Human influenza virus NS1 protein enhances viral pathogenicity and acts as an RNA silencing suppressor in plants

Abstract

RNA silencing has a well-established function as an antiviral defence mechanism in plants and insects. Using an Agrobacterium-mediated transient assay, we report here that NS1 protein from human influenza A virus suppresses RNA silencing in plants in a manner similar to P1/HC-Pro protein of Tobacco etch potyvirus, a well-characterized plant virus silencing suppressor. Moreover, we have shown that NS1 protein expression strongly enhances the symptoms of Potato virus X in three different plant hosts, suggesting that NS1 protein could be inhibiting defence mechanisms activated in the plant on infection. These data provide further evidence that an RNA silencing pathway could also be activated as a defence response in mammals.Higher eukaryotes are involved in a continuing battle against viruses. To minimize the effects of viral infection, a number of defence mechanisms have been developed based on the recognition of specific molecular patterns produced only in infected cells (Plasterk, 2002). This is the case with double-stranded RNA (dsRNA) molecules, which are not normally found in eukaryotic cells but are generated as an intermediate molecule during virus replication (Hutvagner & Zamore, 2002). In mammalian cells, dsRNA appears to play a major role in the induction of the interferon (IFN) response following viral infection (Stark et al., 1998). In recent years, another dsRNA-mediated defence mechanism known as RNA silencing has been described (Dougherty & Parks, 1995;Montgomery & Fire, 1998).RNA silencing is a sequence-specific RNA degradation process that leads to elimination of the targeted RNA mediated by cytoplasmic nucleases and plays a natural antiviral role in plants (for reviews, see Baulcombe, 2002;Carrington et al., 2001;Vance & Vaucheret, 2001;Vazquez Rovere et al., 2002;Waterhouse et al., 2001). Recently, this mechanism of antiviral defence has also been reported in insect cells (Li et al., 2002).In response to these types of host antiviral defences, it is not unexpected that viruses have devised counteracting mechanisms that interfere with them at different levels. Thus, many animal viruses are known to prevent or inhibit IFN-mediated defence (García-Sastre, 2001). The same is true for RNA silencing: many plant viruses (Carrington & Whitham, 1998;Li & Ding, 2001;Voinnet et al., 1999) and one insect virus (Li et al., 2002) have been shown to encode silencing suppressor proteins. Nearly 20 silencing suppressors described so far show no sequence similarity and hence appear to have evolved independently to overcome silencing-mediated defence. Thus, it is not easy to predict which protein is going to behave as an RNA silencing suppressor based on sequence analysis.Since this RNA silencing has been described in species from different kingdoms (fungi, animals and plants), it has been proposed that it can also play an antiviral role in mammalian cells (Cullen, 2002;Gitlin et al., 2002). Here, we have investigated whether the NS1 protein, a multifunctional protein of human influenza A virus, could act as an RN...

Suppressors of RNA silencing encoded by plant viruses and their role in viral infections

“…Interestingly, the first animal virus suppressor of RNA silencing described is the B2 protein encoded by Flock house virus (FHV), an insect virus known to infect plants [38,39]. Subsequently, several dsRNA-binding proteins including the vaccinia virus protein E3L, and the influenza virus protein NS1 have all been shown to suppress RNA silencing using the plant agro-infiltration system and/or in insect cell cultures [39,40]. Even more remarkable is the recent demonstration that the Primate foamy virus type 1(PFV-1), a complex retrovirus similar to Human immunodeficiency virus, encodes a potent silencing suppressor (Tas) that functions in both cultured human cells and in Arabidopsis plants [41].…”

Structural basis for dsRNA recognition by NS1 protein of influenza A virus

“…NS1 protein was reported to have an RNA-silencing suppression activity in plants [18,19], similar to established RNA-silencing suppressor proteins, such as cucumovirus 2b (CMV2b) and flock house virus B2 (FHVB2) [20,21]. To test whether NS1A RBD domain itself is sufficient to suppress RNAi in vitro and whether Arg38 is responsible for dsRNA binding in vivo, we performed Agrobacterium co-infiltration assay in transgenic plants that express green fluorescent protein (GFP), as described previously [22].…”

The NS3 protein of Rice hoja blanca tenuivirus suppresses RNA silencing in plant and insect hosts by efficiently binding both siRNAs and miRNAs

“…Also human-infecting viruses encode proteins that can act as suppressors of RNA silencing. The NS1 protein of Influenza virus A is active in insect cells as well as in plants (Bucher et al 2004;Delgadillo et al 2004;Li et al 2004) and the NSs protein of La Crosse virus (LACV) shows RNA silencing inhibition in human cells (Soldan et al 2005).…”