Cloning, sequencing and expression of the gene encoding the extracellular metalloprotease of Aeromonas caviae

A gene (apk) encoding the extracellular protease of Aeromonas caviae Ae6 has been cloned and sequenced. For cloning the gene, the DNA genomic library was screened using skim milk LB agar. One clone harboring plasmid pKK3 was selected for sequencing. Nucleotide sequencing of the 3.5 kb region of pKK3 revealed a single open reading frame (ORF) of 1,785 bp encoding 595 amino acids. The deduced polypeptide contained a putative 16-amino acid signal peptide followed by a large propeptide. The N-terminal amino acid sequence of purified recombinant protein (APK) was consistent with the DNA sequence. This result suggested a mature protein of 412 amino acids with a molecular mass of 44 kDa. However, the molecular mass of purified recombinant APK revealed 34 kDa by SDS-PAGE, suggesting that further processing at the C-terminal region took place. The 2 motifs of zinc binding sites deduced are highly conserved in the APK as well as in other zinc metalloproteases including Vibrio proteolyticus neutral protease, Emp V from Vibrio vulnificus, HA/P from Vibrio cholerae, and Pseudomonas aeruginosa elastase. Proteolytic activity was inhibited by EDTA, Zincov, 1,10-phenanthroline and tetraethylenepentamine while unaffected by the other inhibitors tested. The protease showed maximum activity at pH 7.0 and was inactivated by heating at 80 C for 15 min. These results together suggest that APK belongs to the thermolysin family of metalloendopeptidases.