When viruses infect cells, intrinsic defensive actions are initiated almost immediately. These defenses include the innate immune system, which provides cytokines to halt virus infection, and modulate the adaptive immune response should the infection proceed unchecked (Janeway and Medzhitov, 2002
). The innate immune system is activated when microbial products, such as lipopolysaccharide or viral nucleic acids, are detected. RNA viruses are recognized as foreign by cellular sensors that are activated by viral proteins or nucleic acids, leading to the production of the critical antiviral type I interferons.
Sensing of RNA virus infection by the innate immune system is carried out by membrane-bound Toll-like receptors, or by cytoplasmic sensors such as PKR, RIG-I, and MDA-5 (reviewed in (Kato et al., 2005
; Yoneyama and Fujita, 2007
; Yoneyama et al., 2004
)). RIG-I and MDA-5 proteins comprise an amino-terminal caspase recruitment domain (CARD) and an RNA helicase domain (Kang et al., 2002
). Results of a recent study on the evolution of RIG-I and MDA-5 indicate that the unique protein domain arrangement evolved independently by domain grafting and not by a simple gene duplication event of the entire four-domain arrangement, which may have been initiated by differential sensitivity of these proteins to viral infection (Sarkar et al., 2008
). Additionally, MDA-5, but not RIG-I, orthologs are found in fish indicating that MDA-5 might have evolved before RIG-I (Sarkar et al., 2008
). After binding viral RNA, these sensors interact with a CARD-containing adaptor protein, IPS-1, located in the outer membrane of mitochondria (Kawai et al., 2005
; Meylan et al., 2005
; Seth et al., 2005
; Xu et al., 2005
). This interaction mediates recruitment and activation of protein kinases that phosphorylate the transcription protein IFN-regulatory factor 3, leading to synthesis of type I IFN.
An important question is how RIG-I and MDA-5 distinguish viral from cellular RNAs. It was originally believed that these proteins recognize dsRNA, which is rarely found in the cytoplasm of cells but is abundant in virus-infected cells (Yoneyama et al., 2004
). More recently it has become clear that RIG-I recognizes RNA with a 5′-triphosphate (Hornung et al., 2006
; Pichlmair et al., 2006
). Because most cellular cytoplasmic RNAs bear a 5′-cap structure, this observation seems to explain the ability of RIG-I to discriminate between host and viral RNA. This substrate specificity is supported by observations that suggest that RIG-I and MDA-5 specialize in recognition of different viruses. Infection of mice lacking the gene encoding either protein reveals that RIG-I is essential for detecting infection by rhabdoviruses, influenza viruses, paramyxoviruses, and flaviviruses (Kato et al., 2006
). Replication of these viruses leads to production of RNAs with a 5′-triphosphate. In contrast, MDA-5 senses infection with picornaviruses, whose RNA 5′-ends are linked to a viral protein, VPg, not a 5′-triphosphate (Gitlin et al., 2006
; Kato et al., 2006
). It has been suggested that dsRNAs produced during picornavirus replication are the substrates for MDA-5 recognition.
Despite these elegant innate mechanisms, virus infections still occur because their genomes encode proteins that antagonize this and every other step of host defense. Examples include inhibition of RIG-I function by binding of the influenza virus NS1 protein (Guo et al., 2007
; Mibayashi et al., 2007
; Opitz et al., 2007
), and cleavage of IPS-1 by proteases encoded in the genomes of picornaviruses and hepatitis C virus (Li et al., 2005
; Lin et al., 2006
) (J. Drahos and V. Racaniello, unpublished data). We previously showed that MDA-5 is degraded in cells infected with different picornaviruses, and suggested that such cleavage might be a mechanism to antagonize production of type I IFN in response to viral infection (Barral et al., 2007
). Here we examine the state of RIG-I during picornavirus infection. We found that RIG-I is degraded in cells infected with poliovirus, rhinoviruses, echovirus, and encephalomyocarditis virus. In contrast to MDA-5, cleavage of RIG-I is not accomplished by cellular caspases or the proteasome (Barral et al., 2007
). Rather, the viral proteinase 3Cpro
cleaves RIG-I, both in vitro and in cells. Cleavage of RIG-I during picornavirus infection may constitute another mechanism for attenuating the innate response to viral infection.