Molecular characterisation of Apolipophorin-III gene in Samia cynthia ricini and its roles in response to bacterial infection.

Apolipophorin-III (ApoLp-III) is an abundant hemolymph protein mainly involved in lipid transport and innate immunity in insects. In the present study, the gene Samia cynthia ricini ApoLp-III (ScApoLp-III) was identified from a transcriptome database, and contained 790 nucleotides with a putative open reading frame (ORF) of 561 bp encoding 186 amino acid residues. Phylogenetic analysis revealed that ScApoLp-III had significant homology with ApoLp-III protein from Antheraea pernyi. Higher ScApoLp-III expression levels were found in the fat body and silk gland by reverse transcription quantitative PCR (RT-qPCR). Injection of Staphylococcus aureus induced up-regulation of ScApoLp-III in the midgut, fat body and hemocytes. However, ScApoLp-III was down-regulated in the midgut and fat body after Pseudomonas aeruginosa injection, indicating that ScApoLp-III may contribute to the host's defense against invading pathogens. Additionally, recombinant ScApoLp-III was found to bind different bacteria, including E. coli, P. aeruginosa, S. aureus and B. subtilis. Bactericidal tests showed that recombinant ScApoLp-III strongly inhibited Gram-negative bacteria, including Escherichia coli and P. aeruginosa. However, it had no obvious influence on Gram-positive bacteria. Taken together, our results suggest that the ScApoLp-III might play an important role in the innate immunity of S. c. ricini.