The RTX cytotoxin-related FrpA protein of Neisseria meningitidis is secreted extracellularly by meningococci and by HlyBD+ Escherichia coli.

Neisseria meningitidis produces proteins (FrpA and FrpC) related to the RTX cytotoxin family. In meningococcal strain FAM20 these proteins were both localized in the outer membrane and secreted into the extracellular medium. An Escherichia coli strain with wild-type hemolysin secretion genes hlyB and hlyD and containing a cloned frpA gene secreted FrpA, whereas an isogenic hlyBD mutant strain did not. Low-stringency DNA hybridization revealed hlyBD-like sequences in N. meningitidis FAM20, suggesting that a similar RTX secretion system exists in meningococci. Structural features found at the C termini of other RTX proteins and thought to be important for their secretion were also found at the C terminus of FrpA. The secretion of FrpA from E. coli by heterologous RTX transport proteins further demonstrates the relation of the FrpA protein to RTX toxins.