The type V secretion pathway encompasses the autotransporter proteins (AT), the two-partner secretion system (TPS) and the type AT-2 family of proteins. TPS systems are composed of two separate proteins, with TpsA being the secreted protein and TpsB its specific transporter. The secreted proteins are exported in a Sec-dependent manner across the inner membrane, after which they cross the outer membrane with the help of their cognate transporters. Translocation appears to be folding-sensitive, indicating that TpsA proteins cross the periplasm and the outer membrane in non-native conformations and fold progressively at the cell surface. A major difference of the TPS pathways compared to the AT arises from the manner in which specificity is established between the secreted protein and its transporter. The TPS pathway has solved the question of specific recognition between the TpsA proteins and their transporters by the addition to the TpsA proteins of an N-proximal module, the conserved TPS domain, which represents a hallmark of the TPS pathway. The exoproteins of the TPS have been reported to be adhesins, haem-binding proteins, antigenic factors or haemolysins/cytolysins. The structure of the TPS domain has been solved and shows to fold as a ß helix. The crystal structure of the transmembrane domain of the Bordetella pertussis TpsB transporter (FhaC) has been resolved revealling a ß-barrel with 16 transmembrane strands.

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