The outer-membrane protein MafA of Neisseria meningitidis constitutes a novel protein secretion pathway specific for the fratricide protein MafB

MafB proteins are toxins secreted by <i>Neisseria</i> spp. which are involved in interbacterial competition. Their secretion mechanism has so far not been elucidated. Each strain can produce several MafB variants. On the chromosome, the <i>mafB</i> genes are localized on genomic islands also containing <i>mafA</i> genes. MafA proteins have a role in virulence with reported activities in adhesion and transcytosis of pathogenic <i>Neisseria, a priori</i> unrelated to MafB activities. In this study, we investigated the possible involvement of MafA in the transport of MafB across the outer membrane of <i>Neisseria meningitidis</i>. In wild-type strains, proteolytic fragments of MafB proteins were detected in the extracellular medium. In the absence of MafA, secretion was abrogated, and, in the case of MafB_I, full-length and truncated polypeptides were detected inside the cells and inside outer-membrane vesicles. MafB_I secretion required its cognate MafA, whereas MafB_III could use any MafA. Heterologous expression in <i>Escherichia coli</i> showed that MafB_III is transported to a cell-surface-exposed, i.e. protease-accessible, location in a MafA-dependent way. MafA itself was found to be localized to the outer membrane, forming large oligomeric complexes. As homologs were found in diverse bacteria, the Maf system represents a new protein secretion system in Gram-negative bacteria.