Outer membrane hemagglutinin secretion protein, FhaC. Functionally important conserved motifs have been identified (Delattre et al., 2010).  The x-ray structure reveals a beta-barrel pore obstructed by two structural elements conserved in all two partner secretion systems, an N-terminal α-helix and an extracellular loop.  FhaC goes from the closed to the open state in the presence of the filamentous haemagglutinin adhesin, FHA.  The N-terminal α-helix is displaced into the periplasm during FHA secretion (Guérin et al. 2014).  With two POTRA domains in the periplasm, a transmembrane beta barrel and a large loop harboring a functionally important motif, FhaC epitomizes the conserved features of the superfamily (Jacob-Dubuisson et al. 2009). The conserved secretion domain of FHA interacts with the POTRA domains, specific extracellular loops and strands of FhaC and the inner beta-barrel surface. The interaction map indicates a funnel-like pathway, with conformationally flexible FHA entering the channel in a non-exclusive manner and exiting along a four-stranded beta-sheet at the surface of the FhaC barrel. This sheet of FhaC guides the secretion domain of FHA along discrete steps of translocation and folding (Baud et al. 2014).  The membrane-proximal POTRA domain exists in several conformations, and the binding of FHA displaces this equilibrium (Guérin et al. 2015).