Here, we show that retroviral GAG and the GAG-interacting RNA packaging signals can recruit AGO2 onto RNA without eliciting translation repression. However, our results also confirm an important contribution of host miRNAs in the interaction of AGO2 with retroviral RNAs (12
). In fact, the majority of HIV-1 RNAs requires host miRNAs to interact with AGO2: 70% of unspliced and 98% of spliced HIV-1 RNAs interact with AGO2 in a miRNA-dependent manner (C). Thus, we can now distinguish at least two ways to recruit AGO2 on retroviral mRNAs: one elicited by host miRNAs (12
) and a second, mediated by GAG and the RNA packaging sequences. These two types of interaction are not exclusive and are probably implicated in distinct steps of the retroviral life cycle. As viruses have co-evolved with the miRNA repertoire of their hosts (14
), the first mode, that is dependent on host miRNAs, could have an impact on retroviral replication: for instance, at a particular time point [e.g.
latent infection (17
)], in specific cells [e.g.
resting cells (17
)] and/or with certain RNAs (e.g.
spliced RNAs, C). On the other hand, mRNA recognition by miRNAs [ and (12
)], interaction with other RNAi proteins [such as DCP1 (A and A) or AGO1 (D and C)] and sequestration in P-bodies (19
) could also represent deleterious consequences of the recruitment of AGO2 or other miRNA-related components on viral RNAs. PFV-1 and HIV-1 could have therefore developed protein- or RNA-based strategies to limit the negative effects of cellular miRNAs (12
). Interestingly, host miRNAs also play both beneficial and detrimental roles in HCV replication (13
) and AGO2 was recently shown to be required for efficient HCV replication (27
). Similar to the retroviral GAG protein, the miR-122 recruits an AGO2-containing complex onto viral mRNAs (27
) with unclear consequences: miR-122 is able to stimulate HCV translation (50
) but this effect is not sufficient to fully explain its actions on HCV replication (14
). Recently, and in accordance with our results, the replication of HCV RNA was shown to depend on recruitment of AGO2 and miR-122 to lipid droplets, not P-bodies, while suppression of HCV RNA by siRNA and AGO2 involves interaction with P-bodies (53
). It has been postulated that the miR-122 interaction with HCV RNA changes during the viral life cycle (28
), as hypothesized here in the case of the interactions between retroviral RNAs and AGO2. Hence, it is possible that HCV and retroviruses similarly hijack some AGO2 functions that are not related to translation regulation. Provided, the considerable differences existing between Retroviridae
, it is tempting to speculate that these AGO2 functions are also implicated in the replication of other viruses. The experiments showing that vaccinia, influenza A or encephalomyocarditis viruses are not affected by the blockade of miRNA biogenesis (15
) do not exclude an authentic contribution of some RNAi-related proteins, in particular AGO2, in their replication.
Taken together, our results shed a new light on the cellular functions of AGO2 as it can be recruited onto messenger RNAs without eliciting RNA silencing. Our results support the idea that AGO2 has original functions that are not related to miRNAs and RNA silencing (54
). In fact, AGO2 has previously been found in specific protein complexes that are not linked to miRNA biogenesis or RNA interference (54
). We anticipate that these functions will be deciphered by further studies on the AGO2-containing complex(es) implicated in viral replication.