mallei strain ATCC 23344 (locus tag # BMA1027) that resembles the adhesins Yersinia enterocolitica YadA [2, 21, 52], Moraxella catarrhalis Hag [8, 53, 54], B. pseudomallei BoaA and BoaB [55], and B. mallei BoaA [55]. These molecules belong to the oligomeric coiled-coil adhesin (Oca) sub-family of oligomeric autotransporter proteins and have a characteristic modular organization consisting of: (i) a surface-exposed NVP-HSP990 cell line region specifying adhesive properties termed passenger domain, (ii) a short linker region predicted to form an α helix, and (iii) a hydrophobic C-terminus composed of four β-strands anchoring the NU7026 solubility dmso autotransporter in the OM designated transporter domain [16, 19–21]. As

shown in Figure  1A, BMA1027 is predicted to possess these structural features. Figure 1 Structural features of BMA1027 and orthologous gene products. Different regions of the protein encoded by B. mallei ATCC 23344 BMA1027 (A), B. pseudomallei K96243 BPSL1631 (B) and the B. pseudomallei DD503 BMA1027 ortholog (C) are depicted

with the positions of residues defining selected domains. Transporter domains (OM anchors) and helical linkers VX-661 were identified using the PSIPRED secondary structure prediction algorithm. The colored boxes, red triangles, and grey crosses show the relative position and number of repeated aa motifs. Searches using the Pfam database revealed that the region encompassing aa 936–1012 of BMA1027 shows similarity to a YadA anchor domain (PF3895.10; expect value of 6.3e−22), which is present in most Oca and described as important for oligomerization and targeting autotransporters to the OM. Pfam searches also indicated that BMA1027 contains four YadA stalk domains (PF05662, formerly designated HIM; expect values ranging from 2.2e−4 to 1.5e−9; grey crosses in Figure  1A). This motif is associated with invasins and haemagglutinins and is present in YadA as well as Hag [2, 8, 52, 53]. YadA contains oxyclozanide one stalk domain, which has been shown to be necessary for protein stability and adhesive properties. Further sequence analysis revealed that the passenger domain of BMA1027 specifies repeated aa motifs, a trait noted in several oligomeric autotransporters including

YadA [2, 52], Hag [8, 53], BoaA and BoaB [55], the B. pseudomallei biofilm factor BbfA [56], and the M. catarrhalis UspA1, UspA2, and UspA2H proteins [57–60]. As illustrated in Figure  1A, the passenger domain of BMA1027 contains nine copies of the 5-mer SLSTS (red triangles) and several repeated elements beginning with residues NSTA (colored boxes). Additional characteristics of the predicted protein are listed in Table  1. Table 1 Characteristics a of BMA1027 orthologous genes and their encoded products Strainb Locus tag Predicted protein (aa) MW (kDa) Potential signal sequence cleavage sitec B. pseudomallei           1026b/DD503* BP1026B_I1575 1,152 107.4 ASA37▼G, AMA69▼A   K96243 BPSL1631 1,124 104.8 ASA37▼G, AMA69▼A B. mallei           ATCC 23344 BMA1027 1,012 94.