Functional study of the gene encoding apolipophorin III in development and immune responses in the beet armyworm, Spodoptera exigua

Apolipophorin III (ApoLpIII) plays a crucial role in lipid transport in insects. Its property of binding to lipid moieties is also used in pathogen recognition, such as its binding to lipopolysaccharides of Gram-negative bacteria. This study identified the cDNA sequence of ApoLpIII from the beet armyworm, Spodoptera exigua. Its cDNA was cloned to be 692 bp and it encoded 188 amino acid residues. The predicted amino acid sequence contained a signal peptide (17 residues) at the amino terminal region and one N-glycosylation site. Its predicted molecular weight was 19,042.18 Da after cleavage of the signal peptide. Secondary structure analysis of ApoLpIII predicted that 85.38% of the protein was composed of five α helix domains. RT-PCR analysis showed that ApoLpIII transcript was expressed in all developmental stages, but its expression was restricted to fat body and hemocyte in S. exigua. The expression of ApoLpIII was highly upregulated by bacterial or fungal challenges based on quantitative RT-PCR analysis. When the expression of ApoLpIII was suppressed by injection of its specific dsRNA, the treated larvae showed significant immunosuppression, which was analyzed by hemocyte nodule formation against bacterial challenge in S. exigua. In addition, larvae treated with the specific dsRNA showed high mortality during development. These results suggest that ApoLpIII is associated with development and immune responses in S. exigua.