Characterization of a functionally expressed dipeptidyl aminopeptidase III from Drosophila melanogaster

A Drosophila melanogaster cDNA clone (GH01916) encoding a putative 723-residue long (82 kDa) protein (CG 7415) and displaying 50% identity with mammalian cytosolic dipeptidyl aminopeptidase (DPP) III was functionally expressed in Schneider S-2 cells. Immunocytochemical studies using anti-(rat liver DPP III) Ig indicated the expression of this putative DPP III at the outer cell membrane and into the cytosol of transfected cells. Two protein bands (82 and 86 kDa) were immunologically detected after PAGE and Western blot of cytosol or membrane prepared from transfected cells. Western blot analysis of partially purified D. melanogaster DPP III confirmed the overexpression of these two protein bands into the cytosol and on the membranes of transfected cells. Despite the identification of six potential glycosylation sites, PAGE showed that these protein bands were not shifted after deglycosylation experiments. The partially purified enzyme hydrolysed the insect myotropic neuropeptide proctolin (Arg-Tyr-Leu-Pro-Thr) at the Tyr-Leu bond (K-m approximate to 4 mum). In addition, low concentration of the specific DPP III inhibitor tynorphin prevented proctolin degradation (IC50 = 0.62 +/- 0.15 mum). These results constitute the first characterization of an evolutionarily conserved insect DPP III that is expressed as a cytosolic and a membrane peptidase involved in proctolin degradation.