A gamma-glutamyl transpeptidase of Aphidius ervi venom induces apoptosis in the ovaries of host aphids.

Parasitism by the endophagous braconid Aphidius ervi (Hymenoptera, Braconidae) has a negative impact on the reproductive activity of its host, Acyrthosiphon pisum (Homoptera, Aphididae). The host castration is induced by the parasitoid venom and is reproduced by the injection of chromatographic fractions highly enriched with two proteins, of 18 (p18) and 36 kDa (p36) in size, respectively. Here we demonstrate that these bioactive proteins trigger apoptosis of the cells in the germaria and ovariole sheath of the host aphid. Both p18 and p36 were internally sequenced and the gathered information was matched against the deduced amino acid sequence of the putative proteins encoded by cDNA clones, randomly selected from a cDNA library, which was raised using mRNA extracted from A. ervi venom glands. The identified cDNA clones contained an insert corresponding to the RNA product of an interrupted gene, made of six exons and five introns, which was found to be transcribed at higher levels in adult females of A. ervi than in males. This gene codes for a putative protein composed of 541 amino acids, with a calculated molecular mass of 56.9 kDa, which contained the amino acid sequences experimentally determined for both p18 and p36. This putative protein showed a significant level of sequence identity with gamma-glutamyl transpeptidases (gamma-GT), and it was named Ae-gamma-GT. The gamma-GTs are enzymes which play a key role in the metabolism of glutathione (GSH) and, as observed in most organisms, they are membrane-bound heterodimers formed by a large and a small subunit, which originate by post-translational processing of a single-chain precursor. The expression in insect cells of Ae-gamma-GT confirmed the occurrence of the expected post-translational processing, and demonstrated that, unlike other gamma-GTs, this protein is secreted in the extracellular environment. A measurable gamma-GT activity was detected in the venom of A. ervi and in the chromatographic fractions containing Ae-gamma-GT. Thus, we suggest that this venom protein may induce apoptosis in the host ovarioles by generating an alteration of the GSH metabolism and a consequent oxidative stress.

Protein tyrosine phosphatases of Toxoneuron nigriceps bracovirus as potential disrupters of host prothoracic gland function.

The genomic sequence of the bracovirus associated with the wasp Toxoneuron nigriceps (Hymenoptera, Braconidae) (TnBV), an endophagous parasitoid of the tobacco budworm larvae, Heliothis virescens (Lepidoptera, Noctuidae), contains a large gene family coding for protein tyrosine phosphatases (PTPs). Here we report the characterization of cDNAs for two of the viral PTPs isolated by screening a cDNA library from haemocytes of parasitized host larvae. The two encoded proteins show 70% amino acid identity and are expressed in the fat body of parasitized hosts. In addition, one was expressed in inactivated prothoracic glands (PTGs), 24 h after parasitoid oviposition. The rapid block of ecdysteroidogenesis does not appear to be due to inhibition of general protein synthesis, as indirectly indicated by the unaltered S6 kinase activity in the cytosolic extracts of basal PTGs from parasitized host larvae. Rather, TnBV PTP over-expression in inactivated host PTGs suggests that gland function may be affected by the disruption of the phosphorylation balance of key proteins regulating points upstream from the ribosomal S6 phosphorylation in the PTTH signaling cascade.