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. The prototype of the last family is the adhesin A (YadA) of Yersinia enterocolitica. Homologs are found in a variety of pathogenic bacteria. Proteins of this family mediate attachment to the surfaces of host cells and protect bacteria against complement and the bactericidal activities of defensins. Adhesins of this family form oligomeric lollipop-like structures anchored in the outer membrane by their C-termini. In contrast to the typical autotransporters, whose C-terminal porin regions are of about 250 residues long and possess 12 transmembrane ß strands, members of the AT-2 family, have their C-terminal pore-forming anchors composed of about only 70 residues, with just 4 possible transmembrane ß strands and form trimers. These facts suggest that the pore-forming translocator units evolved independently and they belong to distinct families. The structure of one such protein, the HiA autotransporter of Haemophilus influenzae has been solved revealing a ß-barrel with 12 transmembrane ß-strands, comprised by 4 strands from each subunit. The central channel has a pore of 1.8 nm in diameter that is traversed by three N-terminal alpha helices, one from each subunit.

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