Cap-dependent binding of mRNA to the 40 S ribosomal subunit during translational initiation requires the association of eukaryotic initiation factor 4G (eIF4G; formerly eIF-4γ and p220) with other initiation factors, notably eIF4E, eIF4A, and eIF3. Infection of cells by picornaviruses results in proteolytic cleavage of eIF4G and generation of a cap-independent translational state. Rhinovirus 2A protease and foot-and-mouth-disease virus L protease were used to analyze the association of eIF4G with eIF4A, eIF4E, and eIF3. Both proteases bisect eIF4G into N-and C-terminal fragments termed cp N and cp c . cp N was shown to contain the eIF4E-binding site, as judged by retention on m 7 GTP-Sepharose, whereas cp c was bound to eIF3 and eIF4A, based on ultracentrifugal co-sedimentation. Further proteolysis of cp N by L protease produced an 18-kDa polypeptide termed cp N2 which retained eIF4E binding activity and corresponded to amino acid residues 319-479 of rabbit eIF4G. Further proteolysis of cp c yielded several smaller fragments. cp C2 (~887-1402) contained the eIF4A binding site, whereas cp C3 (~480-886) contained the eIF3 binding site. These results suggest that cleavage by picornaviral proteases at residues 479-486 separates eIF4G into two domains, one required for recruiting capped mRNAs and one for attaching mRNA to the ribosome and directing helicase activity. Only the latter would appear to be necessary for internal initiation of picornaviral RNAs.

Enteroviruses such as Coxsackievirus B3 can cause dilated cardiomyopathy, but the mechanism of this pathology is unknown. Mutations in cytoskeletal proteins such as dystrophin cause hereditary dilated cardiomyopathy, but it is unclear if similar mechanisms underlie acquired forms of heart failure. We demonstrate here that purified Coxsackievirus protease 2A cleaves dystrophin in vitro as predicted by computer analysis. Dystrophin is also cleaved during Coxsackievirus infection of cultured myocytes and in infected mouse hearts, leading to impaired dystrophin function. In vivo, dystrophin and the dystrophin-associated glycoproteins alpha-sarcoglycan and beta-dystroglycan are morphologically disrupted in infected myocytes. We suggest a molecular mechanism through which enteroviral infection contributes to the pathogenesis of acquired forms of dilated cardiomyopathy.

Recognition of the 5-cap structure of mRNA by eIF4E is a critical step in the recruitment of most mRNAs to the ribosome. In Caenorhabditis elegans, ϳ70% of mRNAs contain an unusual 2,2,7-trimethylguanosine cap structure as a result of trans-splicing onto the 5 end of the pre-mRNA. The characterization of three eIF4E isoforms in C. elegans (IFE-1, IFE-2, and IFE-3) was reported previously. The present study describes two more eIF4E isoforms expressed in C. elegans, IFE-4 and IFE-5. We analyzed the requirement of each isoform for viability by RNA interference. IFE-3, the most closely related to mammalian eIF4E-1, binds only 7-methylguanosine caps and is essential for viability. In contrast, three closely related isoforms (IFE-1, IFE-2, and IFE-5) bind 2,2,7-trimethylguanosine caps and are partially redundant, but at least one functional isoform is required for viability. IFE-4, which binds only 7-methylguanosine caps, is most closely related to an unusual eIF4E isoform found in plants (nCBP) and mammals (4E-HP) and is not essential for viability in any combination of IFE knockout. ife-2, ife-3, ife-4, and ife-5 mRNAs are themselves trans-spliced to SL1 spliced leaders. ife-1 mRNA is transspliced to an SL2 leader, indicating that its gene resides in a downstream position of an operon.Eukaryotic mRNAs and small nuclear RNAs synthesized by RNA polymerase II are posttranscriptionally modified to form a 5Ј-5Ј GpppN linkage (1). The 5Ј-terminal G is methylated at N7 while still in the nucleus to yield an MMG 1 cap. The cap of small nuclear RNAs is then further methylated at N2 in the cytoplasm to yield a TMG cap (2). Methylation of small nuclear RNAs is dependent upon the binding of Sm proteins to form small nuclear ribonucleoproteins. Formation of the TMG cap is the targeting signal for import of small nuclear ribonucleoproteins back into the nucleus to take part in pre-mRNA splicing (3, 4). mRNAs, on the other hand, which possess only the MMG cap, remain in the cytoplasm.In some primative eukaryotes, including Caenorhabditis elegans, mRNAs acquire a TMG cap through the process of trans-splicing (5). Primary transcripts from approximately 70% of protein-coding genes are trans-spliced to 22-nt SL sequences, such that the original MMG caps are replaced with the TMG caps from the SL small nuclear RNAs (6, 7). Also common in C. elegans is the organization of genes into operons that are transcribed from a single promotor into a polycistronic RNA (8). trans-Splicing results in the processing of these primary transcripts into monocistronic mRNAs. Generally, the mRNA from the first cistron is trans-spliced to SL1,whereas mRNAs from downstream cistrons are trans-spliced to SL2 or SL2 variants (8). mRNAs that are not trans-spliced retain the original MMG cap. Thus, both MMG-and TMG-capped mRNAs are found in the cytoplasm of C. elegans. Both types of mRNA enter polyribosomes and are translated, indicating that they are competent to interact with the translational machinery (9).The recruitment of mRNAs to ribosomes is catalyzed by...