Encephalomyocarditis viral protein 2A localizes to nucleoli and inhibits cap-dependent mRNA translation.

Panels of monoclonal antibodies were raised against viral non-structural proteins of encephalomyocarditis virus (EMCV) and used to probe infected cells in laser confocal microscopy experiments and Western analyses. Surprisingly, all Mengovirus and EMCV-infected cells showed strong targeting of protein 2A, 3B(VPg), 3C(pro), and 3D(pol) signals to cellular nuclei, in particular to nucleoli, from the earliest times of infection. Viral capsid proteins (1AB, 1C, and 1D) and other non-structural proteins (2B, 2C, and 3A) did not target nuclei and remained cytoplasmic throughout the infection. The cardioviral 2A protein (subject of this article) has a novel 143 amino acid sequence, terminating in a 19 amino acid COOH-terminal processing cassette (PCC) that participates in autocatalytic, co-translational primary cleavage of the viral polyprotein. The remainder of the 2A protein shares only limited similarity with other viral or cellular sequences, except for a short motif (KRvRPFRLP) near PCC resembling the nuclear localization signals (NLS) common to many yeast ribosomal proteins. Deletions within the EMCV 2A protein that impinge on this region have been reported to diminish the ability of virus to inhibit cap-dependent translation of cellular mRNAs. We have now observed that these same deletions prevented nuclear localization. Cellular expression of 2A protein from RNA transcripts or cDNAs confirmed that it does not require other viral proteins or activities for nuclear transport; even when expressed as a single protein, 2A protein effectively shuts off translation from capped reporter mRNAs. Within infected, transfected, or DNA vector-transformed cells, the 2A protein was always found in close association with the nucleolar ribosomal chaperone protein B23, which may help the traffic 2A into nucleoli like a surrogate ribosomal protein, by virtue of the putative nucleolar localization signal (NoLS). The data are consistent with a novel mechanism for virus-induced host protein shut off in cardioviruses, whereby 2A helps to upregulate the synthesis of new and modified ribosomes that have an inherent preference for internal ribosomal entry site (IRES)-dependent viral genome translation over cap-dependent host mRNA translation.

Encephalomyocarditis virus (EMCV) proteins 2A and 3BCD localize to nuclei and inhibit cellular mRNA transcription but not rRNA transcription.

We have followed the viral processing cascade and polyprotein precursor fates during encephalomyocarditis virus (EMCV) infection of HeLa cells using a panel of monoclonal antibodies (mAbs). Within the first 2-4 h of infection, signals of antibodies specific for the 2A, 3B(VPg), 3C(pro) and 3D(pol) proteins were found to co-localize in nucleoli at the rRNA synthesis and cellular protein B23 (nucleophosmin) sites. Cellular fractionation identified viral protein precursor 3BCD as the common source of the P3-region antibody signals. Previously thought to be a minor product of the polymerase region cleavage pathways, the nuclear targeting of this precursor was localized with engineered mutations to five P2 and P3 region polyprotein processing sites. A nuclear localization motif (NLS), similar to that in many yeast ribosomal proteins, was identified near the N-terminus of the 3D(pol) sequence. Point mutations within this motif prevented nuclear and nucleolar localization by all forms of 3B(VPg), 3C(pro) and 3D(pol), and were lethal to the virus because they also prevented genome replication. However, viral RNA synthesis was not required for nucleolar transport and 3BCD was found in nuclei, even when the 3D(pol) was inactivated. Co-immunoprecipitation experiments showed a tight association between 3BCD and B23 (nucleophosmin), suggesting a possible ribosomal protein-like mechanism for nuclear transport. Infected cell extracts analyzed with microarrays, quantitative slot-blots and pulse-labeling experiments confirmed a nearly complete shutoff of host pol-II-dependent mRNA synthesis during EMCV infection, in reactions that depended on wild-type 2A protein. In contrast to human rhinovirus-16 infection, rRNA synthesis by pol-I and pol-III were not turned off by EMCV, although the cellular concentration of rRNA decreased during infection, relative to control samples. The data suggest that nuclear targeting by 2A and 3BCD may be responsible for regulating cellular mRNA and rRNA transcription during infection, perhaps via a proteolytic mechanism catalyzed by the endogenous 3C(pro) sequence.