Functional characterization of the aspartic proteinase cathepsin D in the beet armyworm (Spodoptera exigua)

In insects, proteolytic enzymes are involved in food digestion and the metamorphosis process. In the present study, the full-length cDNA of an aspartic proteinase, Spodoptera exigua cathepsin D (SeCatD), was cloned, and its functions in metamorphosis were characterized. SeCatD contains an open reading frame of 1152 nucleotides, encoding a 384-amino acid polypeptide including a signal peptide and two functional domains (family A1 propeptide of amino acids (19-45) and a cathepsin D-like domain of 327 amino acids (55-381)). Three-dimensional structure analysis indicated that Asp66 and Asp251 may play important role in hydrolysis. Recombinant SeCatD was expressed in Sf9 insect cells and verified via SDS-PAGE and Western blot, the molecular mass of the expressed SeCatD was approximately 42kDa. The enzyme had an optimal pH value of 3 for activity. In addition, the tissue expression profile of SeCatD during metamorphosis was obtained, and the data demonstrated that SeCatD was expressed increasingly in the fat body and midgut, but not in the epidermis. Finally, injection of dsRNA-SeCatD into the fifth-instar larvae significantly reduced SeCatD expression and larvae survival rate compared to a dsRNA-GFP treatment. These data imply that SeCatD may function during metamorphosis and may represent a target for insect control.