FIGURE 1.

(A) Schematic representation of bacterial serine proteases of the HtrA family. The highly conserved domains of the HtrA serine proteases are indicated. Sig (TM)-signal sequence and trans-membrane domain; Trypsin-the proteolytic domain. Bacterial HtrA proteases may exhibit one or two PDZ domains. The export signal peptide, trypsin and unique PDZ domain of HtrA_BA as well as the H153, D183, and S255 residues forming the catalytic triad of the proteolytic domain were unambiguously identified by alignment with amino-acid sequences of 30 different bacterial serine proteases of the HtrA family using the T-coffee multiple sequence alignment program (Notredame et al., 2000; http://tcoffee.crg.cat). Similar alignments was previously documented (for example, Pallen and Wren, 1997; Kim and Kim, 2005; Singh et al., 2011). Boxed amino-acid sequence alignment: the high sequence conservation around the catalytic H, D, and S residues (shown in red with yellow background) is exemplified for six different bacterial HtrA serine proteases (hydrophobic residues are shown in yellow; polar non-charged residues are shown in green; positively charged residues are shown in blue; negatively charged residues are shown in pink). The respective bacteria are indicated on the left of the alignment. (B) Schematic description of the three forms of HtrA_BA used in trans-complementation experiments for determining the role of the PDZ domain and the proteolytic activity of HtrA_BA: full-length (FL), truncated (ΔPDZ) or mutated (htrAS255A). The trans-complementation HtrA_BA forms were engineered to exhibit a C terminal stretch of 6 histidine residues (6 × His, brown box) enabling zinc-affinity purification and Western blot visualization using anti-His antibodies.