Member of a two partner secretion pathway (TPS) in which it mediates the secretion of filamentous hemagglutinin FHA (fhaB). Monomer. In the crystal structure alpha helix H1 passes through the middle of the pore with its C-terminus in the periplasm and loop L6 also blocks the channel. Both of the POTRA domains are periplasmic; POTRA stands for polypeptide-transport associated. Belongs to the TPS (TC 1.B.20) family.