The type V secretion pathway encompasses the autotransporter proteins (AT-1), the two-partner secretion system (TPS) and the recently described type AT-2 family of proteins. AT proteins contain their own transporter domain, covalently attached to the C-terminal extremity of the secreted passenger domain. 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 AT passenger domains cross the periplasm and the outer membrane in non-native conformations and fold progressively at the cell surface. A major difference between AT and TPS pathways arises from the manner by which specificity is established between the secreted protein and its transporter. To date, all of the functionally characterized autotransporters have been implicated in bacterial virulence by displaying enzymatic activity (protease, peptidase, lipase, esterase), mediating actin-promoted bacterial motility or acting as adhesins, toxins/cytotoxins, immunomodulatory proteins or, as more recently discovered, permitting maturation of another virulent protein. The structures of several translocator domains have been solved, suggesting that the &ß-barrel consists of 12 strands. FabF is a new class of secretion system similar to type V secretion in the autotransporter proteins (AT-1), characterized by a C-terminal, 12-stranded ß barrel with a helix-blocked pore in the closed state. However, in the truncated FapF structure, the N terminus exits the barrel on the periplasmic side rather than passing completely through the pore as in autotransporters. Regarding Hapb, it represents the ß barrel domain of self-associating autotransporters (SAATs). SAATs is a set of virulence factors that enhance bacterial biofilm 87. The Haemophilus influenzae Hap SAAT consists of three domains, the residues 1 - 25 correspond to the signal peptide, the residues 26 - 1036 refer to the Haps that called the passenger domain, while the residues 1037 - 1394 form the 14 ß strands at the C-terminal.

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