1 Institute of Biology, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
2 Department of Biochemistry, Boyce Hall, University of California, Riverside, CA 92521, USA
Correspondence
John F. Bol
j.bol{at}chem.leidenuniv.nl
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Although many viral RNAs lack a cap structure and/or a poly(A) tail, RNA viruses have evolved sophisticated strategies to allow their messengers to compete with those of the host. Picornaviruses do not have a cap structure, but the initiation factors bind to an internal ribosome entry site in the 5' untranslated region (UTR) of the RNA and mediate the interaction with PABP (Svitkin et al., 2001). Messengers of rotaviruses (family Reoviridae) do have a cap structure, but no poly(A) tail. The rotavirus non-structural NSP3 protein binds to the 3' end of the viral mRNA and interacts with eIF4G to enhance translation (Piron et al., 1998
; Vende et al., 2000
). The RNAs of Barley yellow dwarf virus (family Luteoviridae) have neither a cap structure nor a poly(A) tail, but long-distance base pairing between a stemloop structure in the 5' UTR and a translation enhancer in the 3' UTR have been proposed to ensure circularization of the RNAs and transfer of initiation factors to the 5' end (Guo et al., 2001
).
The replication strategy of Alfalfa mosaic virus (AMV), a virus with a tripartite positive-strand RNA genome, was studied in this report. RNAs 1 and 2 encode the replicase proteins P1 and P2, respectively, whereas RNA 3 encodes the movement protein (MP) and coat protein (CP). CP is translated from the subgenomic RNA 4. AMV is the type species of the genus Alfamovirus and a member of the family Bromoviridae. In this family, a mixture of the three genomic RNAs of bromo-, cucumo- or oleaviruses is infectious as such, whereas the RNAs of alfamo- and ilarviruses require the addition of a few molecules of CP per RNA molecule to initiate infection (Bol, 1999, 2003
; Jaspars, 1999
). The 3' termini of the RNAs of bromo- and cucumoviruses contain a tRNA-like structure (TLS) that can be charged with tyrosine. It has been shown that the 3' termini of the RNAs of AMV can adopt two alternative structures: a linear array of hairpins with a high affinity for CP (CPB conformation) or a pseudoknotted structure that resembles the TLS of bromo-, cucumo- and oleaviruses, although it cannot be charged with an amino acid (TLS conformation) (Olsthoorn et al., 1999
). The TLS conformation was required for minus-strand promoter activity of AMV RNA in an in vitro assay with purified AMV replicase (Olsthoorn et al., 1999
, 2004
). Binding of CP to the CPB conformation strongly enhanced translation of AMV RNAs in vivo (Neeleman et al., 2001
). Transfection of tobacco protoplasts with AMV RNA 4 resulted in synthesis of CP only when the RNA encoded wild-type or mutant CP that was able to bind to the 3' end of its messenger (Neeleman et al., 2001
). Translation of RNA 4 encoding a CP that was defective in RNA binding could be rescued by expression of functional CP in trans or by replacing the 3' UTR of the RNA with the 3' UTR of Brome mosaic virus (BMV, genus Bromovirus) (Neeleman et al., 2001
, 2004
). From these experiments, it was concluded that the AMV 3' UTR stimulates translation in a CP-dependent manner, whereas the BMV 3' UTR stimulates translation independently of CP. Because extension of the AMV RNAs with a poly(A) tail circumvented the requirement for CP to initiate infection, it was proposed that, in a wild-type infection, CP mimics the function of PABP (Neeleman et al., 2001
).
It has been shown that introduction of the 3' UTR of BMV and Tobacco mosaic virus (TMV) downstream of a reporter gene strongly enhances translational efficiency of the chimeric RNAs in carrot protoplasts, whereas introduction of the AMV 3' UTR does not (Gallie & Kobayashi, 1994). Here, the effect of CP on message stability and translation of luciferase transcripts with 3'-terminal sequences consisting of the AMV 3' UTR or a poly(A) tail was analysed. In carrot protoplasts, CP strongly enhanced translation efficiency of the message with the AMV sequence, but not that of the polyadenylated RNA. Glutathione S-transferase (GST) pull-down and Far-Western assays revealed a specific interaction of AMV CP with eIF4G and eIFiso4G from wheatgerm. The data support the model in which CP stimulates translation by converting the viral RNAs into a closed-loop structure.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The GSTCP fusion construct was created by PCR amplification of the CP gene from plasmid pAL3 (Neeleman et al., 1991), using oligonucleotide 5'-CGAGATCTGAGAACCTGTACTTCCAGAGTTCTTCACAAAAGAAAGCT-3', which specifies a BglII site (underlined), the tobacco etch virus protease-recognition motif and the N terminus of the CP gene (in italics), and oligonucleotide 5'-GGAATTCAATGACGATCAAGATCGTCA-3', which contains an EcoRI site (underlined) adjacent to the CP stop codon (italics complementary to the 5'-terminal sequence of the CP gene). The amplified fragment was introduced as a BglIIEcoRI fragment into pGEX-2T between the BamHI and EcoRI sites and verified by sequencing.
Protein extracts.
Expression of GSTCP fusion protein from pGEX-CP was done in Escherichia coli strain DH5 at 19 °C. Expression was induced with 7·5 µM IPTG at a starting OD600 of 0·04 and the suspension was further incubated until saturation of the culture. Cells were disrupted by sonication in 25 mM HEPES/KOH (pH 7·5), 100 mM potassium acetate, 2 mM EDTA, 5 mM dithiothreitol (DTT), 0·2 % Sarkosyl and 0·5 mM PMSF. These conditions yielded >90 % soluble GSTCP in the supernatant. Cell debris was removed by centrifugation at 30 g for 20 min and the supernatant was mixed with glutathioneSepharose 4B (Pharmacia). After 30 min incubation with gentle shaking at 4 °C, the Sepharose was pelleted by centrifugation and washed five times with the same buffer as above without Sarkosyl. Finally, the pelleted Sepharose was mixed with 4 vols 25 mM HEPES/KOH (pH 7·5), 100 mM potassium acetate, 5 mM DTT and 75 % glycerol. The excess supernatant was removed and the beads with bound GSTCP were stored at 20 °C. The unfused GST control was prepared similarly from E. coli strain DH5
containing pGEX-2T.
Expression of wheat eIF4E, eIFiso4E, eIF4G and eIFiso4G in E. coli strain DH5 and the preparation of S30 extracts was done as described previously (Gazo et al., 2004
; van Heerden & Browning, 1994
). AMV CP was purified from virus particles as described previously (Neeleman et al., 1993
) and kept frozen in small aliquots at 80 °C. Wheatgerm extract (WGE) was purchased from Promega.
Carrot-cell electroporation and luciferase or -glucuronidase (GUS) assays.
Carrot (Daucus carota) protoplasts were prepared from a carrot-cell suspension as described previously (Gallie et al., 1995). Electroporation was done as described previously (Gallie et al., 1995
) with the following modifications: RNA samples (9 µg or approx. 2·5 pmol) in a small volume of water (24 µl) were mixed with 10 µl of a dilution of purified AMV CP (2·530·0 pmol) in 5 mM HEPES (pH 7·5) and pre-incubated for 5 min on ice before mixing with 400 µl protoplast suspension and immediate electroporation (settings: 350 µF, 400 V with a 5 mm gap electrode). Luciferase and GUS assays were performed essentially as described previously (Gallie et al., 1995
).
Luciferase-expression kinetics and functional half-life determination.
The rate of Luc translation was determined from the kinetics of luciferase production in carrot cells following delivery of mRNA constructs by electroporation, during the phase of linear increase after recruitment of RNA onto polysomes. This rate was used as a measure of translational efficiency and the length of time over which Luc continued to accumulate was used to determine message stability, as described previously (Gallie et al., 1996; Ling et al., 2002
).
Pull-down assays and Western analysis.
Our standard pull-down buffer P was 25 mM HEPES/KOH (pH 7·5), 100 mM potassium acetate, 2·5 mM magnesium acetate, 0·5 mM EDTA, 5 mM DTT and 0·02 % Triton X-100. GlutathioneSepharose 4B containing GSTCP or GST in 50 % glycerol buffer was prepared by adding 2·6 vol. buffer P and 0·4 vol. 10 % solution of blocking reagent (Roche) in 100 mM maleate (pH 7·5) plus 150 mM NaCl and incubating for 30 min on ice. Excess supernatant was then removed and, for the GSTCP/native CP pull-downs, 35 µg purified CP was added per 50 µl resin [containing 0·75 µg GSTCP (µl resin)1]; preparations were incubated for 30 min at 30 °C, then washed three times with 1·3 ml buffer P containing 1 % blocking reagent. WGE (9 µl; 350 µg total protein) and 0·5 µl 100 mM PMSF were added to each 50 µl aliquot of glutathioneSepharose resin. After 30 min incubation on ice with occasional mixing, resin aliquots were washed two to four times with 1 ml buffer P as indicated. GST or GSTCP was eluted from each washed aliquot with 3x100 µl 10 mM reduced glutathione in 5 mM HEPES (pH 7·5), incubating for 2 min at room temperature before taking the supernatant. Combined successive eluates were precipitated with 1·5 ml acetone at 20 °C, incubating for 45 min at 20 °C before centrifuging for 20 min at 10 000 r.p.m. and 4 °C. Pellets were air-dried and redissolved in 35 µl SDS-PAGE loading buffer at 80 °C. Eight microlitres of each sample was separated by SDS-PAGE and transferred to Hybond P membrane (Amersham Biosciences). Detection of initiation factors by Western analysis was done as described by Ling et al. (2002), using antisera detecting eIF4E, eIF4G, eIFiso4G, eIF4A, eIF4B or eIF3 (Browning et al., 1990
; Gallie et al., 1998
). The antisera are specific and no cross-reactivity is observed (Gallie et al., 1997
, 1998
; Le et al., 1998
).
Far-Western assays.
Proteins were run in 10 % acrylamide gels made in 375 mM Tris/HCl (pH 8·8) containing 0·02 % SDS. A relatively low concentration of SDS was used to facilitate renaturing of the proteins. Gels were blotted onto Hybond P membrane (Amersham Biosciences) and the membranes were probed with GSTCP fusion protein or native CP and analysed with GST or CP antisera, using a previously described procedure (Le et al., 1997).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
Stimulation of translational efficiency by CP is cap-dependent
The experiments described so far were done with transcripts containing a 5'-cap structure. To see whether the stimulatory effect of CP is cap-dependent, the experiments shown in Figs 1 and 2 were repeated with uncapped LucAMV and Lucpoly(A) transcripts. In the absence of CP or RNA 4, Luc expression in protoplasts by uncapped LucAMV and Lucpoly(A) was 28-fold and 93-fold lower, respectively, than expression observed with the corresponding capped transcripts (Fig. 4
). Addition of CP or RNA 4 only marginally affected (approx. twofold) Luc expression by uncapped LucAMV (Fig. 4
). This demonstrates that efficient (approx. 40-fold) CP-mediated stimulation of translation is cap-dependent.
|
To analyse a possible interaction between CP and initiation factors, an N-terminal GST fusion with CP was bound to glutathioneSepharose 4B and used to pull down possible initiation factors from WGE. This extract contains five to ten times more eIFiso4F than eIF4F (Browning, 1996; Browning et al., 1990
). In solution, CP occurs as a dimer (Jaspars, 1985
) and dimer formation is required for stimulation of translation of RNA 4 by CP (Neeleman et al., 2004
). Therefore, two different baits were used in the pull-down assays: the glutathioneSepharose 4B/GSTCP complex and the same complex pre-incubated with unfused CP. Elution of this pre-incubated complex with glutathione after it had been washed several times resulted in elution of both the GSTCP fusion protein and unfused CP (data not shown). This indicates that pre-incubation resulted in the interaction of at least some of the added CP dimers with the GSTCP fusion protein. Unfused GST bound to glutathioneSepharose 4B was used as a negative control. The baits were incubated with WGE, Sepharose beads were washed two, three or four times with washing buffer and, finally, proteins bound to the beads were eluted with buffer containing glutathione. Analysis of the eluate on a stained gel showed bands corresponding to GSTCP, unfused CP when added during pre-incubation and a few, hardly visible bands of proteins selected from the WGE (data not shown). Fig. 5
shows Western blotting analysis of the eluted proteins using antisera that specifically recognized eIF4G, eIFiso4G and eIF4A (Browning et al., 1990
; Gallie et al., 1998
).
|
The lane loaded with total WGE shows that eIF4G in the extract is mostly degraded to the 75 kDa product, whereas little degradation of eIFiso4G is observed (Fig. 5a, b
; lane 1). Longer exposure times revealed minor amounts of the
45 kDa fragment of eIF4G in the total WGE. It is not known whether these fragments of eIF4G are still present in an eIF4F-like complex. Thus, it is possible that all eIF4A detected in Fig. 5(c)
is pulled down as part of the EIFiso4F complex and that the eIF4G fragments in Fig. 5(a)
are pulled down by direct interaction with CP. Enrichment of the 45 kDa fragment in the pull-down assay may reflect a relatively high affinity of CP for this fragment or an enhanced degradation of eIF4G during the pull-down assay.
As with eIF4G and eIFiso4G, eIF4A was pulled down with the GSTCP bait, but not with the GST control (Fig. 5c). eIF4A is associated weakly with the eIF4F and eIFiso4F complexes (Browning, 1996
; Lax et al., 1985
) and was washed off more readily than eIF4G and eIFiso4G. Initiation factors eIF4B and eIF3 interact with eIF4F and eIFiso4F (Gallie, 2002
), but these two factors were not pulled down by the GSTCP bait above background levels (results not shown).
AMV CP interacts with eIF4G and eIFiso4G
Initiation factors involved in the interaction of the eIF4F and eIFiso4F complexes with CP were identified by Far-Western assays. eIF4E, eIFiso4E, eIF4G and eIFiso4G were expressed separately in E. coli from a pET3d vector and eIF4E and eIFiso4E were purified from a bacterial S30 extract by binding the recombinant proteins to m7GTPSepharose beads as described previously (van Heerden & Browning, 1994). Fig. 6(a)
shows analysis on a stained gel of eIF4E (lane 1) and eIFiso4E (lane 2) eluted from the beads with m7GTP. In the gel system used, eIFiso4E co-migrates with the 34 kDa marker, whereas eIF4E migrates ahead of this marker. Beads complexed with eIF4E were mixed with an S30 extract of E. coli expressing eIF4G and the eIF4EeIF4G complex was eluted from the beads with m7GTP (van Heerden & Browning, 1994
). The eIF4E protein selected a few proteins migrating in the 200 kDa region of the gel that are expressed specifically from the eIF4G vector (Fig. 6a
, lane 3). By using a similar procedure, beads complexed with eIFiso4E were used to purify eIFiso4G from an S30 extract of E. coli expressing this recombinant protein (Fig. 6a
, lane 4). Identities of the purified eIF4E, eIF4G and eIFiso4G were confirmed by Western blotting (Fig. 7
).
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
From the time-course studies presented in Fig. 3, it can be concluded that CP enhances the translational efficiency of LucAMV in plant cells by about 40-fold. CP did not stimulate translation of Lucpoly(A) or Luccontrol transcripts, demonstrating that the effect of CP is specific for messengers with the AMV 3' UTR. CP mutants defective in RNA binding did not stimulate translation (Neeleman et al., 2001
). Apparently, CP has to bind to the 3' end of the viral RNA to enhance translation efficiency. Addition of RNA 4 to LucAMV in a molar ratio of 1 : 1 stimulated Luc expression by approximately sixfold (Fig. 2
). In a previous study, it was reported that addition of a 50-fold molar excess of RNA 4 to LucAMV only marginally affected Luc expression in carrot protoplasts (Gallie & Kobayashi, 1994
). However, in this earlier study, the RNA 4 was not capped. More recently, it has been shown that uncapped RNA 4 is translated poorly in protoplasts (Neeleman et al., 2001
). The observation that uncapped RNA 4 does not stimulate LucAMV expression is in line with the conclusion that RNA 4 stimulates translation through its encoded CP (Neeleman et al., 2001
). As with the RNA 4 transcript, the LucAMV transcript must be capped to allow stimulation of its translation by CP (Fig. 4
). In the absence of CP, Luc expression from capped LucAMV is not zero, but amounts to 2530 % of the Luc expression obtained with Lucpoly(A) (Fig. 2
). After co-transfection of protoplasts with LucAMV and RNA 4, both RNAs are probably translated initially at relatively low levels until CP expressed from RNA 4 stimulates translation of the two RNAs. The observation that Luc expression from LucAMV is stimulated more efficiently by CP than by RNA 4 (Fig. 2
) is consistent with the notion that translation of RNA 4 must occur prior to stimulation of translation of LucAMV.
How does binding of CP to the viral RNA stimulates translation? It has been reported previously that extension of the 3' termini of AMV genomic RNAs with a poly(A) tail of 40 or 80 residues permitted initiation of infection of tobacco plants and protoplasts at a level that was 5 % of the CP-mediated initiation of infection (Neeleman et al., 2001). Here, it has been shown that, in the presence of CP, the AMV 3' UTR stimulates Luc expression in plant cells much more efficiently than a poly(A) tail of 50 residues does. After entry into a plant cell, CP may allow the AMV RNAs to compete efficiently with the approximately 300 000 cellular messengers for the translation machinery. The poly(A) tail of cellular messengers binds PABP, which enhances translation efficiency via interaction with the eIF4G subunit of eIF4F. Our pull-down and Far-Western assays revealed that AMV CP interacts specifically with the eIF4G and eIFiso4G subunits from wheat eIF4F and eIFiso4F, respectively. This supports the proposal that CP acts as a PABP mimic (Neeleman et al., 2001
).
The role of AMV CP in the translation of viral RNA may resemble the function of the rotavirus NSP3 protein. Both proteins have to bind as dimers to the 3' end of their non-polyadenylated mRNAs to enhance translation and both proteins interact with components of the eIF4F complex (Neeleman et al., 2004; Piron et al., 1998
; Vende et al., 2000
). The N-terminal domains of NSP3 and AMV CP are involved in binding of the cognate viral RNAs (Bol, 1999
; Deo et al., 2002
). NSP3 evicts PABP from eIF4F during rotavirus infection, leading to enhanced translation of viral mRNAs and the concomitant inhibition of the translation of cellular mRNAs (Piron et al., 1998
). AMV infection does not inhibit cellular protein synthesis (Hooft van Huijsduijnen et al., 1986
). Preliminary experiments showed that, when GSTCP (or GSTCP plus native CP) was used to pull down eIF4G and eIFiso4G, PABP was also bound to the Sepharose beads (results not shown). This indication that AMV CP does not evict PABP from eIF4F or eIFiso4F will be further investigated. In addition to the similarity with NSP3, the early function of AMV CP may resemble the role of the stemloop-binding protein (SLBP), which binds to the 3' end of non-polyadenylated animal histone mRNAs. Interaction of SLBP with the mRNA and with eIF4G is required for efficient translation of the histone mRNA (Ling et al., 2002
).
Like bromo- and cucumoviruses in the family Bromoviridae, the 3' termini of the RNAs of TMV (genus Tobamovirus) and Turnip yellow mosaic virus (TYMV, genus Tymovirus) can be folded into a TLS that can be charged with an amino acid. The TLS of TMV and TYMV have been shown to bind the elongation factor eEF1A (Matsuda & Dreher, 2004; Zeenko et al., 2002
) and, for TYMV, this interaction enhanced translation of the viral RNA in plant cells (Matsuda & Dreher, 2004
). By analogy, eEF1A may stimulate translation of the RNAs of bromo-, cucumo- and possibly oleaviruses in the family Bromoviridae. CP inhibits AMV minus-strand RNA synthesis in vitro (Houwing & Jaspars, 1986
), whereas eEF1A inhibits TYMV minus-strand RNA synthesis in vitro (Matsuda et al., 2004
). CP and eEF1A may have similar regulatory roles in the early steps of the replication cycles of CP-dependent and CP-independent plant viruses, respectively (Olsthoorn et al., 1999
; Matsuda et al., 2004
).
![]() |
ACKNOWLEDGEMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bol, J. F. (2003). Alfalfa mosaic virus: coat protein-dependent initiation of infection. Mol Plant Pathol 4, 18.[CrossRef]
Bol, J. F., van Vloten-Doting, L. & Jaspars, E. M. J. (1971). A functional equivalence of top component a RNA and coat protein in the initiation of infection by alfalfa mosaic virus. Virology 46, 7385.[CrossRef][Medline]
Browning, K. S. (1996). The plant translational apparatus. Plant Mol Biol 32, 107144.[CrossRef][Medline]
Browning, K. S., Humphreys, J., Hobbs, W., Smith, G. B. & Ravel, J. M. (1990). Determination of the amounts of the protein synthesis initiation and elongation factors in wheat germ. J Biol Chem 265, 1796717973.
Browning, K. S., Webster, C., Roberts, J. K. M. & Ravel, J. M. (1992). Identification of an isozyme form of protein synthesis initiation factor 4F in plants. J Biol Chem 267, 1009610100.
Deo, R. C., Groft, C. M., Rajashankar, K. R. & Burley, S. K. (2002). Recognition of the rotavirus mRNA 3' consensus by an asymmetric NSP3 homodimer. Cell 108, 7181.[CrossRef][Medline]
Fraser, C. S., Pain, V. M. & Morley, S. J. (1999). The association of initiation factor 4F with poly(A)-binding protein is enhanced in serum-stimulated Xenopus kidney cells. J Biol Chem 274, 196204.
Gallie, D. R. (1991). The cap and poly(A) tail function synergistically to regulate mRNA translational efficiency. Genes Dev 5, 21082116.[Abstract]
Gallie, D. R. (2002). Protein-protein interactions required during translation. Plant Mol Biol 50, 949970.[CrossRef][Medline]
Gallie, D. R. & Kobayashi, M. (1994). The role of the 3'-untranslated region of non-polyadenylated plant viral mRNAs in regulating translational efficiency. Gene 142, 159165.[CrossRef][Medline]
Gallie, D. R. & Browning, K. S. (2001). eIF4G functionally differs from eIFiso4G in promoting internal initiation, cap-independent translation, and translation of structured mRNAs. J Biol Chem 276, 3695136960.
Gallie, D. R., Lucas, W. J. & Walbot, V. (1989). Visualizing mRNA expression in plant protoplasts: factors influencing efficient mRNA uptake and translation. Plant Cell 1, 301311.
Gallie, D. R., Caldwell, C. & Pitto, L. (1995). Heat shock disrupts cap and poly(A) tail function during translation and increases mRNA stability of introduced reporter mRNA. Plant Physiol 108, 17031713.
Gallie, D. R., Lewis, N. J. & Marzluff, W. F. (1996). The histone 3'-terminal stem-loop is necessary for translation in Chinese hamster ovary cells. Nucleic Acids Res 24, 19541962.
Gallie, D. R., Le, H., Caldwell, C., Tanguay, R. L., Hoang, N. X. & Browning, K. S. (1997). The phosphorylation state of translation initiation factors is regulated developmentally and following heat shock in wheat. J Biol Chem 272, 10461053.
Gallie, D. R., Le, H., Tanguay, R. L. & Browning, K. S. (1998). Translation initiation factors are differentially regulated in cereals during development and following heat shock. Plant J 14, 715722.[CrossRef]
Gazo, B. G., Murphy, P., Gatchel, J. R. & Browning, K. S. (2004). A novel interaction of cap-binding protein complexes eukaryotic initiation factor (eIF) 4F and eIF(iso)4F with a region in the 3'-untranslated region of satellite tobacco necrosis virus. J Biol Chem 279, 1358413592.
Guo, L., Allen, E. M. & Miller, W. A. (2001). Base-pairing between untranslated regions facilitates translation of uncapped, nonpolyadenylated viral RNA. Mol Cell 7, 11031109.[CrossRef][Medline]
Hooft van Huijsduijnen, R. A. M., Alblas, S. W., de Rijk, R. H. & Bol, J. F. (1986). Induction by salicylic acid of pathogenesis-related proteins and resistance to alfalfa mosaic virus infection in various plant species. J Gen Virol 67, 21352143.
Houwing, C. J. & Jaspars, E. M. J. (1986). Coat protein blocks the in vitro transcription of the virion RNAs of alfalfa mosaic virus. FEBS Lett 209, 284288.[CrossRef]
Imataka, H., Gradi, A. & Sonenberg, N. (1998). A newly identified N-terminal amino acid sequence of human eIF4G binds poly(A)-binding protein and functions in poly(A)-dependent translation. EMBO J 17, 74807489.
Jaspars, E. M. J. (1985). Interaction of alfalfa mosaic virus nucleic acid and protein. In Molecular Plant Virology, vol. 1, pp. 155225. Edited by J. W. Davies. Boca Raton, FL: CRC Press.
Jaspars, E. M. J. (1999). Genome activation in alfamo- and ilarviruses. Arch Virol 144, 843863.[CrossRef][Medline]
Jobling, S. A. & Gehrke, L. (1987). Enhanced translation of chimaeric messenger RNAs containing a plant viral untranslated leader sequence. Nature 325, 622625.[CrossRef][Medline]
Lax, S., Fritz, W., Browning, K. & Ravel, J. (1985). Isolation and characterization of factors from wheat germ that exhibit eukaryotic initiation factor 4B activity and overcome 7-methylguanosine 5'-triphosphate inhibition of polypeptide synthesis. Proc Natl Acad Sci U S A 82, 330333.
Le, H., Tanguay, R. L., Balasta, M. L., Wei, C.-C., Browning, K. S., Metz, A. M., Goss, D. J. & Gallie, D. R. (1997). Translation initiation factors eIF-iso4G and eIF-4B interact with the poly(A)-binding protein and increase its RNA binding activity. J Biol Chem 272, 1624716255.
Le, H., Browning, K. S. & Gallie, D. R. (1998). The phosphorylation state of the wheat translation initiation factors eIF4B, eIF4A, and eIF2 is differentially regulated during seed development and germination. J Biol Chem 273, 2008420089.
Ling, J., Morley, S. J., Pain, V. M., Marzluff, W. F. & Gallie, D. R. (2002). The histone 3'-terminal stem-loop-binding protein enhances translation through a functional and physical interaction with eukaryotic initiation factor 4G (eIF4G) and eIF3. Mol Cell Biol 22, 78537867.
Matsuda, D. & Dreher, T. W. (2004). The tRNA-like structure of turnip yellow mosaic virus RNA is a 3'-translational enhancer. Virology 321, 3646.[CrossRef][Medline]
Matsuda, D., Yoshinari, S. & Dreher, T. W. (2004). eEF1A binding to aminoacylated viral RNA represses minus strand synthesis by TYMV RNA-dependent RNA polymerase. Virology 321, 4756.[CrossRef][Medline]
Neeleman, L., van der Kuyl, A. C. & Bol, J. F. (1991). Role of alfalfa mosaic virus coat protein gene in symptom formation. Virology 181, 687693.[CrossRef][Medline]
Neeleman, L., van der Vossen, E. A. G. & Bol, J. F. (1993). Infection of tobacco with alfalfa mosaic virus cDNAs sheds light on the early function of the coat protein. Virology 196, 883887.[CrossRef][Medline]
Neeleman, L., Olsthoorn, R. C. L., Linthorst, H. J. M. & Bol, J. F. (2001). Translation of a nonpolyadenylated viral RNA is enhanced by binding of viral coat protein or polyadenylation of the RNA. Proc Natl Acad Sci U S A 98, 1428614291.
Neeleman, L., Linthorst, H. J. M. & Bol, J. F. (2004). Efficient translation of alfamovirus RNAs requires the binding of coat protein dimers to the 3' termini of the viral RNAs. J Gen Virol 85, 231240.
Olsthoorn, R. C. L., Mertens, S., Brederode, F. T. & Bol, J. F. (1999). A conformational switch at the 3' end of a plant virus RNA regulates viral replication. EMBO J 18, 48564864.
Olsthoorn, R. C. L., Haasnoot, P. C. J. & Bol, J. F. (2004). Similarities and differences between the subgenomic and minus-strand promoters of an RNA plant virus. J Virol 78, 40484053.
Piron, M., Vende, P., Cohen, J. & Poncet, D. (1998). Rotavirus RNA-binding protein NSP3 interacts with eIF4GI and evicts the poly(A) binding protein from eIF4F. EMBO J 17, 58115821.
Pokrovskaya, I. D. & Gurevich, V. V. (1994). In vitro transcription: preparative RNA yields in analytical scale reactions. Anal Biochem 220, 420423.[CrossRef][Medline]
Preiss, T. & Hentze, M. W. (1998). Dual function of the messenger RNA cap structure in poly(A)-tail-promoted translation in yeast. Nature 392, 516520.[CrossRef][Medline]
Reusken, C. B. E. M., Neeleman, L. & Bol, J. F. (1994). The 3'-untranslated region of alfalfa mosaic virus RNA 3 contains at least two independent binding sites for viral coat protein. Nucleic Acids Res 22, 13461353.[Abstract]
Svitkin, Y. V., Imataka, H., Khaleghpour, K., Kahvejian, A., Liebig, H.-D. & Sonenberg, N. (2001). Poly(A)-binding protein interaction with eIF4G stimulates picornavirus IRES-dependent translation. RNA 7, 17431752.
Tarun, S. Z., Jr & Sachs, A. B. (1996). Association of the yeast poly(A) tail binding protein with translation initiation factor eIF-4G. EMBO J 15, 71687177.[Abstract]
van Heerden, A. & Browning, K. S. (1994). Expression in Escherichia coli of the two subunits of the isozyme form of wheat germ protein synthesis initiation factor 4F. Purification of the subunits and formation of an enzymatically active complex. J Biol Chem 269, 1745417457.
Vende, P., Piron, M., Castagné, N. & Poncet, D. (2000). Efficient translation of rotavirus mRNA requires simultaneous interaction of NSP3 with the eukaryotic translation initiation factor eIF4G and the mRNA 3' end. J Virol 74, 70647071.
Wells, S. E., Hillner, P. E., Vale, R. D. & Sachs, A. B. (1998). Circularization of mRNA by eukaryotic translation initiation factors. Mol Cell 2, 135140.[CrossRef][Medline]
Zeenko, V. V., Ryabova, L. A., Spirin, A. S., Rothnie, H. M., Hess, D., Browning, K. S. & Hohn, T. (2002). Eukaryotic elongation factor 1A interacts with the upstream pseudoknot domain in the 3' untranslated region of tobacco mosaic virus RNA. J Virol 76, 56785691.
Received 30 November 2004;
accepted 8 February 2005.