A Functional Two-Partner Secretion System Contributes to Adhesion of Neisseria meningitidis to Epithelial Cells

Abstract

Neisseria meningitidis is a frequent commensal of the human nasopharynx causing severe invasive infections in rare cases. A functional two-partner secretion (TPS) system in N. meningitidis, composed of the secreted effector protein HrpA and its cognate transporter HrpB, is identified and characterized in this study. Although all meningococcal strains harbor at least one TPS system, the hrpA genes display significant C-terminal sequence variation. Meningococcal genes encoding the TPS effector proteins and their transporters are closely associated and transcribed into a single mRNA. HrpA proteins are translocated across the meningococcal outer membrane by their cognate transporters HrpB and mainly released into the environment. During this process, HrpA is proteolytically processed to a mature 180-kDa form. In contrast to other known TPS systems, immature HrpA proteins are stable in the absence of HrpB and accumulate within the bacterial cell. A small percentage of mature HrpA remains associated with the bacteria and contributes to the interaction of meningococci with epithelial cells.Export of proteins to the surface and protein secretion are implicated in many aspects of bacterial life, including cell-tocell communication, motility, and virulence. Highly sophisticated systems have evolved to ensure correct secretion of bacterial proteins, and many of them have been recognized as central virulence determinants. These include the type IV secretion system of Helicobacter pylori (2), type III secretion systems of members of the family Enterobacteriaceae (32), and bacterial autotransporters (18). Filamentous hemagglutinin (FHA) of Bordetella pertussis is the prototype of a family of large exoproteins in gram-negative bacteria that are secreted via the two-partner secretion (TPS) pathway (22,23). FHA and related proteins, termed TpsA for two-partner secretion protein A, are translocated across the inner membrane via the universal Sec machinery (8). Transport across the outer membrane requires a specific transporter termed TpsB. In B. pertussis, this transporter is referred to as FhaC (23). The Nterminal region of the TpsA proteins contains a signal peptide for secretion via Sec and the adjacent TPS signal or secretion domain of approximately 245 amino acids (aa) that targets the TpsA protein to its cognate transporter, TpsB (8,10,22,23). The genes encoding TpsA and TpsB are often found in close vicinity. Several TpsA proteins have been shown to contribute to virulence in plant and animal pathogens (5,13,38,43,48). B. pertussis FHA itself is both a major adhesion protein essential for establishing pathogen-host contact (29) and an important immunogen (26), representing the main constituent of the pertussis vaccine (35). By subtractive hybridization of DNA regions specific for Neisseria meningitidis strain Z2491 and absent in Neisseria gonorrhoeae, Klee et al. were the first to describe a homologue of FHA in meningococci (27). All three available meningococcal genomes contain open reading frames (ORFs) encoding putative...

Molecular Aspects ofMoraxella catarrhalisPathogenesis

“…A TPS typically consists of a transporter protein located in the outer membrane and the transported/ secreted protein partner(s). Well-known examples include the FHA protein of Bordetella pertussis (69) and Hag-related protein (Hrp) of Neisseria meningitidis (127), both of which have also been shown to be functionally involved in adhesion. The MhaC protein is predicted to form a ␤-barrel porin-like structure in the outer membrane, facilitating the transport of MhaB1 and MhaB2 across the bacterial outer membrane.…”

Comparative Genome Biology of a Serogroup B Carriage and Disease Strain Supports a Polygenic Nature of Meningococcal Virulence

“…TpsA proteins display significant C-terminal sequence variation and are translocated across the meningococcal outer membrane by their cognate transporters (TpsB). Since it was shown recently that a small percentage of mature TpsA remains associated with the bacteria and contributes to the interaction with epithelial cells, differences in the sequence and repertoire of TpsA proteins between strains might result in differences in the interaction with host cells (63). Finally, this class of most-variable proteins includes a glycosyl transferase (NMB0846), an MafB-3 alternative C-terminal cassette (NMB0655) (see below), the lactoferrin-binding protein B (LbpB/NMB1541), a transcription-repair coupling factor (NMB1281), and RNA-binding protein (NMB1826).…”

“…2). In particular, given that TPS proteins recently were found to contribute to the adhesion of N. meningitidis to epithelial cells (63), it was surprising that the TPS-encoding gene NMB1214 (TpsA3) and other TPS proteins on TPS1 (IHT-C) and TPS2 (IHT-B) ( Fig. 2; also see Table S5 in the supplemental material) were significantly downregulated upon adhesion.…”

The Polypeptide Transport-associated (POTRA) Domains of TpsB Transporters Determine the System Specificity of Two-partner Secretion Systems

“…Functions have only been attributed to system 1. TpsA1 (HrpA) promotes adherence to and the intracellular survival and escape from cultured human epithelial cell lines (21,22), is involved in biofilm formation (23), and acts as contact-dependent toxin involved in bacterial fratricide against other meningococcal strains (24).…”