ABSTRACT
Three goose-type (g-type) lysozymes, designated as OHLysG1, OHLysG2 and OHLysG3 were identified from expressed sequence tags (ESTs) of a gastropod Oncomelania hupensis, the intermediate host of Schistosoma japonicum. The full cDNA sequences of OHLysG1, OHLysG2 and OHLysG3 consisted of 735, 909 and 808 nucleotides, with an open reading frame of 198, 214 and 249 codons containing a 21, 7 and 8 amino acid (aa) signal peptide at the N-terminus, respectively. The three g-type lysozymes shared conserved features with other g-type lysozymes, such as the substrate binding sites, the catalytic residues critical for the fundamental structure and function of g-type lysozymes. It seems possible that g-type lysozymes in molluscs shared one conserved cysteine with those in birds and mammals, and six conserved cysteines were observed for mollusc g-type lysozymes, with two unique cysteines present in the g-type lysozymes of O. hupensis. The three lysozyme genes were expressed mainly in hepatopancreas, with relatively low expression level observed in head-foot muscle and intestine. When comparing S. japonicum-infected and uninfected snails, significant increase (P<0.05) was observed for all the three lysozymes in infected snails, with the highest increase detected in hepatopancreas, and lowest in intestine, implying their defensive role in the host-parasite, i.e. snail-trematode system. The three recombinant lysozymes expressed in Escherichia coli strain M15 showed lytic activity against Aeromonas hydrophila, Vibrio fluvialis, Aeromonas sobria and Micrococcus lysodeikticus. In conclusion, the finding of three g-type lysozymes in O. hupensis provides structural and functional evidence of multiple g-type lysozymes in gastropod, which may have evolutional implication in the snail-trematode system.
