No abstract

The interferon (IFN) system is a major effector of the innate immunity that allows time for the subsequent establishment of an adaptive immune response against a wide-range of pathogens. Their diverse biological actions are thought to be mediated by the products of specific but usually overlapping sets of cellular genes induced in the target cells. Ubiquitin ligase members of the tripartite motif (TRIM) protein family have emerged as IFN-induced proteins involved in both innate and adaptive immunity. In this report, we provide evidence that TRIM22 is a functional E3 ubiquitin ligase that is also ubiquitinated itself. We demonstrate that TRIM22 expression leads to a viral protection of HeLa cells against encephalomyocarditis virus infections. This effect is dependent upon its E3 ubiquitinating activity, since no antiviral effect was observed in cells expressing a TRIM22-deletion mutant defective in ubiquitinating activity. Consistent with this, TRIM22 interacts with the viral 3C protease (3C PRO ) and mediates its ubiquitination. Altogether, our findings demonstrate that TRIM22 E3 ubiquitin ligase activity represents a new antiviral pathway induced by IFN against picornaviruses. INTRODUCTIONPicornaviruses include important human pathogens such as rhinovirus, poliovirus and hepatitis A virus, as well as significant insect, plant and agricultural pathogens, such as foot-and-mouth disease virus (Whitton et al., 2005). Encephalomyocarditis virus (EMCV) is the prototype of the cardiovirus subgroup of picornaviruses. Its genome consists of a single-strand of messenger-sense RNA. Viral proteins are expressed from a large open reading frame encoding a giant precursor polyprotein (approx. 255 kDa), which is processed through a series of proteolytic cleavages to produce both capsid and non-structural proteins (Palmenberg, 1990). The majority of the processing reactions are carried out by the 3C protease (3C PRO ), which acts both inter-and intramolecularly to produce polyprotein precursors as well as the mature 3C PRO polypeptide (Palmenberg et al., 1979). Cleavage of the polyprotein normally occurs between glutamine-glycine or glutamine-serine amino acid pairs, which are usually flanked by proline residues (Palmenberg et al., 1984). Besides its commitment to the processing of the viral polyprotein, picornaviral 3C PRO can also facilitate virus replication by altering fundamental cellular processes such as transcription and translation of host cell mRNA (Barral et al., 2007;Ehrenfeld, 1982;Kuyumcu-Martinez et al., 2004; Neznanov et al., 2005;Shen et al., 1996;Yalamanchili et al., 1996Yalamanchili et al., , 1997. Because the regulated proteolysis mediated by 3C PRO is a critical feature in both the processing of picornaviral polyprotein and the inhibition of cellular defences, 3C PRO constitutes an important target for host antiviral innate pathways. Consistent with this, the degradation of EMCV 3C PRO by the ubiquitin/26S proteasome system has been reported (Losick et al., 2003;Schlax et al., 2007). Although the precise ro...