1.B.40 The Autotransporter-2 (AT-2) Family
The Yersinia adhesin -A (YadA) of Y. enterocolitica is a trimeric autotransporter adhesion with three major domains: an N-terminal head mediating adherence to host cells, a stalk involved in serum resistance, and a C-terminal anchor that forms a membrane pore and is responsible for the autotransport function. The anchor contains a glycine residue, nearly invariant throughout trimeric autotransporter adhesins, that faces the pore lumen: This conserved glycine residue affects both the export and the stability of YadA, and consequently some of its putative functions in pathogenesis (Reggenkamp et al., 2003; Grosskinsky et al., 2007). YadA mediates attachment to the surfaces of host cells and protects the bacteria against complement and the bactericidal activities of defensins (El Tahir and Skurnik, 2001). This protein is the prototype of a family of bacterial adhesins that form oligomeric lollypop-like structures anchored in the outer membrane by their C-termini. The AT-2 family has also been called the oligomeric coiled-coil adhesin (Oca) family (Desvaux et al. 2004). Leo et al. (2012) review these and other (putative) autotransporters. Several reports suggest that AT2 proteins may not alone translocate their passenger domains to the outer surface of the outer membrane, but instead may use the β-barrel assembly machinery (BAM complex) to accomplish this task (Peterson et al. 2018). This conclusion has been confirmed (van Ulsen et al. 2018).
The C-terminal region (residues 355-422) of YadA consists of four amphipathic β-strands and is necessary and sufficient for the outer membrane insertion of this trimeric adhesin. It is also responsible for oligomerization. The linker region (residues 331-369) is required for outer membrane translocation of the N-terminal passenger domain. In these functional respects, YadA resembles the autotransporters of the AT family (TC #1.B.12). The C-terminal translator domains of AT-2 Oca proteins of different origin are efficient translocators of the YadA passenger-domain. Further, the cognate TLD of YadA is essential for bacterial survival in human serum and mouse virulence (Ackermann et al., 2008).
YadA shows no sequence similarity with AT proteins in the C-terminal AT domain. Moreover, while the C-terminal porin regions of AT family members are of about 250 residues, exhibit about 14 transmembrane β-strands and form large oligomers (8-10 mers), the C-terminal pore-forming anchors of AT-2 family members are only of about 70 residues, have just 4 transmembrane β-strands, and form trimers. These facts suggest that the pore-forming units that translocate their N-terminal passenger domains evolved independently. They therefore belong to distinct families.
Hundreds of homologues of YadA have been sequenced. They have been termed invasins, immunoglobulin binding proteins, serum resistance proteins and hemagglutinins. They are encoded in the genomes of a wide variety of Gram-negative bacteria and their phage. They vary in size (340- >4000 aas) with multiple (2-8) repeat domains of about 150 residues, and these may consist of smaller repeat units.Tthe translocation process of autotransporters and the structural motifs that are unique to this class of proteins have been reviewed (Kiessling et al. 2019).
NhhA, Neisseria hia/hsf homologue, or GNA0992, is an oligomeric outer membrane protein of Neisseria meningitidis, included in the family of trimeric autotransporter adhesins. The last 72 C-terminal residues allow trimerization and localization of the N-terminal protein domain to the bacterial surface. E. coli strains expressing NhhA were able to adhere to epithelial cells. NhhA is a multifunctional adhesin, able to promote the bacterial adhesion to host cells and extracellular matrix components (Scarselli et al., 2006).
Haemagglutinins of B. xenovorans (1.B.40.1.2) contain repeat sequences that are homologous to repeat sequences in AT1 proteins and the toxins of TC# 1.C.11.1.4, 1.C.57.3.4 and 1.C.75.1.1, members of the RTX superfamily, as well as other toxins in these families, and TolA (2.C.1.2.1). These repeat sequences probably mediate protein-protein interacts and comprise parts of toxins.
The reaction catalyzed by YadA is:
N-terminal substrate domain (periplasm) → N-terminal substrate domain (outer surface of outer membrane)
Membrane anchored cell surface haemagglutinin (4726aas)
Haemagglutinin of Burkholderia xenovorans (Q13U92)
The YadB adhesin (364 aas) (Forman et al., 2008)
YadB of Yersinia pestis (Q7CHJ4)
The YadC adhesin (622 aas) (Forman et al., 2008)
YadC of Yersinia pestis (Q7CHJ5)
Cha of Haemophilus sp. (B3FNS7)
Aegerolysin domain-containing protein of 314 aas
UP of Streptomyces griseus
Auto transporter adhesin, BpaC, of 1125 aas. BpaC plays a central role in the initiation of the infectious process (Kiessling et al. 2019).
BpaC of Burkholderia pseudomallei
Trimeric autotransporter, HadA, of 256 aas. It is an atypical coliled-coil multifunctional adhesin of Haemophilus influenzae biogroup aegyptius, which promotes entry of the bacteria into host cells (Serruto et al. 2009).
HadA of Haemophilus influenzae
NhhA of Neisseria meningitidis
The extracellular matrix/adhesin autotransporter, EmaA, (collagen-binding adhesin of 1965 aas) (Tang et al., 2007). The extended signal peptide of the trimeric autotransporter EmaA modulates secretion (Jiang et al., 2011).
EmaA of Aggregatibacter (Actinobacillus) actinomycetemcomitans (Q6VBQ2)
The trimeric AT adhesin, essential for virulence, UpaG (1674aas) (Valle et al., 2008). The high resolution structure has been solved using the "dictionary" approach (Hartmann et al. 2012).
UpaG of EPEC E. coli (A8A667)
Adhesin (Hia) The 3-d structure is available (PDB#2GR7). Mediates bacterial adhesion to the respiratory epithelium. The crystal structure of the C-terminal end of Hia, corresponding to the entire Hia translocator domain and part of the passenger domain (residues 992-1098) shows that this domain forms a beta-barrel with 12 transmembrane beta-strands, including four strands from each subunit. The beta-barrel has a central channel of 1.8 nm in diameter that is traversed by three N-terminal alpha-helices, one from each subunit. Mutagenesis studies demonstrated that the transmembrane portion of the three alpha-helices and the loop region between the alpha-helices and the neighboring beta-strands are essential for stability, and that trimerization of the translocator domain is a prerequisite for translocator activity (Meng et al. 2006). Electrostatic repulsion between the positive charges of Arg1077 is important to prevent the formation of misassembled oligomers by the Hia transmembrane domain in vitro (Aoki et al. 2017).
Hia Adhesin of Haemophilus influenzae (Q8GM76)
The trimeric AT adhesin, essential for virulence, SadA (1461 aas). The high resolution structure has been solved using the "dictionary" approach (Hartmann et al. 2012). It's insertion into the outer membrane may be dependent on the BAM complex (TC# 1.B.33) as well as a small inner membrane lipoprotein, SadB (Grin et al. 2013).
SadA of Salmonella enterica
Adhesin Aha (Acinetobacter trimeric autotransporter) of 1873 aas. Ata contains all of the typical features of trimeric autotransporters, including a long signal peptide followed by an N-terminal, surface-exposed passenger domain and a C-terminal domain encoding 4 β-strands. Ata plays a role in biofilm formation and binds to various extracellular matrix/basal membrane (ECM/BM) components, including collagen types I, III, IV, and V and laminin (Bentancor et al. 2012).
Aha of Acinetobacter baumannii
Carbohydrate-binding autotransporter of 879 aas and 1 N-terminal TMS.
AT of Streptococcus salivarius
Outer membrane haemaglutinin autotransporter of 2012 aas
AT2 of Veillonella parvula
Putataive cell surface membrane anchored adhesin; haemagglutinin
Adhesin of Parachlamydia acanthamoebae (F8KWP8)
HP of Mycoplasma penetrans (Q8EWJ7)
Autotransporter of 516 aas, BimA. A polarly localized iron binding protein, BimC, determines the polar targeting as well as polar actin tail formation for motility (Lu et al. 2015).
BimA of Burkholderia pseudomallei (Pseudomonas pseudomallei)