Cleavage of eukaryotic translation initiation factor 4GII within foot-and-mouth disease virus-infected cells: identification of the L-protease cleavage site in vitro

Foot-and-mouth disease virus (FMDV) induces a very rapid inhibition of host cell protein synthesis within infected cells. This is accompanied by the cleavage of the eukaryotic translation initiation factor 4GI (eIF4GI). The cleavage of the related protein eIF4GII has now been analyzed. Within FMDV-infected cells, cleavage of eIF4GI and eIF4GII occurs with similar kinetics. Cleavage of eIF4GII is induced in cells and in cell extracts by the FMDV leader protease (L(pro)) alone, generating cleavage products similar to those induced by enterovirus and rhinovirus 2A protease (2A(pro)). By the use of a fusion protein containing residues 445 to 744 of human eIF4GII, it was demonstrated that the FMDV L(pro) specifically cleaves this protein between residues G700 and S701, immediately adjacent to the site (V699/G700) cleaved by rhinovirus 2A(pro) in vitro. The G700/S701 cleavage site does not correspond, by amino acid sequence alignment, to that cleaved in eIF4GI by the FMDV L(pro) in vitro. Knowledge of the cleavage sites and the three-dimensional structures of the FMDV L(pro) and rhinovirus 2A(pro) enabled mutant forms of the eIF4GII sequence to be generated that are differentially resistant to either one of these proteases. These results confirmed the specificity of each protease and showed that the mutant forms of the fusion protein substrate retained their correct sensitivity to other proteases.     

Poliovirus 2A(Pro) increases viral mRNA and polysome stability coordinately in time with cleavage of eIF4G

Poliovirus (PV) 2A protease (2A(Pro)) cleaves eukaryotic initiation factors 4GI and 4GII (eIF4GI and eIF4GII) within virus-infected cells, effectively halting cap-dependent mRNA translation. PV mRNA, which does not possess a 5' cap, is translated via cap-independent mechanisms within viral protease-modified messenger ribonucleoprotein (mRNP) complexes. In this study, we determined that 2A(Pro) activity was required for viral polysome formation and stability. 2A(Pro) cleaved eIF4GI and eIF4GII as PV polysomes assembled. A 2A(Cys109Ser) (2A(Pro) with a Cys109Ser mutation) protease active site mutation that prevented cleavage of eIF4G coordinately inhibited the de novo formation of viral polysomes, the stability of viral polysomes, and the stability of PV mRNA within polysomes. 2A(Cys109Ser)-associated defects in PV mRNA and polysome stability correlated with defects in PV mRNA translation. 3C(Pro) activity was not required for viral polysome formation or stability. 2A(Pro)-mediated cleavage of eIF4G along with poly(rC) binding protein binding to the 5' terminus of uncapped PV mRNA appear to be concerted mechanisms that allow PV mRNA to form mRNP complexes that evade cellular mRNA degradation machinery.     

Foot-and-mouth disease virus 3C protease induces cleavage of translation initiation factors eIF4A and eIF4G within infected cells

Infection of cells by foot-and-mouth disease virus (FMDV) results in the rapid inhibition of host cell protein synthesis. This process is accompanied by the early cleavage of the translation initiation factor eIF4G, a component of the cap-binding complex eIF4F. This cleavage is mediated by the leader (L) protease. Subsequently, as the virus proteins accumulate, secondary cleavages of eIF4G occur. Furthermore, eIF4A (46 kDa), a second component of eIF4F, is also cleaved in these later stages of the infection cycle. The 33-kDa cleavage product of eIF4A has lost a fragment from its N terminus. Transient-expression assays demonstrated that eIF4A was not cleaved in the presence of FMDV L or with the poliovirus 2A protease (which also mediates eIF4G cleavage) but was cleaved when the FMDV 3C protease was expressed. The FMDV 3C protease was also shown in such assays to induce cleavage of eIF4G, resulting in the production of cleavage products different from those generated by the L protease. Consistent with these results, within cells infected with a mutant FMDV lacking the L protease or within cells containing an FMDV replicon lacking L-P1 coding sequences it was again shown that eIF4A and eIF4G were cleaved.     

Recognition of eukaryotic initiation factor 4G isoforms by picornaviral proteinases

The leader proteinase (L(pro)) of foot and mouth disease virus is a papain-like cysteine proteinase. After processing itself from the polyprotein, L(pro) then cleaves the host protein eukaryotic initiation factor (eIf) 4GI, thus preventing protein synthesis from capped mRNA in the infected cell. We have investigated L(pro) interaction with eIF4GI and its isoform, eIF4GII. L(pro), expressed as a catalytically inactive fusion protein with glutathione S-transferase, binds specifically to eIF4G isomers in rabbit reticulocyte lysates. Deletion and specific mutagenesis were used to map the binding domain on L(pro) to residues 183-195 of the C-terminal extension and to residue Cys(133). These residues of the C-terminal extension and Cys(133) are adjacent in the crystal structure but lie about 25 A from the active site. The region on eIF4GI recognized by the L(pro) C-terminal extension was mapped to residues 640-669 using eIF4GI fragments generated by proteolysis or by in vitro translation. The L(pro) cleavage site at Gly(674) downward arrow Arg(675) was not necessary for binding. Similar experiments with human rhinovirus 2A proteinase (2A(pro)), a chymotrypsin-like cysteine proteinase that also cleaves eIF4G isoforms, revealed that 2A(pro) can also bind to eIF4GI fragments lacking its cleavage site. These experiments strongly suggest a novel interaction between picornaviral proteinases and eIF4G isoforms.     

The eukaryotic translation initiation factor 4GI is cleaved by different retroviral proteases

The initiation factor eIF4G plays a central role in the regulation of translation. In picornaviruses, as well as in human immunodeficiency virus type 1 (HIV-1), cleavage of eIF4G by the viral protease leads to inhibition of protein synthesis directed by capped cellular mRNAs. In the present work, cleavage of both eIF4GI and eIF4GII has been analyzed by employing the proteases encoded within the genomes of several members of the family Retroviridae, e.g., Moloney murine leukemia virus (MoMLV), mouse mammary tumor virus, human T-cell leukemia virus type 1, HIV-2, and simian immunodeficiency virus. All of the retroviral proteases examined were able to cleave the initiation factor eIF4GI both in intact cells and in cell-free systems, albeit with different efficiencies. The eIF4GI hydrolysis patterns obtained with HIV-1 and HIV-2 proteases were very similar to each other but rather different from those obtained with MoMLV protease. Both eIF4GI and eIF4GII were cleaved very efficiently by the MoMLV protease. However, eIF4GII was a poor substrate for HIV proteases. Proteolytic cleavage of eIF4G led to a profound inhibition of cap-dependent translation, while protein synthesis driven by mRNAs containing internal ribosome entry site elements remained unaffected or was even stimulated in transfected cells.     

Differential cleavage of eIF4GI and eIF4GII in mammalian cells. Effects on translation

Two isoforms of the translation initiation factor eIF4G, eIF4GI and eIF4GII, have been described in eukaryotic cells. The exact function of each isoform during the initiation of protein synthesis is still under investigation. We have developed an efficient and reliable method of expressing poliovirus 2Apro, which differentially proteolyzes eIF4GI and eIF4GII in a time- and dose-dependent manner. This system is based on the electroporation of an in vitro transcribed mRNA that contains the encephalomyocarditis virus internal ribosome entry site followed by the sequence of poliovirus 2Apro. In contrast to HeLa cells, expression of this protease in BHK-21 cells induces delayed hydrolysis kinetics of eIF4GI with respect to eIF4GII. Moreover, under these conditions the polyadenylate binding protein is not cleaved. Interestingly, translation of de novo synthesized luciferase mRNA is highly dependent on eIF4GI integrity, whereas ongoing translation is inhibited at the same time as eIF4GII cleavage. Moreover, reinitiation of a preexisting mRNA translation after polysome run-off is dependent on the integrity of eIF4GII. Notably, de novo translation of heat shock protein 70 mRNA depends little on eIF4GI integrity but is more susceptible to eIF4GII hydrolysis. Finally, translation of an mRNA containing encephalomyocarditis virus internal ribosome entry site when the two isoforms of eIF4G are differentially hydrolyzed has been examined.     

Human rhinovirus 2A proteinase cleavage sites in eukaryotic initiation factors (eIF) 4GI and eIF4GII are different

Several picornaviruses shut down host cellular protein synthesis by proteolytic cleavage of the eukaryotic initiation factor (eIF) 4GI and eIF4GII isoforms. Viral RNA translation is maintained by a cap-independent mechanism. Here, we identify the human rhinovirus 2 2A(pro) cleavage site in eIF4GII in vitro as PLLNV(699)*GSR; this sequence lies seven amino acids C-terminal to the cleavage site previously identified in eIF4GI (LSTR681*GPP).     

Visualizing and quantifying the differential cleavages of the eukaryotic translation initiation factors eIF4GI and eIF4GII in the enterovirus‐infected cell

Enterovirus (EV) infection has been shown to cause a marked shutoff of host protein synthesis, an event mainly achieved through the cleavages of eukaryotic translation initiation factors eIF4GI and eIF4GII that are mediated by viral 2A protease (2A^pro). Using fluorescence resonance energy transfer (FRET), we developed genetically encoded and FRET‐based biosensors to visualize and quantify the specific proteolytic process in intact cells. This was accomplished by stable expression of a fusion substrate construct composed of the green fluorescent protein 2 (GFP²) and red fluorescent protein 2 (DsRed2), with a cleavage motif on eIF4GI or eIF4GII connected in between. The FRET biosensor showed a real‐time and quantifiable impairment of FRET upon EV infection. Levels of the reduced FRET closely correlated with the cleavage kinetics of the endogenous eIF4Gs isoforms. The FRET impairments were solely attributed to 2A^pro catalytic activity, irrespective of other viral‐encoded protease, the activated caspases or general inhibition of protein synthesis in the EV‐infected cells. The FRET biosensors appeared to be a universal platform for several related EVs. The spatiotemporal and quantitative imaging enabled by FRET can shed light on the protease–substrate behaviors in their normal milieu, permitting investigation into the molecular mechanism underlying virus‐induced host translation inhibition. Biotechnol. Bioeng. 2009; 104: 1142–1152. © 2009 Wiley Periodicals, Inc.     

Efficient cleavage of ribosome-associated poly(A)-binding protein by enterovirus 3C protease

Poliovirus (PV) causes a rapid and drastic inhibition of host cell cap-dependent protein synthesis during infection while preferentially allowing cap-independent translation of its own genomic RNA via an internal ribosome entry site element. Inhibition of cap-dependent translation is partly mediated by cleavage of an essential translation initiation factor, eIF4GI, during PV infection. In addition to cleavage of eIF4GI, cleavage of eIF4GII and poly(A)-binding protein (PABP) has been recently proposed to contribute to complete host translation shutoff; however, the relative importance of eIF4GII and PABP cleavage has not been determined. At times when cap-dependent translation is first blocked during infection, only 25 to 35% of the total cellular PABP is cleaved; therefore, we hypothesized that the pool of PABP associated with polysomes may be preferentially targeted by viral proteases. We have investigated what cleavage products of PABP are produced in vivo and the substrate determinants for cleavage of PABP by 2A protease (2A(pro)) or 3C protease (3C(pro)). Our results show that PABP in ribosome-enriched fractions is preferentially cleaved in vitro and in vivo compared to PABP in other fractions. Furthermore, we have identified four N-terminal PABP cleavage products produced during PV infection and have shown that viral 3C protease generates three of the four cleavage products. Also, 3C(pro) is more efficient in cleaving PABP in ribosome-enriched fractions than 2A(pro) in vitro. In addition, binding of PABP to poly(A) RNA stimulates 3C(pro)-mediated cleavage and inhibits 2A(pro)-mediated cleavage. These results suggest that 3C(pro) plays a major role in processing PABP during virus infection and that the interaction of PABP with translation initiation factors, ribosomes, or poly(A) RNA may promote its cleavage by viral 2A and 3C proteases.     

Identification of caspase 3-mediated cleavage and functional alteration of eukaryotic initiation factor 2alpha in apoptosis

Induction of apoptosis in a variety of cell types leads to inhibition of protein synthesis. Recently, the cleavage of eukaryotic translation initiation factor 4G (eIF4G) by caspase 3 was described as a possible event contributing to translation inhibition. Here, we report the cleavage of another initiation factor in apoptotic cells, eIF2alpha, that could contribute to regulation of translation during apoptosis. This cleavage event could be completely inhibited by pretreatment of HeLa cells with Z-VAD-fmk. In vitro analysis using purified eIF2 and purified caspases showed cleavage of eIF2alpha by caspase 3, 6, 8, and 10 but not 9. Caspase 3 most efficiently cleaved eIF2alpha and this could be inhibited by addition of Ac-DEVD-CHO in vitro. Comparison of cleavage of phosphorylated versus nonphosphorylated eIF2alpha revealed a modest preference of the caspases for the nonphosphorylated form. When eIF2. 2B complex was used as substrate, only caspase 3 was capable of eIF2alpha cleavage, which was not affected by phosphorylation of the alpha subunit. The eIF2.GDP binary complex was cleaved much less efficiently by caspase 3. Sequence analysis of the cleavage fragment suggested that the cleavage site is located in the C-terminal portion of the protein. Analysis showed that after caspase cleavage, exchange of GDP bound to eIF2 was very rapid and no longer dependent upon eIF2B. Furthermore, in vitro translation experiments indicated that cleavage of eIF2alpha results in functional alteration of the eIF2 complex, which no longer stimulated upstream AUG selection on a mRNA containing a viral internal ribosome entry site and was no longer capable of stimulating overall translation. In conclusion, we describe here the cleavage of a translation initiation factor, eIF2alpha that could contribute to inhibition or alteration of protein synthesis during the late stages of apoptosis.     

Depletion of the cap-associated isoform of translation factor eIF4G induces germline apoptosis in C. elegans

During mammalian programmed cell death, cleavage of the translation initiation factor 4G proteins (eIF4GI and eIF4GII) by caspase-3 induces the cap-independent synthesis of pro-apoptotic proteins. Apoptosis occurs naturally in the gonad to remove germ cells that are not selected to grow as oocytes and mature into eggs. Here, we describe two major isoforms of Caenorhabditis elegans eIF4G that are derived from a single gene (ifg-1) and their separate roles in germline homeostasis. Full length IFG-1 protein (170 kDa isoform) differs from the shorter isoform (130 kDa) by the inclusion of the N-terminal domain containing the putative eIF4E-binding site required for mRNA cap recognition. Depletion of the cap-associated p170 isoform induced CED-4 expression in oocytes and markedly increased germline apoptotic events, but did not prevent early mitotic germ cell proliferation. Loss of both p170 and p130 suppressed germ cell proliferation and arrested larval development. Evidence suggests that eIF4G isoforms are differentially utilized during oogenesis to regulate germ cell apoptosis. We propose that an alternative mechanism to eIF4G cleavage may be employed in germ cells by changing the availability of the p170 isoform.     

Translation of mRNAs from Vesicular Stomatitis Virus and Vaccinia Virus Is Differentially Blocked in Cells with Depletion of eIF4GI and/or eIF4GII

Cytolytic viruses abrogate host protein synthesis to maximize the translation of their own mRNAs. In this study, we analyzed the eukaryotic initiation factor (eIF) 4G requirement for translation of vesicular stomatitis virus (VSV) and vaccinia virus (VV) mRNAs in HeLa cells using two different strategies: eIF4G depletion by small interfering RNAs or cleavage of eIF4G by expression of poliovirus 2A protease. Depletion of eIF4GI or eIF4GII moderately inhibits cellular protein synthesis, whereas silencing of both factors has only a slightly higher effect. Under these conditions, the extent of VSV protein synthesis is similar to that of nondepleted control cells, whereas VV expression is substantially reduced. Similar results were obtained when eIF4E was depleted. On the other hand, eIF4G cleavage by poliovirus 2A protease strongly inhibits translation of VV protein expression, whereas translation directed by VSV mRNAs is not abrogated, even though VSV mRNAs are capped. Therefore, the requirement for eIF4F activity is different for VV and VSV, suggesting that the molecular mechanism by which their mRNAs initiate their translation is also different. Consistent with these findings, eIF4GI does not colocalize with ribosomes in VSV-infected cells, while eIF2α locates at perinuclear sites coincident with ribosomes.     

Cap-independent translation initiation in Xenopus oocytes.

Eukaryotic cellular mRNAs contain a cap at their 5'-ends, but some viral and cellular mRNAs bypass the cap-dependent mechanism of translation initiation in favor of internal entry of ribosomes at specific RNA sequences. Cap-dependent initiation requires intact initiation factor eIF4G (formerly eIF-4gamma, eIF-4Fgamma or p220), whereas internal initiation can proceed with eIF4G cleaved by picornaviral 2A or L proteases. Injection of recombinant coxsackievirus B4 protease 2A into Xenopus oocytes led to complete cleavage of endogenous eIF4G, but protein synthesis decreased by only 35%. Co-injection of edeine reduced synthesis by >90%, indicating that eIF4G-independent synthesis involved ongoing initiation. The spectrum of endogenous proteins synthesized was very similar in the presence or absence of intact eIF4G. Translation of exogenous rabbit globin mRNA, by contrast, was drastically inhibited by eIF4G cleavage. The N-terminal cleavage product of eIF4G (cpN), which binds eIF4E, was completely degraded within 6-12 h, while the C-terminal cleavage product (cpC), which binds to eIF3 and eIF4A, was more stable over the same period. Thus, translation initiation of most endogenous mRNAs inXenopusoocytes requires no eIF4G, or perhaps only cpC, suggesting a cap-independent mechanism.     

Translation of Sindbis virus 26S mRNA does not require intact eukariotic initiation factor 4G

The infection of baby hamster kidney (BHK) cells by Sindbis virus gives rise to a drastic inhibition of cellular translation, while under these conditions the synthesis of viral structural proteins directed by the subgenomic 26S mRNA takes place efficiently. Here, the requirement for intact initiation factor eIF4G for the translation of this subgenomic mRNA has been examined. To this end, SV replicons that contain the protease of human immunodeficiency virus type 1 (HIV-1) or the poliovirus 2A(pro) replacing the sequences of SV glycoproteins have been constructed. BHK cells electroporated with the different RNAs synthesize protein C and the corresponding protease at late times. Notably, the proteolysis of eIF4G by both proteases has little effect on the translation of the 26S mRNA. In addition, recombinant viable SVs were engineered that encode HIV-1 PR or poliovirus 2A protease under the control of a duplicated late promoter. Viral protein synthesis at late times of infection by the recombinant viruses is slightly affected in BHK cells that contain proteolysed eIF4G. The translatability of SV genomic 49S mRNA was assayed in BHK cells infected with a recombinant virus that synthesizes luciferase and transfected with a replicon that expresses poliovirus 2Apro. Under conditions where eIF4G has been hydrolysed significantly the translation of genomic SV RNA was deeply inhibited. These findings indicate a different requirement for intact eIF4G in the translation of genomic and subgenomic SV mRNAs. Finally, the translation of the reporter gene that encodes green fluorescent protein, placed under the control of a second duplicate late promoter, is also resistant to the cleavage of eIF4G. In conclusion, despite the presence of a cap structure in the 5' end of the subgenomic SV mRNA, intact eIF4G is not necessary for its translation.     

The processing of eIF4GI by human rhinovirus type 2 2A(pro): relationship to self-cleavage and role of zinc

The 2A proteinase (2A(pro)) of human rhinoviruses (HRVs) is a cysteine protease containing a structurally important zinc ion. In the viral polyprotein, the enzyme cleaves between the C terminus of VP1 and its own N terminus. 2A(pro) also processes the two isoforms of the cellular protein, eukaryotic initiation factor 4G (eIF4G). We have shown that mature HRV2 2A(pro), when translated in vitro in rabbit reticulocyte lysates, efficiently cleaves eIF4GI, although the enzyme was not immediately active upon synthesis. Here, we examine the relationship between self-processing and eIF4GI cleavage. The onset of both reactions first occurred at least 10 min after initiation of protein synthesis. Furthermore, when self-processing was prevented by a specific mutation between VP1 and 2A(pro), the VP1-2A(pro) precursor was essentially unable to cleave eIF4GI, implying that self-processing is a prerequisite for eIF4GI cleavage. 2A(pro) synthesized in the presence of a potent zinc chelator is inactive; however, upon addition of excess zinc, HRV2 2A(pro) rapidly gained activity. Finally, the presence of the zinc chelator in the culture medium can protect HeLa cells from HRV infection.     

Multiple eIF4GI-Specific Protease Activities Present in Uninfected and Poliovirus-Infected Cells

Cleavage of eukaryotic translation initiation factor 4GI (eIF4GI) is required for shutoff of host cell translation during poliovirus (PV) infection of HeLa cells. Reports published by several groups have led to confusion whether this cleavage is mediated by viral 2A protease (2A(pro)) or a putative cellular enzyme (termed eIF4Gase) which is activated by 2A(pro) or other aspects of viral infection. Here we have further investigated eIF4Gase activities in PV-infected cells. Column purification of eIF4GI cleavage activity separated two activities which generated N-terminal cleavage products of different lengths. Both activities were detected using either native eIF4G or radiolabeled recombinant eIF4G as the substrate. Analysis of cleavage products formed by each activity on native and mutant substrates suggests that one activity cleaves eIF4G1 at or very near the 2A(pro) cleavage site and the other activity cleaves approximately 40 residues upstream of the 2A(pro) cleavage site. When PV infections in HeLa cells were supplemented with 2 mM guanidine, which indirectly limits expression of 2A(pro), two distinct C-terminal cleavage fragments of eIF4GI were detected. These C-terminal cleavage fragments of eIF4GI were purified from infected cells, and a new eIF4GI cleavage site was mapped to a unique site 43 amino acids upstream of the known 2A(pro) cleavage site. Further, eIF4GI cleavage in vivo could be blocked by addition of zVAD to PV-guanidine infections. zVAD is a broad-spectrum caspase inhibitor which had no effect on 2A(pro) cleavage activity or PV polyprotein processing. Lastly, similar types of eIF4Gase cleavage activities were also detected in uninfected cells under various conditions, including early apoptosis or during cell cycle transit. The data suggest that the same types of eIF4GI cleavage activities which are generated in PV-infected cells can also be generated in the absence of virus. Taken together, the data support a model in which multiple cellular activities process eIF4GI in PV-infected cells, in addition to 2A(pro).     

Cleavage of Poly(A)-Binding Protein by Coxsackievirus 2A Protease In Vitro and In Vivo: Another Mechanism for Host Protein Synthesis Shutoff?

Infection of cells by picornaviruses of the rhinovirus, aphthovirus, and enterovirus groups results in the shutoff of host protein synthesis but allows viral protein synthesis to proceed. Although considerable evidence suggests that this shutoff is mediated by the cleavage of eukaryotic translation initiation factor eIF4G by sequence-specific viral proteases (2A protease in the case of coxsackievirus), several experimental observations are at variance with this view. Thus, the cleavage of other cellular proteins could contribute to the shutoff of host protein synthesis and stimulation of viral protein synthesis. Recent evidence indicates that the highly conserved 70-kDa cytoplasmic poly(A)-binding protein (PABP) participates directly in translation initiation. We have now found that PABP is also proteolytically cleaved during coxsackievirus infection of HeLa cells. The cleavage of PABP correlated better over time with the host translational shutoff and onset of viral protein synthesis than did the cleavage of eIF4G. In vitro experiments with purified rabbit PABP and recombinant human PABP as well as in vivo experiments with Xenopus oocytes and recombinant Xenopus PABP demonstrate that the cleavage is catalyzed by 2A protease directly. N- and C-terminal sequencing indicates that cleavage occurs uniquely in human PABP at ⁴⁸²VANTSTQTM↓GPRPAAAAAA⁵⁰⁰, separating the four N-terminal RNA recognition motifs (80%) from the C-terminal homodimerization domain (20%). The N-terminal cleavage product of PABP is less efficient than full-length PABP in restoring translation to a PABP-dependent rabbit reticulocyte lysate translation system. These results suggest that the cleavage of PABP may be another mechanism by which picornaviruses alter the rate and spectrum of protein synthesis.     

Cleavage of poly(A)-binding protein by poliovirus 3C protease inhibits host cell translation: a novel mechanism for host translation shutoff

Cleavage of eukaryotic translation initiation factor 4GI (eIF4GI) by viral 2A protease (2Apro) has been proposed to cause severe translation inhibition in poliovirus-infected cells. However, infections containing 1 mM guanidine-HCl result in eIF4GI cleavage but only partial translation shutoff, indicating eIF4GI cleavage is insufficient for drastic translation inhibition. Viral 3C protease (3Cpro) cleaves poly(A)-binding protein (PABP) and removes the C-terminal domain (CTD) that interacts with several translation factors. In HeLa cell translation extracts that exhibit cap-poly(A) synergy, partial cleavage of PABP by 3Cpro inhibited translation of endogenous mRNAs and reporter RNA as effectively as complete cleavage of eIF4GI and eIF4GII by 2Apro. 3Cpro-mediated translation inhibition was poly(A) dependent, and addition of PABP to extracts restored translation. Expression of 3Cpro in HeLa cells resulted in partial PABP cleavage and similar inhibition of translation. PABP cleavage did not affect eIF4GI-PABP interactions, and the results of kinetics experiments suggest that 3Cpro might inhibit late steps in translation or ribosome recycling. The data illustrate the importance of the CTD of PABP in poly(A)-dependent translation in mammalian cells. We propose that enteroviruses use a dual strategy for host translation shutoff, requiring cleavage of PABP by 3Cpro and of eIF4G by 2Apro.     

Unique characteristics of a picornavirus internal ribosome entry site from the porcine teschovirus-1 talfan

The teschoviruses constitute a recently defined picornavirus genus. Most of the genome sequence of the porcine teschovirus-1 (PTV) Talfan and several other strains is known. We now demonstrate that initiation of protein synthesis occurs at nucleotide (nt) 412 on the PTV Talfan RNA and that nt 1 to 405 contains an internal ribosome entry site (IRES) that functions efficiently in vitro and within mammalian cells. In comparison with other picornavirus IRES elements, the PTV IRES is relatively short and lacks a significant polypyrimidine tract near the 3' end. Expression of an enterovirus 2A protease, which induces cleavage of eIF4G within the translation initiation complex eIF4F, has little effect on the PTV IRES activity within BHK cells. The PTV IRES has a unique set of properties and represents a new class of picornavirus IRES element.