Multiple amino acid sequence alignment of PA serine proteases. CLUSTALW was used to align amino acid sequences of PA serine proteases for which their structures were determined experimentally or predicted computationally (highlighted in yellow). Bovine chymotrypsin B (CTRB, highlighted in magenta) is used as a standard reference for residue numbering. Only the regions showing the conserved catalytic residues His (H), Asp (D) and Ser (S) are shown. Amino acid residues with 100% conservation (*) between aligned sequences are either highlighted in blue (catalytic residues) or red (other). Other residues showing high (:) conservation (highlighted in gray) or medium (.) conservation are also indicated.