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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 beta 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 beta 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 beta-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.
The Haemophilus influenzae Hia autotransporter contains an unusually short trimeric translocator domain
J Biol Chem. 2004 Apr 9;279(15):14679-85. doi: 10.1074/jbc.M311496200. Epub 2004 Jan 15.
Molecular analysis of transport and oligomerization of the Yersinia enterocolitica adhesin YadA
J Bacteriol. 2003 Jul;185(13):3735-44. doi: 10.1128/jb.185.13.3735-3744.2003.
YadA, the multifaceted Yersinia adhesin
Int J Med Microbiol. 2001 Aug;291(3):209-18. doi: 10.1078/1438-4221-00119.
Structure of the outer membrane translocator domain of the Haemophilus influenzae Hia trimeric autotransporter
EMBO J. 2006 Jun 7;25(11):2297-304. doi: 10.1038/sj.emboj.7601132. Epub 2006 May 11.