The eukaryotic mRNA 3′ poly(A) tail and the 5′ cap cooperate to synergistically enhance translation. This interaction is mediated, at least in part, by eIF4G, which bridges the mRNA termini by simultaneous binding the poly(A)-binding protein (PABP) and the cap-binding protein, eIF4E. The poly(A) tail also stimulates translation from the internal ribosome binding sites (IRES) of a number of picornaviruses. eIF4G is likely to mediate this translational stimulation through its direct interaction with the IRES. Here, we support this hypothesis by cleaving eIF4G to separate the PABP-binding site from the portion that promotes internal initiation. eIF4G cleavage abrogates the stimulatory effect of poly(A) tail on translation. In addition, translation in extracts in which eIF4G is cleaved is resistant to inhibition by the PABP-binding protein 2 (Paip2). The eIF4G cleavage-induced loss of the stimulatory effect of poly(A) on translation was mimicked by the addition of the C-terminal portion of eIF4G. Thus, PABP stimulates picornavirus translation through its interaction with eIF4G.

The translation of picornavirus genomic RNAs occurs by a cap-independent mechanism that requires the formation of specific ribonucleoprotein complexes involving host cell factors and highly structured regions of picornavirus 5′ noncoding regions known as internal ribosome entry sites (IRES). Although a number of cellular proteins have been shown to be involved in picornavirus RNA translation, the precise role of these factors in picornavirus internal ribosome entry is not understood. In this report, we provide evidence for the existence of distinct mechanisms for the internal initiation of translation between type I and type II picornavirus IRES elements. In vitro translation reactions were conducted in HeLa cell cytoplasmic translation extracts that were depleted of the cellular protein, poly(rC) binding protein 2 (PCBP2). Upon depletion of PCBP2, these extracts possessed a significantly diminished capacity to translate reporter RNAs containing the type I IRES elements of poliovirus, coxsackievirus, or human rhinovirus linked to luciferase; however, the addition of recombinant PCBP2 could reconstitute translation. Furthermore, RNA electrophoretic mobility-shift analysis demonstrated specific interactions between PCBP2 and both type I and type II picornavirus IRES elements; however, the translation of reporter RNAs containing the type II IRES elements of encephalomyocarditis virus and foot-and-mouth disease virus was not PCBP2 dependent. These data demonstrate that PCBP2 is essential for the internal initiation of translation on picornavirus type I IRES elements but is dispensable for translation directed by the structurally distinct type II elements.

The encephalomyocarditis virus of the diabetogenic M-strain (EMC-M) is known to cause diabetes in mice. The EMC-M virus has also been shown to cause paresis in some of the infected animals. The clinical features include an acute ascending predominantly motor paralysis, developing within days. This resembles acute idiopathic polyneuritis. The alpha motor neurons would be a possible target for the virus, so two parameters, the total number and the size distribution of motor neurons, were therefore selected for further investigation in 6 mice with neurological involvement and compared with 6 control mice. The optical fractionator method was applied for estimating the total number of motor neurons and the 3D size distribution was estimated using the rotator method in a vertical design. No difference was found in the total number of motor neurons and the size distributions were similar in the 2 groups. This design can be used as a model for the estimation of the total number of motor neurons and their size distribution in other experimental animal models.