Juvenile hormone induces the expression of the SnoSP2 gene encoding a methionine-rich hexamerin in Sesamia nonagrioides (Lepidoptera)

A gene encoding a methionine-rich storage protein, SnoSP2, was cloned and its complete cDNA sequence was determined in the corn stalk borer, Sesamia nonagrioides (Lepidoptera: Noctuidae). Potentially, SnoSP2 encoded a 748-amino acid protein, with a calculated molecular weight of 87.9 kDa and an isoelectric point pI = 9.41. Signal peptide of 15 amino acids is present at the N-terminus and the protein contained conserved insect larval storage protein signature sequence patterns. The deduced amino acid sequence of SnoSP2 showed the highest identity to the methionine-rich storage protein from Spodoptera litura (77%) and other methionine-rich storage proteins. SnoSP2 belongs to the subfamily of methionine-rich storage proteins (6.8% methionine, 9.5% aromatic amino acid), according to criteria of amino acid composition and phylogenetic analysis. Expression of SnoSP2 mRNA was determined using semi-quantitative RT-PCR and real-time PCR. When larvae were treated with juvenile-hormone analog, methroprene, SnoSP2 transcripts were induced. In non-diapausing conditions, the SnoSP2 mRNA presents in the beginning of fifth instar, increased dramatically during the sixth instar, peaked in the end of sixth instar, decreased in the early pupae and were very low at the late pupae. In diapausing conditions, SnoSP2 remain abundant through the pre-diapause, persists through deep diapause and disappear in the end of post-diapause phase.