PDVs associated with Cotesia spp. and the closely related Glyptapanteles indiensis encode cystatins which are tight-binding reversible inhibitors of C1A cysteine proteases, represented in insects by cathe-psins B, L, and F, and 26/29 kDa proteases. In insects these proteases have been associated with developmental processes such as embryogenesis, moulting, and metamorphosis (Liu et al., 2006), but there is also growing evidence that they may be involved in the host immune response (Saito et al., 1992; De Gregorio et al., 2001; Levy et al., 2003; Attardo et al., 2006). Several lines of evidence suggest that cystat-ins are important PDV virulence factors: cystatins are among the most highly expressed genes in the CcBV-Manduca interaction and recombinant cysta-tin 1 proved to be a functional and potent C1 cyst-eine protease inhibitor in vitro (Espagne et al, 2005). Furthermore, approaches combining molecular evolution and three-dimensional modelling have revealed that bracovirus cystatins are subject to strong diversifying selection acting in key active sites which are important for the interaction with target proteases. This particular selection, which is probably imposed by host defences, emphasizes the potential role of cystatins as pathogenic factors and suggests that cystatins co-evolve with host cysteine proteases (Serbielle et al., 2008). The characterization of host targets in this system and their function will enable us to understand the consequences of their inactivation during parasitism.
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