A comparison of the amino acid sequence of the serine protease of the fish pathogen Aeromonas salmonicida subsp. salmonicida with those of other subtilisin-type enzymes relative to their substrate-binding sites.

The amino acid sequence of the so-called 70 kDa (actually 64 kDa) serine protease secreted by the Gram-negative fish pathogen Aeromonas salmonicida has been determined. It shows a high degree of homology with the complete sequence of other bacterial serine proteases which, with molecular masses of approximately 30 kDa, are less than half its size. This homology is particularly marked in regions adjacent to the catalytic triad Asp32, His64 and Ser221 of subtilisin BPN'. Significant features of the A. salmonicida enzyme, a new member of the group of cysteine-containing subtilisin-type serine proteases, are the presence of six cysteine residues in the mature enzyme, a 37 amino acid extension at the N-terminus and 215 amino acids at the C-terminus when compared with subtilisin BPN'. In addition to a number of smaller peptide insertions there is a non-aligned 32 amino acid sequence in a position corresponding to its introduction between Lys213 and Tyr214 of subtilisin BPN'. This sequence is highly hydrophilic, with Asp/Asn accounting for 10 of the 32 amino acids. Further, the possession of two Cys residues separated by 24 amino acids provides the capacity for stabilizing the peptide as an externalized loop.