serogroup B strain of N. meningitidis. Tandem mass-spectrometry data for trypsinised N. lactamica OMV vaccine were matched to an incompletely assembled genome sequence from the same strain to give 65 robust protein identifications and a further 122 single- or two-peptide matches. Fifty-seven N. meningitidis K454 proteins were identified robustly (and a further 68 from single- or two-peptide matches) by inference from the N. meningitidis MC58 genome. The results suggest that OMVs have a hitherto unappreciated complexity and pinpoint novel candidate antigens for further characterisation.

Outer membrane vesicles from group BNeisseria meningitidis Δgna33 mutant: Proteomic and immunological comparison with detergent-derived outer membrane vesicles

We compared the proteome of detergent-derived group B Neisseria meningitidis (MenB) outer membrane vesicles (DOMVs) with the proteome of outer membrane vesicles (m-OMVs) spontaneously released into culture supernatant by MenB delta gna33, a mutant in which the gene coding for a lytic transglycosylase homologous to the E. coli MltA was deleted. In total, 138 proteins were identified in DOMVs by 1- and 2-DE coupled with MS; 64% of these proteins belonged to the inner membrane and cytoplasmic compartments. By contrast, most of the 60 proteins of m-OMVs were classified by PSORT as outer membrane proteins. When tested for their capacity to elicit bactericidal antibodies, m-OMVs elicited a broad protective activity against a large panel of MenB strains. Therefore, the identification of mutations capable of conferring an OMV-releasing phenotype in bacteria may represent an attractive approach to study bacterial membrane composition and organization, and to design new efficacious vaccine formulations.

Protein secretion and secreted proteins in pathogenicNeisseriaceae

Secreted proteins of pathogenic bacteria are often essential virulence factors. They are involved, for example, in the adherence of the bacteria to host cells or required to suppress the host's defence mechanisms. Until recently, only IgA1 protease had been studied in detail in the NeisseriaceaeNeisseria meningitidis and Neisseria gonorrhoeae. The availability of their genome sequences, however, has boosted research in this area. Here, we present a survey of the secretome of the pathogenic Neisseriaceae, based on the available genome sequences, and the current knowledge of the functions and structures of the secreted proteins. Of the six protein-secretion pathways that are widely disseminated among Gram-negative bacteria, three pathways appear to be present among the Neisseriaceae, i.e. the autotransporter-, the two-partner- and the type I-secretion mechanisms. Comparison of the predicted secretomes reveals a considerable flexibility. As compared with N. meningitidis and the nonpathogen N. lactamica, N. gonorrhoeae appears to have a considerably degenerated secretome, which may reflect its altered niche occupancy. The flexibility of the secretome may be enhanced by the presence of ORFs in the genomes potentially encoding fragments of secreted proteins. We hypothesize that these ORFs may substitute for the corresponding fragments in the full-length genes through genetic recombination, thereby changing the host-cell receptor specificity of the secreted protein.

The impact of genomics on vaccine design

After 200 years of practice, vaccinology has gained new perspectives for preventing infectious diseases. Sequencing of complete bacterial genomes led to the development of new large-scale technologies, such as bioinformatics, proteomics and DNA microarrays. By examining genetic content, as well as transcription and expression profiles, a more detailed understanding of bacterial pathogenesis can be reached. Moreover, the whole-genome perspective is expected to provide an instrumental contribution to vaccine development, particularly to target those pathogens for which the traditional approaches have failed so far. In this review, we describe how genomic approaches can be used to identify novel vaccine candidates or create safer live-attenuated vaccines.

Unusual genetic organization of a functional type I protein secretion system in Neisseria meningitidis

Proteins secreted by Neisseria meningitidis are thought to play important roles in the pathogenesis of meningococcal disease. These proteins include the iron-repressible repeat-in-toxin (RTX) exoprotein FrpC. Related proteins in other pathogens are secreted via a type I secretion system (TOSS), but such a system has not been demonstrated in N. meningitidis. An in silico search of the group B meningococcal genome suggested the presence of a uniquely organized TOSS. Genes encoding homologs of the Escherichia coli HlyB (ATP-binding), HlyD (membrane fusion), and TolC (outer membrane channel) proteins were identified. In contrast to the cistronic organization of the secretion genes in most other rtx operons, the hlyD and tolC genes were adjacent but unlinked to hlyB; neither locus was part of an operon containing genes encoding putative TOSS substrates. Both loci were flanked by genes normally associated with mobile genetic elements. The three genes were shown to be expressed independently. Mutation at either locus resulted in an inability to secrete FrpC and a related protein, here called FrpC2. Successful complementation of these mutations at an ectopic site confirmed the observed phenotypes were caused by loss of function of the putative TOSS genes. We show that genes scattered in the meningococcal genome encode a functional TOSS required for secretion of the meningococcal RTX proteins.

Protein secretion through autotransporter and two-partner pathways

Two distinct protein secretion pathways, the autotransporter (AT) and the two-partner secretion (TPS) pathways are characterized by their apparent simplicity. Both are devoted to the translocation across the outer membrane of mostly large proteins or protein domains. As implied by their name, AT proteins contain their own transporter domain, covalently attached to the C-terminal extremity of the secreted passenger domain, while TPS systems are composed of two separate proteins, with TpsA being the secreted protein and TpsB its specific transporter. In both pathways, the secreted proteins are exported in a Sec-dependent manner across the inner membrane, after which they cross the outer membrane with the help of their cognate transporters. The AT translocator domains and the TpsB proteins constitute distinct families of protein-translocating, outer membrane porins of Gram-negative bacteria. Both types of transporters insert into the outer membrane as beta-barrel proteins possibly forming oligomeric pores in the case of AT and serve as conduits for their cognate secreted proteins or domains across the outer membrane. Translocation appears to be folding-sensitive in both pathways, indicating that AT passenger domains and TpsA proteins cross the periplasm and the outer membrane in non-native conformations and fold progressively at the cell surface. A major difference between AT and TPS pathways arises from the manner by which specificity is established between the secreted protein and its transporter. In AT, the covalent link between the passenger and the translocator domains ensures the translocation of the former without the need for a specific molecular recognition between the two modules. In contrast, the TPS pathway has solved the question of specific recognition between the TpsA proteins and their transporters by the addition to the TpsA proteins of an N-proximal module, the conserved TPS domain, which represents a hallmark of the TPS pathway.

Meningococcal vaccines

Meningococcal disease is one of the most feared and serious infections in the young and its prevention by vaccination is an important goal. The high degree of antigenic variability of the organism makes the meningococcus a challenging target for vaccine prevention. Meningococcal polysaccharide vaccines against serogroup A and C are efficacious and have been widely used, often in combination with serogroup Y and W135 components. Their relative lack of immunogenicity in young children and infants can be overcome by conjugation to a protein carrier. The effectiveness of serogroup C glycoconjugate vaccines in children of all ages has been demonstrated and they have now been introduced into routine vaccination schedules. Conjugate vaccines against other serogroups, including A, Y, and W135 will soon be available and it is hoped they may emulate this success. Prevention of serogroup B disease has proven more elusive. Several serogroup B vaccines based on outer membrane vesicles have been shown to be immunogenic and reasonably effective in adults and older children, but the protection offered by them is chiefly strain-specific. Multivalent recombinant PorA vaccines have been developed to broaden the protective effect, but no efficacy data are available as yet. Intensive efforts have been directed at other outer membrane protein vaccine candidates and lipopolysaccharide, and some of these have been shown to offer protection in experimental animal models. Nonpathogenic Neisseriae spp. such as Neisseria lactamica are also possible vaccine candidates. Previously unknown proteins have been identified from in silico analysis of the meningococcal genome and their vaccine potential explored. However, none of these has yet been presented as the 'universal' protective antigen and work in this field continues to be held back by our limited knowledge concerning the mechanisms of natural protection against serogroup B meningococci.

Neisseria meningitidis NadA is a new invasin which promotes bacterial adhesion to and penetration into human epithelial cells

Neisseria meningitidis is a human pathogen, which is a major cause of sepsis and meningitis. The bacterium colonizes the upper respiratory tract of approximately 10% of humans where it lives as a commensal. On rare occasions, it crosses the epithelium and reaches the bloodstream causing sepsis. From the bloodstream it translocates the blood-brain barrier, causing meningitis. Although all strains have the potential to cause disease, a subset of them, which belongs to hypervirulent lineages, causes disease more frequently than others. Recently, we described NadA, a novel antigen of N. meningitidis, present in three of the four known hypervirulent lineages. Here we show that NadA is a novel bacterial invasin which, when expressed on the surface of Escherichia coli, promotes adhesion to and invasion into Chang epithelial cells. Deletion of the N-terminal globular domain of recombinant NadA or pronase treatment of human cells abrogated the adhesive phenotype. A hypervirulent strain of N. meningitidis where the nad A gene was inactivated had a reduced ability to adhere to and invade into epithelial cells in vitro. NadA is likely to improve the fitness of N. meningitidis contributing to the increased virulence of strains that belong to the hypervirulent lineages.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.

Biotechnology and vaccines: application of functional genomics to Neisseria meningitidis and other bacterial pathogens

Since its introduction, vaccinology has been very effective in preventing infectious diseases. However, in several cases, the conventional approach to identify protective antigens, based on biochemical, immunological and microbiological methods, has failed to deliver successful vaccine candidates against major bacterial pathogens. The recent development of powerful biotechnological tools applied to genome-based approaches has revolutionized vaccine development, biological research and clinical diagnostics. The availability of a genome provides an inclusive virtual catalogue of all the potential antigens from which it is possible to select the molecules that are likely to be more effective. Here, we describe the use of "reverse vaccinology", which has been successful in the identification of potential vaccines candidates against Neisseria meningitidis serogroup B and review the use of functional genomics approaches as DNA microarrays, proteomics and comparative genome analysis for the identification of virulence factors and novel vaccine candidates. In addition, we describe the potential of these powerful technologies in understanding the pathogenesis of various bacteria.

The autotransporter secretion system

The type V secretion system includes the autotransporter family, the two-partner system and the Oca family. The autotransporter secretion process involving first the translocation of the precursor through the inner membrane and then its translocation through the outer membrane via a pore formed by a beta-barrel is reviewed.

Mucosal vaccination against serogroup B meningococci: induction of bactericidal antibodies and cellular immunity following intranasal immunization with NadA of Neisseria meningitidis and mutants of Escherichia coli heat-labile enterotoxin

Conjugated polysaccharide vaccines protect against serogroup C meningococci. However, this approach cannot be applied to serogroup B, which is still a major cause of meningitis. We evaluated the immunogenicity of three surface-exposed proteins from serogroup B Neisseria meningitidis (App, NhhA, and NadA) identified during whole-genome sequencing. Mice were immunized intranasally with individual proteins in the presence of wild-type Escherichia coli heat-labile enterotoxin (LTwt), LTR72, a partially inactivated mutant, or LTK63, a completely nontoxic mutant, as the adjuvant. Each of the meningococcal proteins induced significant cellular responses; NhhA and NadA induced strong antibody responses, but only NadA induced bactericidal antibody when administered intranasally with mucosal adjuvants. In addition, immunoglobulin A and bactericidal antibodies were detected in the respiratory tract following intranasal delivery of NadA. Analysis of antigen-specific cytokine production by T cells from immunized mice revealed that intranasal immunization with NadA alone failed to generate detectable cellular immune responses. In contrast, LTK63, LTR72, and LTwt significantly augmented NadA-specific gamma interferon, interleukin-4 (IL-4), IL-5, and IL-10 production by spleen and lymph node cells, suggesting that both Th1 and Th2 cells were induced in vivo. The strongest cellular responses and highest bactericidal antibody titers were generated with LTR72 as the adjuvant. These findings demonstrate that the quality and magnitude of the immune responses generated by mucosal vaccines are influenced by the antigen as well as the adjuvant and suggest that nasal delivery of NadA with mucosal adjuvants has considerable potential in the development of a mucosal vaccine against serogroup B meningococci.

A Neisserial autotransporter NalP modulating the processing of other autotransporters

Autotransporters constitute a relatively simple secretion system in Gram-negative bacteria, depending for their translocation across the outer membrane only on a C-terminal translocator domain. We have studied a novel autotransporter serine protease, designated NalP, from Neisseria meningitidis strain H44/76, featuring a lipoprotein motif at the signal sequence cleavage site. Indeed, lipidation of NalP could be demonstrated, but the secreted 70 kDa domain of NalP lacked the lipid-moiety as a result of additional N-terminal processing. A nalP mutant showed a drastically altered profile of secreted proteins. Mass-spectrometric analysis of tryptic fragments identified the autotransporters IgA protease and App, a homologue of the adhesin Hap of Haemophilus influenzae, as the major secreted proteins. Two forms of both of these proteins were found in the culture supernatant of the wild-type strain, whereas only the lower molecular-weight forms predominated in the culture supernatant of the nalP mutant. The serine-protease active site of NalP was required for the modulation of the processing of these autotransporters. We propose that, apart from the autoproteolytic processing, NalP can process App and IgA protease and hypothesize that this function of NalP could contribute to the virulence of the organism.