A PUTATIVE DIGESTIVE CYSTEINE PROTEINASE FROM DROSOPHILA-MELANOGASTERIS PREDOMINANTLY EXPRESSED IN THE EMBRYONIC AND LARVAL MIDGUT

Plant seeds have biodefense systems for protection against insects. On e of these systems may be based on the occurrence of phytocystatins, c ysteine proteinase inhibitors of plant origin, that probably inhibit i nsect cysteine proteinases involved in digestive functions. To elucida te a molecular mechanism for this biodefense phenomenon, we isolated a gene encoding a putative digestive cysteine proteinase from Drosophil a melanogaster, a suitable model species. The cloned genomic DNA fragm ent contained a sequence encoding a cysteine proteinase. The mature en zyme 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 cyste ine proteinase-3 contained in the digestive juice. This enzyme also sh owed significant similarities to cysteine proteinases of animal origin such as cathepsins H and L, and to proteinases of plant origin such a s rice oryzains alpha and beta. In situ hybridization studies for the embryo showed that the mRNA for Drosophila cysteine proteinase-1 was p redominantly expressed in the midgut. Larval alimentary organs, such a s the salivary gland and the midgut including the gastric caeca, also expressed the mRNA at significant levels. These observations, suggesti ng that Drosophila cysteine proteinase-1 is a digestive cysteine prote inase which can be used as a model target of phytocystatins, will hope fully lead to new strategies for the regulation of pest insects.