Cloning and characterization of an invertebrate type lysozyme from Venerupis philippinarum

Lysozymes are key proteins to invertebrates in the innate immune responses against bacterial infections and providing nutrition as digestion enzymes. In the present study, an invertebrate type lysozyme (denoted as VpLYZ) was identified from Venerupis philippinarum haemocytes by cDNA library and RACE approaches. The full-length cDNA of VpLYZ consisted of 805 nucleotides with a canonical polyadenylation signal sequence AATAAA and a polyA tail, and an open-reading frame of 558bp encoding a polypeptide of 185 amino acids with a calculated molecular mass of 20.87kD and theoretical pI of 8.44. The high similarity of VpLYZ with other i-type lysozymes from mollusk indicated that VpLYZ should be a new member of i-type lysozyme family. Similar to most i-type lysozymes, VpLYZ possessed all conserved features critical for the fundamental structure and function of i-type lysozymes, such as three catalytic residues (Glu19, Asn72 and Ser75) and i-type specific motif CL(E/L/R/H)C(I/M)C. By semi-quantitative RT-PCR analysis, mRNA transcript of VpLYZ was found to be most abundantly expressed in the tissues of gills, hepatopancreas and haemocytes, weakly expressed in the tissues of muscle, foot and mantle. After clams were challenged by Vibrio anguillarum, the mRNA level of VpLYZ in overall haemocyte population was recorded by quantitative real-time RT-PCR. VpLYZ mRNA was down-regulated sharply from 6h to 12h post-infection. Then, the expression level increased to the peak at 72h and recovered to the original level at 96h. All these results indicated that VpLYZ was involved in the immune response against microbe infection and contributed to the clearance of bacterial pathogens.