A Putative Digestive Cysteine Proteinase from Drosophila melanogaster is Predominantly Expressed in the Embryonic and Larval Midgut

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Abstract
Plant seeds have biodefense systems for protection against insects. One of these systems may be based on the occurrence of phytocystatins, cysteine proteinase inhibitors of plant origin, that probably inhibit insect cysteine proteinases involved in digestive functions. To elucidate a molecular mechanism for this biodefense phenomenon, we isolated a gene encoding a putative digestive cysteine proteinase from Drosophila melanogaster, a suitable model species. The cloned genomic DNA fragment contained a sequence encoding a cysteine proteinase. The mature enzyme from the encoded protein, termed Drosophila cysteine proteinase‐1, consisted of 218 amino acid residues. Drosophila cysteine proteinase‐1 showed 67% similarity in its amino acid sequence to a lobster cysteine proteinase‐3 contained in the digestive juice. This enzyme also showed significant similarities to cysteine proteinases of animal origin such as cathepsins H and L, and to proteinases of plant origin such as rice oryzains α and β. In situ hybridization studies for the embryo showed that the mRNA for Drosophila cysteine proteinase‐1 was predominantly expressed in the midgut. Larval alimentary organs, such as the salivary gland and the midgut including the gastric caeca, also expressed the mRNA at significant levels. These observations, suggesting that Drosophila cysteine proteinase‐1 is a digestive cysteine proteinase which can be used as a model target of phytocystatins, will hopefully lead to new strategies for the regulation of pest insects.