cDNA sequences and mRNA levels of two hexamerin storage proteins PinSP1 and PinSP2 from the Indianmeal moth, Plodia interpunctella.

In insects, storage proteins or hexamerins accumulate apparently to serve as sources of amino acids during metamorphosis and reproduction. Two storage protein-like cDNAs obtained from a cDNA library prepared from fourth instar larvae of the Indianmeal moth (Plodia interpunctella) were cloned and sequenced. The first clone, PinSP1, contained 2431 nucleotides with a 2295 nucleotide open reading frame (ORF) encoding a protein with 765 amino acid residues. The second cDNA, PinSP2, consisted of 2336 nucleotides with a 2250-nucleotide ORF encoding a protein with 750 amino acid residues. PinSP1 and PinSP2 shared 59% nucleotide sequence identity and 44% deduced amino acid sequence identity. A 17-amino acid signal peptide and a molecular mass of 90.4 kDa were predicted for the PinSP1 protein, whereas a 15-amino acid signal peptide and a mass of 88 kDa were predicted for PinSP2. Both proteins contained conserved insect larval storage protein signature sequence patterns and were 60-70% identical to other lepidopteran larval storage proteins. Expression of mRNA for both larval storage proteins was determined using the quantitative reverse transcription polymerase chain reaction method. Only very low levels were present in the second instar, but both mRNAs dramatically increased during the third instar, peaked in the fourth instar, decreased dramatically late in the same instar and pupal stages, and were undetectable during the adult stage. Males and females exhibited similar mRNA expression levels for both storage proteins during the pupal and adult stages. The results support the hypothesis that P. interpunctella, a species that does not feed after the larval stage, accumulates these two storage proteins as reserves during larval development for subsequent use in the pupal and adult stages.