Filamentous hemagglutinin (FHA), the major 230-kDa adhesin of the whooping cough agent Bordetella pertussis, is one of the most efficiently secreted proteins in Gram-negative bacteria. FHA is secreted by means of the two-partner secretion (TPS) pathway. A TPS system is composed of two separate proteins, with TpsA the secreted protein and TpsB its associated specific outer-membrane transporter. All TPS-secreted proteins contain a distinctive N-proximal module essential for secretion, the TPS domain. The TPS domain folds into a beta-helix. This domain is found in a wide variety of secreted proteins from bacterial pathogens.

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