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Characterization of the IB-like gene family in polydnaviruses associated with wasps belonging to different Braconid subfamilies

Eric Espagne^3,

¹ Dipartimento di Biologia, Difesa e Biotecnologie Agro-Forestali, Università della Basilicata, Potenza, Italy
² Istituto di Genetica e Biofisica, CNR, Napoli, Italy
³ Institut de Recherche sur la Biologie de l'Insecte, CNRS, Université de Tours, Faculté des Sciences, Tours, France
⁴ Dipartimento di Biologia Strutturale e Funzionale, Università dell'Insubria, Varese, Italy
⁵ Dipartimento di Entomologia e Zoologia Agraria ‘F. Silvestri’, Università di Napoli ‘Federico II’, Via Università 100, 80055 Portici (NA), Italy

Correspondence
Francesco Pennacchio
f.pennacchio{at}unina.it

Polydnaviruses (PDVs) are obligate symbionts of hymenopteran parasitoids of lepidopteran larvae that induce host immunosuppression and physiological redirection. PDVs include bracoviruses (BVs) and ichnoviruses (IVs), which are associated with braconid and ichneumonid wasps, respectively. In this study, the gene family encoding IB-like proteins in the BVs associated with Cotesia congregata (CcBV) and Toxoneuron nigriceps (TnBV) was analysed. PDV-encoded IB-like proteins (ANK) are similar to insect and mammalian IB, an inhibitor of the transcription factor nuclear factor B (NF-B), but display shorter ankyrin domains and lack the regulatory domains for signal-mediated degradation and turnover. Phylogenetic analysis of ANK proteins indicates that those of IVs and BVs are closely related, even though these two taxa are believed to lack a common ancestor. Starting from a few hours after parasitization, the transcripts of BV ank genes were detected, at different levels, in several host tissues. The structure of the predicted proteins suggests that they may stably bind NF-B/Rel transcription factors of the tumour necrosis factor (TNF)/Toll immune pathway. Accordingly, after bacterial challenge of Heliothis virescens host larvae parasitized by T. nigriceps, NF-B immunoreactive material failed to enter the nucleus of host haemocytes and fat body cells. Moreover, transfection experiments in human HeLa cells demonstrated that a TnBV ank1 gene product reduced the efficiency of the TNF--induced expression of a reporter gene under NF-B transcriptional control. Altogether, these results suggest strongly that TnBV ANK proteins cause retention of NF-B/Rel factors in the cytoplasm and may thus contribute to suppression of the immune response in parasitized host larvae.

Present address: Institut de Génétique et Microbiologie, Université Paris Sud, Bat. 400, 91405 Orsay cedex, France.