Striped murrel S1 family serine protease: immune characterization,antibacterial property and enzyme activities

We report a molecular characterization of S1 family serine protease (SP-1) from snakehead murrel (or called striped murrel) Channa striatus (Cs). CsSP-1 polypeptide contained a catalytic core domain (otherwise known as serine protease trypsin domain) between H20 and I237 along with a catalytic triad at H⁶¹, D¹⁰⁴ and S¹⁹⁷. Phylogenetic analysis confirmed that CsSP-1 belongs to serine protease S1 family. The tertiary structure showed that CsSP-1 contains 14 β-sheets as 2 separate β-barrels (the first β-barrel consists of 8 β-sheets in the N-terminal region and the second β-barrel consists of 6 β-sheets in the C-terminal region) and 3 α-helical regions. Significantly (P < 0.05) the highest CsSP-1 mRNA expression was observed in intestine, liver and kidney, moderate expression was seen in spleen, head kidney, skin and blood, and the lowest one in brain, gill, muscle and heart. Further, the expression was induced in intestine with fungus Aphanomyces invadans and bacteria Aeromonas hydrophila. The recombinant CsSP-1 protein showed antibacterial activity against both gram-negative and gram-positive bacteria. The optimum CsSP-1 enzyme activity against the substrate casein was determined at 8 mM casein concentration. Moreover, the activity was highly influenced by 5 mM phenyl-methylsulfonyl fluoride followed by ethylenediaminetetraacetic acid, 4-(2-aminoethyl)benzenesulfonylfluoride hydrochloride and calpain inhibitor I. The CsSP-1 enzyme exhibited the highest activity at pH 7.5 and temperature 35°C. The overall results showed the potential involvement of CsSP-1 in the immune system of murrels. However, further research is necessary to study the mechanism of implicit trypsin association in the defence process.