Identification and evaluation as a DNA vaccine candidate of a virulence-associated serine protease from a pathogenic Vibrio parahaemolyticus isolate

A putative serine protease gene was cloned from the genomic DNA of Vibrio parahaemolyticus FYZ8621.4. The gene consisted of 1779 base pairs and encoded a 592 amino acid protein. The gene was expressed in Escherichia coli. The expressed protease was purified by Ni-NTA His-Bind Resin column and showed a 63 kDa band on SDS–PAGE. The protease exhibited proteolytic activity on gelatin agar plate and showed maximal proteolytic activity at pH 8.0 and 37 °C. It hydrolyzed N-α-benzoyl-l-tyrosine p-nitroanilide (BAPNA), but did not N-benzoyl-l-arginine ethylester (BAEE), N-benzoyl-l-tyrosine ethylester (BTEE) and N-acetyl-l-tyrosine ethylester (ATEE). Mutants at conserved residues Asp51 (Asp51–Asn), His89 (His89–Asp) and Ser318 (Ser318–Leu, Ser318–Pro) lost proteolytic activities completely. The protein was confirmed to belong to serine protease. The purified serine protease was toxic to zebrafish with a LD50 of 15.4 μg/fish. A DNA vaccine was constructed by inserting the mutated serine protease (Ser318–Pro) gene into pEGFP-N1 plasmid. The pEGFP-N1/m-vps was transfected in HeLa cells. The serine protease was confirmed to be expressed by fluorescence microscopy observation and Western blotting analysis. The pEGFP-N1/m-vps was further observed to express in muscle of the injected turbot (Scophthalmus maximus) by Western blotting seven days after immunization. Efficient protection against lethal V. parahaemolyticus challenge was observed on the vaccinated turbot with pEGFP-N1/m-vps, with the highest relative percent survival (RPS) of 96.11%. Significant specific antibody responses were also observed in the turbot vaccinated with the DNA vaccine. The results indicated that the serine protease might be a potential virulence factor and could be used as an efficient vaccine candidate for the disease control caused by V. parahaemolyticus.