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...