Isolation and sequence of the cDNAs encoding the subunits of the isozyme form of wheat protein synthesis initiation factor 4F

Poly(A)-binding protein promotes VPg-dependent translation of potyvirus through enhanced binding of phosphorylated eIFiso4F and eIFiso4F∙eIF4B

The phosphorylation of eukaryotic translational initiation factors has been shown to play a significant role in controlling the synthesis of protein. Viral infection, environmental stress, and growth circumstances cause phosphorylation or dephosphorylation of plant initiation factors. Our findings indicate that casein kinase 2 can phosphorylate recombinant wheat eIFiso4E and eIFiso4G generated from E. coli in vitro. For wheat eIFiso4E, Ser-207 was found to be the in vitro phosphorylation site. eIFiso4E lacks an amino acid that can be phosphorylated at the position corresponding to Ser-209, the phosphorylation site in mammalian eIF4E, yet phosphorylation of eIFiso4E has effects on VPg binding affinity that are similar to those of phosphorylation of mammalian eIF4E. The addition of VPg and phosphorylated eIFiso4F to depleted wheat germ extract (WGE) leads to enhancement of translation of both uncapped and capped viral mRNA. The addition of PABP together with eIFiso4Fp and eIF4B to depleted WGE increases both uncapped and capped mRNA translation. However, it exhibits a translational advantage specifically for uncapped mRNA, implying that the phosphorylation of eIFiso4F hinders cap binding while promoting VPg binding, thereby facilitating uncapped translation. These findings indicate TEV virus mediates VPg-dependent translation by engaging a mechanism entailing phosphorylated eIFiso4Fp and PABP. To elucidate the molecular mechanisms underlying these observed effects, we studied the impact of PABP and/or eIF4B on the binding of VPg with eIFiso4Fp. The inclusion of PABP and eIF4B with eIFiso4Fp resulted in about 2-fold increase in affinity for VPg (Kd = 24 ± 1.7 nM), as compared to the affinity of eIFiso4Fp alone (Kd = 41.0 ± 3.1 nM). The interactions between VPg and eIFiso4Fp were determined to be both enthalpically and entropically favorable, with the enthalpic contribution accounting for 76-97% of the ΔG at 25°C, indicating a substantial role of hydrogen bonding in enhancing the stability of the complex. The binding of PABP to eIFiso4Fp·4B resulted in a conformational alteration, leading to a significant enhancement in the binding affinity to VPg. These observations suggest PABP enhances the affinity between eIFiso4Fp and VPg, leading to an overall conformational change that provides a stable platform for efficient viral translation.

Functional analogs of mammalian 4E-BPs reveal a role for TOR in global plant translation

Mammalian/mechanistic target of rapamycin (mTOR) regulates global protein synthesis through inactivation of eIF4E-binding proteins (m4E-BPs) in response to nutrient and energy availability. Until now, 4E-BPs have been considered as metazoan inventions, and how target of rapamycin (TOR) controls cap-dependent translation initiation in plants remains obscure. Here, we present short unstructured 4E-BP-like Arabidopsis proteins (4EBP1/4EBP2) that are non-homologous to m4E-BPs except for the eIF4E-binding motif and TOR phosphorylation sites. Unphosphorylated 4EBPs exhibit strong affinity toward eIF4Es and can inhibit formation of the cap-binding complex. Upon TOR activation, 4EBPs are phosphorylated, probably when bound directly to TOR, and likely relocated to ribosomes. 4EBPs can suppress a distinct set of mRNAs; 4EBP2 predominantly inhibits translation of core cell-cycle regulators CycB1;1 and CycD1;1, whereas 4EBP1 interferes with chlorophyll biosynthesis. Accordingly, 4EBP2 overexpression halts early seedling development, which is overcome by induction of Glc/Suc-TOR signaling. Thus, TOR regulates cap-dependent translation initiation by inactivating atypical 4EBPs in plants.

Mechanism of cytoplasmic mRNA translation

Protein synthesis is a fundamental process in gene expression that depends upon the abundance and accessibility of the mRNA transcript as well as the activity of many protein and RNA-protein complexes. Here we focus on the intricate mechanics of mRNA translation in the cytoplasm of higher plants. This chapter includes an inventory of the plant translational apparatus and a detailed review of the translational processes of initiation, elongation, and termination. The majority of mechanistic studies of cytoplasmic translation have been carried out in yeast and mammalian systems. The factors and mechanisms of translation are for the most part conserved across eukaryotes; however, some distinctions are known to exist in plants. A comprehensive understanding of the complex translational apparatus and its regulation in plants is warranted, as the modulation of protein production is critical to development, environmental plasticity and biomass yield in diverse ecosystems and agricultural settings.

Pokeweed antiviral protein, a ribosome inactivating protein: activity, inhibition and prospects

Viruses employ an array of elaborate strategies to overcome plant defense mechanisms and must adapt to the requirements of the host translational systems. Pokeweed antiviral protein (PAP) from Phytolacca americana is a ribosome inactivating protein (RIP) and is an RNA N-glycosidase that removes specific purine residues from the sarcin/ricin (S/R) loop of large rRNA, arresting protein synthesis at the translocation step. PAP is thought to play an important role in the plant's defense mechanism against foreign pathogens. This review focuses on the structure, function, and the relationship of PAP to other RIPs, discusses molecular aspects of PAP antiviral activity, the novel inhibition of this plant toxin by a virus counteraction-a peptide linked to the viral genome (VPg), and possible applications of RIP-conjugated immunotoxins in cancer therapeutics.

Plant cap-binding complexes eukaryotic initiation factors eIF4F and eIFISO4F: molecular specificity of subunit binding

The initiation of translation in eukaryotes requires a suite of eIFs that include the cap-binding complex, eIF4F. eIF4F is comprised of the subunits eIF4G and eIF4E and often the helicase, eIF4A. The eIF4G subunit serves as an assembly point for other initiation factors, whereas eIF4E binds to the 7-methyl guanosine cap of mRNA. Plants have an isozyme form of eIF4F (eIFiso4F) with comparable subunits, eIFiso4E and eIFiso4G. Plant eIF4A is very loosely associated with the plant cap-binding complexes. The specificity of interaction of the individual subunits of the two complexes was previously unknown. To address this issue, mixed complexes (eIF4E-eIFiso4G or eIFiso4E-eIF4G) were expressed and purified from Escherichia coli for biochemical analysis. The activity of the mixed complexes in in vitro translation assays correlated with the large subunit of the respective correct complex. These results suggest that the eIF4G or eIFiso4G subunits influence translational efficiency more than the cap-binding subunits. The translation assays also showed varying responses of the mRNA templates to eIF4F or eIFiso4F, suggesting that some level of mRNA discrimination is possible. The dissociation constants for the correct complexes have K(D) values in the subnanomolar range, whereas the mixed complexes were found to have K(D) values in the ∼10 nm range. Displacement assays showed that the correct binding partner readily displaces the incorrect binding partner in a manner consistent with the difference in K(D) values. These results show molecular specificity for the formation of plant eIF4F and eIFiso4F complexes and suggest a role in mRNA discrimination during initiation of translation.

Turnip mosaic virus (TuMV) is able to use alleles of both eIF4E and eIF(iso)4E from multiple loci of the diploid Brassica rapa

Three copies of eIF4E and three copies of eIF(iso)4E have been identified and sequenced from a Turnip mosaic virus (TuMV)-susceptible, inbred, diploid Brassica rapa line, R-o-18. One of the copies of eIF4E lacked exons 2 and 3 and appeared to be a pseudogene. The two other copies of eIF4E and two of the three copies of eIF(iso)4E were isolated from a bacterial artificial chromosome library of R-o-18. Using an Arabidopsis line (Col-0::dSpm) with a transposon knock-out of the eIF(iso)4E gene which resulted in a change from complete susceptibility to complete resistance to TuMV, complementation experiments were carried out with the two versions of eIF4E and the two versions of eIF(iso)4E. When transformed into Col-0::dSpm, all four Brassica transgenes complemented the Arabidopsis eIF(iso)4E knock-out, conferring susceptibility to both mechanical and aphid challenge with TuMV. One of the copies of eIF4E did not appear to support viral replication as successfully as the other copy of eIF4E or the two copies of eIF(iso)4E. The results show that TuMV can use both eIF4E and eIF(iso)4E from B. rapa for replication and, for the first time, that a virus can use eIF4E and eIF(iso)4E from multiple loci of a single host plant.

Translational control of eukaryotic gene expression

Translational control mechanisms are, besides transcriptional control and mRNA stability, the most determining for final protein levels. A large number of accessory factors that assist the ribosome during initiation, elongation, and termination of translation are required for protein synthesis. Cap-dependent translational control occurs mainly during the initiation step, involving eukaryotic initiation factors (eIFs) and accessory proteins. Initiation is affected by various stimuli that influence the phosphorylation status of both eIF4E and eIF2 and through binding of 4E-binding proteins to eIF4E, which finally inhibits cap- dependent translation. Under conditions where cap-dependent translation is hampered, translation of transcripts containing an internal ribosome entry site can still be supported in a cap-independent manner. An interesting example of translational control is the switch between cap-independent and cap-dependent translation during the eukaryotic cell cycle. At the G1-to-S transition, translation occurs predominantly in a cap-dependent manner, while during the G2-to-M transition, cap-dependent translation is inhibited and transcripts are predominantly translated through a cap-independent mechanism.

Structure of a viral cap-independent translation element that functions via high affinity binding to the eIF4E subunit of eIF4F

RNAs of many positive strand RNA viruses lack a 5' cap structure and instead rely on cap-independent translation elements (CITEs) to facilitate efficient translation initiation. The mechanisms by which these RNAs recruit ribosomes are poorly understood, and for many viruses the CITE is unknown. Here we identify the first CITE of an umbravirus in the 3'-untranslated region of pea enation mosaic virus RNA 2. Chemical and enzymatic probing of the approximately 100-nucleotide PEMV RNA 2 CITE (PTE), and mutagenesis revealed that it forms a long, bulged helix that branches into two short stem-loops, with a possible pseudoknot interaction between a C-rich bulge at the branch point and a G-rich bulge in the main helix. The PTE inhibited translation in trans, and addition of eIF4F, but not eIFiso4F, restored translation. Filter binding assays revealed that the PTE binds eIF4F and its eIF4E subunit with high affinity. Tight binding required an intact cap-binding pocket in eIF4E. Among many PTE mutants, there was a strong correlation between PTE-eIF4E binding affinity and ability to stimulate cap-independent translation. We conclude that the PTE recruits eIF4F by binding eIF4E. The PTE represents a different class of translation enhancer element, as defined by its structure and ability to bind eIF4E in the absence of an m(7)G cap.

Expression and purification of recombinant wheat translation initiation factors eIF1, eIF1A, eIF4A, eIF4B, eIF4F, eIF(iso)4F, and eIF5

Protein synthesis initiation factors from wheat germ were cloned into E. coli expression vectors for expression and purification. The ability to obtain large amounts of functional initiation factors and mutants of the factors will facilitate the biophysical and biochemical analysis of the process of initiation in plants. The initiation factors, eIF1, eIF1A, eIF4A, eIF4B, eIF4F, eIF(iso)4F, and eIF5, were successfully expressed and purified from E. coli. In most cases, the use of 6X-histidine tags was avoided to prevent any possible artifacts of folding or activity because of the presence of the tag. The amounts of highly purified wheat initiation factors obtained ranged from 0.5 to 24mg of protein per liter of culture, depending on the particular initiation factor. The initiation factors were of very high purity, and the activities of the wheat recombinant factors purified from E. coli were found to be comparable to or better than those purified from wheat germ.

The 3' cap-independent translation element of Barley yellow dwarf virus binds eIF4F via the eIF4G subunit to initiate translation

The 3' cap-independent translation element (BTE) of Barley yellow dwarf virus RNA confers efficient translation initiation at the 5' end via long-distance base pairing with the 5'-untranslated region (UTR). Here we provide evidence that the BTE functions by recruiting translation initiation factor eIF4F. We show that the BTE interacts specifically with the cap-binding initiation factor complexes eIF4F and eIFiso4F in a wheat germ extract (wge). In wge depleted of cap-interacting factors, addition of eIF4F (and to a lesser extent, eIFiso4F) allowed efficient translation of an uncapped reporter construct (BLucB) containing the BTE in its 3' UTR. Translation of BLucB required much lower levels of eIF4F or eIFiso4F than did a capped, nonviral mRNA. Both full-length eIF4G and the carboxy-terminal half of eIF4G lacking the eIF4E binding site stimulated translation to 70% of the level obtained with eIF4F, indicating a minor role for the cap-binding protein, eIF4E. In wge inhibited by either BTE in trans or cap analog, eIF4G alone restored translation nearly as much as eIF4F, while addition of eIF4E alone had no effect. The BTE bound eIF4G (Kd = 177 nm) and eIF4F (Kd = 37 nm) with high affinity, but very weakly to eIF4E. These interactions correlate with the ability of the factors to facilitate BTE-mediated translation. These results and previous observations are consistent with a model in which eIF4F is delivered to the 5' UTR by the BTE, and they show that eIF4G, but not eIF4E, plays a major role in this novel mechanism of cap-independent translation.

Potyvirus genome-linked protein, VPg, directly affects wheat germ in vitro translation: interactions with translation initiation factors eIF4F and eIFiso4F

Potyvirus genome linked protein, VPg, interacts with translation initiation factors eIF4E and eIFiso4E, but its role in protein synthesis has not been elucidated. We show that addition of VPg to wheat germ extract leads to enhancement of uncapped viral mRNA translation and inhibition of capped viral mRNA translation. This provides a significant competitive advantage to the uncapped viral mRNA. To understand the molecular basis of these effects, we have characterized the interaction of VPg with eIF4F, eIFiso4F, and a structured RNA derived from tobacco etch virus (TEV RNA). When VPg formed a complex with eIF4F, the affinity for TEV RNA increased more than 4-fold compared with eIF4F alone (19.4 and 79.0 nm, respectively). The binding affinity of eIF4F to TEV RNA correlates with translation efficiency. VPg enhanced eIFiso4F binding to TEV RNA 1.6-fold (178 nm compared with 108 nm). Kinetic studies of eIF4F and eIFiso4F with VPg show approximately 2.6-fold faster association for eIFiso4F.VPg as compared with eIF4F.VPg. The dissociation rate was approximately 2.9-fold slower for eIFiso4F than eIF4F with VPg. These data demonstrate that eIFiso4F can kinetically compete with eIF4F for VPg binding. The quantitative data presented here suggest a model where eIF4F.VPg interaction enhances cap-independent translation by increasing the affinity of eIF4F for TEV RNA. This is the first evidence of direct participation of VPg in translation initiation.

Functional diversity of the eukaryotic translation initiation factors belonging to eIF4 families

Protein synthesis in eukaryotic cells is fundamental for gene expression. This process involves the binding of an mRNA molecule to the small ribosomal subunit in a group of reactions catalyzed by eukaryotic translation initiation factors (eIF) eIF4. To date, the role of each of the four eIF4, i.e. eIF4E, eIF4G, eIF4A and eIF4B, is well established. However, with the advent of genome-wide sequencing projects of various organisms, families of genes for each translation initiation factor have been identified. Intriguingly, recent studies have now established that certain eIF4 proteins can promote or inhibit translation of specific mRNAs, and also that some of them are active in processes other than translation. In addition, there is evidence of tissue- and developmental-stage-specific expression for some of these proteins. These new findings point to an additional level of complexity in the translation initiation process. In this review, we analyze the latest advances concerning the functionality of members of the eIF4 families in eukaryotic organisms and discuss the implications of this in the context of our current understanding of regulation of the translation initiation process.

Binding analyses for the interaction between plant virus genome-linked protein (VPg) and plant translational initiation factors

The turnip mosaic virus (TuMV) genome-linked protein (VPg) and Arabidopsis thaliana translation initiation factors were expressed and purified in order to investigate their binding properties and kinetics. Affinity chromatography on m(7)GTP-sepharose showed that bound A. thaliana eIF(iso)4E was eluted with crude TuMV VPg. Further column studies with purified VPg and other A. thaliana eIF4E isoforms showed that VPg preferentially bound eIF(iso)4E. Structural data implicate Trp-46 and Trp-92 in eIF(iso)4E in cap recognition. When Trp-46 or Trp-92 were changed to Leu, eIF(iso)4E lost the ability to form a complex with both VPg and m(7)GTP-sepharose. This suggests that the VPg-binding site is located in or near the cap-recognition pocket on eIF(iso)4E. Affinity constants for the interactions with eIF(iso)4E of VPg and capped RNA oligomer were determined using surface plasmon resonance (SPR). The K(D) values showed that the binging affinity of VPg for eIF(iso)4E is stronger than that of capped RNA. This suggests that viral VPg can interfere with formation of a translational initiation complex on host plant cellular mRNA by sequestering eIF(iso)4E. Further experiments with affinity chromatography showed that VPg forms a ternary complex with eIF(iso)4E and eIF(iso)4G. Thus, VPg may participate in viral translational initiation by functioning as an alternative cap-like structure.

Selective involvement of members of the eukaryotic initiation factor 4E family in the infection of Arabidopsis thaliana by potyviruses

Arabidopsis thaliana plants with mutations in the genes encoding eukaryotic initiation factor (eIF4E) or isoform of eIF4E (eIF(iso)4E) were tested for susceptibility to Clover yellow vein virus (ClYVV), a member of the genus Potyvirus. ClYVV accumulated in both inoculated and upper uninoculated leaves of mutant plants lacking eIF(iso)4E, but not in mutant plants lacking eIF4E. In contrast, Turnip mosaic virus (TuMV), another member of the genus Potyvirus, multiplied in mutant plants lacking eIF4E but not in mutant plants lacking eIF(iso)4E. These results suggest the selective involvement of members of the eIF4E family in infection by potyviruses.

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

Satellite tobacco necrosis virus (STNV) RNA is naturally uncapped at its 5' end and lacks polyadenylation at its 3' end. Despite lacking these two hallmarks of eukaryotic mRNAs, STNV-1 RNA is translated very efficiently. A approximately 130-nucleotide translational enhancer (TED), located 3' to the termination codon, is necessary for efficient cap-independent translation of STNV-1 RNA. The STNV-1 TED RNA fragment binds to the eukaryotic cap-binding complexes, initiation factor (eIF) 4F and eIF(iso)4F, as measured by nitrocellulose binding and fluorescence titration. STNV-1 TED is a potent inhibitor of in vitro translation when added in trans. This inhibition is reversed by the addition of eIF4F or eIF(iso)4F, and the subunits of eIF4F and eIF(iso)4F cross-link to STNV-1 TED, providing additional evidence that these factors interact directly with STNV-1 TED. Deletion mutagenesis of the STNV-1 TED indicates that a minimal region of approximately 100 nucleotides is necessary to promote cap-independent translation primarily through interaction with the cap binding subunits (eIF4E or eIF(iso)4E) of eIF4F or eIF(iso)4F.

Eukaryotic initiation factors 4A (eIF4A) and 4G (eIF4G) mutually interact in a 1:1 ratio in vivo

mRNA translation in eukaryotic cells involves a set of proteins termed translation initiation factors (eIFs), several of which are involved in the binding of ribosomes to mRNA. These include eIF4G, a modular scaffolding protein, and eIF4A, an RNA helicase, of which two closely related forms are known in mammals, eIF4A(I) and eIF4A(II). In mammals, eIF4G possesses two independent sites for binding eIF4A, whereas in other eukaryotes (e.g. yeast) only one site appears to be present, thus raising the issue of the stoichiometry of eIF4G.eIF4A complexes in different eukaryotes. We show that in human embryonic kidney cells eIF4G is associated with eIF4A(I) or eIF4A(II) but not with both simultaneously, suggesting a stoichiometry of 1:1 rather than 1:2. To confirm this, eIF4A(I) or eIF4A(II) was expressed in a tagged form in these cells, and complexes with eIF4G were again isolated. Complexes containing tagged eIF4A(I) or eIF4A(II) contained no endogenous eIF4A, supporting the notion that eIF4G binds only one molecule of eIF4A. Each binding site in eIF4G can bind either eIF4A(I) or eIF4A(II). The data imply that the second binding site in mammalian eIF4A does not bind an additional eIF4A molecule and that initiation factor complexes in different eukaryotes contain one eIF4A per eIF4G.

Characterization of the two eIF4A-binding sites on human eIF4G-1

Eukaryotic translation initiation factor 4G-1 (eIF4G) plays a critical role in the recruitment of mRNA to the 43 S preinitiation complex. eIF4G has two binding sites for the RNA helicase eIF4A, one in the central domain and one in the COOH-terminal domain. Recombinant eIF4G fragments that contained each of these sites separately bound eIF4A with a 1:1 stoichiometry, but fragments containing both sites bound eIF4A with a 1:2 stoichiometry. eIF3 did not interfere with eIF4A binding to the central site. Interestingly, at the same concentration of free eIF4A, more eIF4A was bound to an eIF4G fragment containing both eIF4A sites than the sum of binding to fragments containing the single sites, indicating cooperative binding. Binding of eIF4A to an immobilized fragment of eIF4G containing the COOH-terminal site was competed by a soluble eIF4G fragment containing the central site, indicating that a single eIF4A molecule cannot bind simultaneously to both sites. The association rate constant, dissociation rate constant, and dissociation equilibrium constant for each site were determined by surface plasmon resonance and found to be, respectively, 1.2 x 10(5) m(-1) s(-1), 2.1 x 10(-3) s(-1), and 17 nm for the central site and 5.1 x 10(3) m(-1) s(-1), 1.7 x 10(-3) s(-1), and 330 nm for the COOH-terminal site.

A conserved HEAT domain within eIF4G directs assembly of the translation initiation machinery

The X-ray structure of the phylogenetically conserved middle portion of human eukaryotic initiation factor (eIF) 4GII has been determined at 2.4 A resolution, revealing a crescent-shaped domain consisting of ten alpha helices arranged as five HEAT repeats. Together with the ATP-dependent RNA helicase eIF4A, this HEAT domain suffices for 48S ribosomal complex formation with a picornaviral RNA internal ribosome entry site (IRES). Structure-based site-directed mutagenesis was used to identify two adjacent features on the surface of this essential component of the translation initiation machinery that, respectively, bind eIF4A and a picornaviral IRES. The structural and biochemical results provide mechanistic insights into both cap-dependent and cap-independent translation initiation.

Cleavage of eukaryotic translation initiation factor 4GII correlates with translation inhibition during apoptosis

Eukaryotic translation initiation factor 4G (eIF4G), which has two homologs known as eIF4GI and eIF4GII, functions in a complex (eIF4F) which binds to the 5' cap structure of cellular mRNAs and facilitates binding of capped mRNA to 40S ribosomal subunits. Disruption of this complex in enterovirus-infected cells through eIF4G cleavage is known to block this step of translation initiation, thus leading to a drastic inhibition of cap-dependent translation. Here, we show that like eIF4GI, the newly identified homolog eIF4GII is cleaved during apoptosis in HeLa cells and can serve as a substrate for caspase 3. Proteolysis of both eIF4GI and eIF4GII occurs with similar kinetics and coincides with the profound translation inhibition observed in cisplatin-treated HeLa cells. Both eIF4GI and eIF4GII can be cleaved by caspase 3 with similar efficiency in vitro, however, eIF4GII is processed into additional fragments which destroy its core central domain and likely contributes to the shutoff of translation observed in apoptosis. Cell Death and Differentiation (2000) 7, 1234 - 1243.

Precocious sporulation and developmental lethality in yelA null mutants of Dictyostelium

A novel developmental gene, yelA, has been found that plays as essential role in regulating terminal differentiation of Dictyostelium discoideum. Strains in which yelA is disrupted by plasmid insertion are arrested at the tight mound stage but accumulate the bright yellow pigment characteristic of mature sori. Although these mutant strains do not form fruiting bodies, many of the cells encapsulate within the mounds. Sporulation occurs about 6 hours earlier in yelA- cells than in wild-type cells, accompanied by precocious expression of a prespore gene, spiA. However, the spores are defective and lose viability over a period of several hours. Unencapsulated cells also die unless they are dissociated from the mounds and shaken in suspension. The yelA gene was isolated by plasmid rescue and found to encode a protein of 102 kDa in which the N-terminal sequence shows significant similarity to domains found in the eIF-4G subunits of the translational initiation complex eIF-4F. In wild-type cells yelA mRNA first accumulates at 8 hours of development and is maintained in both prespore and prestalk cells until culmination when it is found only is stalk cells. Mutations in yelA can partially suppress the block to sporulation in mutant strains in which either of the prestalk genes tagB or tagC is disrupted such that an encapsulation signal is not produced. It appears that premature encapsulation is normally inhibited by YelA until a signal is received from prestalk cells during culmination.

Wheat germ poly(A)-binding protein increases the ATPase and the RNA helicase activity of translation initiation factors eIF4A, eIF4B, and eIF-iso4F

Recent studies demonstrated that wheat germ poly(A)-binding protein (PABP) interacted with translation eukaryotic initiation factor (eIF)-iso4G and eIF4B, and these interactions increased the poly(A) binding activity of PABP (Le, H., Tanguay, R. L., Balasta, M. L., Wei, C. C., Browning, K. S., Metz, A. M., Goss, D. J., and Gallie, D. R. (1997) J. Biol. Chem. 272, 16247-16255) and the cap binding activity of eIF-iso4F (Wei, C. C., Balasta, M. L., Ren, J., and Goss, D. J. (1998) Biochemistry 37, 1910-1916). We report here that the interaction between PABP and eIF-iso4G has a substantial effect on the ATPase activity and RNA helicase activity of (eIF4A + eIF4B + eIF-iso4F) complex. ATPase kinetic assays show, in the presence of poly(U), PABP can increase the parameter (k(cat)/K(m)) by 3.5-fold with a 2-fold decrease of K(m) for the (eIF4A + eIF-iso4F) complex. In the presence of globin messenger RNA, the ATPase activity of the complex (eIF4A + eIF-iso4F) was increased 2-fold by the presence of PABP. RNA helicase assays demonstrated that the presence of PABP enhanced the RNA duplex unwinding activity of the initiation factor complex. These results suggest that, in terms of the scanning model of translation initiation, PABP may enhance the mRNA scanning rate of the complex formed by eIF4A, eIF4B, and eIF4F or eIF-(iso)4F and increase the rate of translation.

Functional characterization of five eIF4E isoforms in Caenorhabditis elegans

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, approximately 70% 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 trans-spliced to an SL2 leader, indicating that its gene resides in a downstream position of an operon.

Eukaryotic translation initiation factor 4E (eIF4E) binding site and the middle one-third of eIF4GI constitute the core domain for cap-dependent translation, and the C-terminal one-third functions as a modulatory region

The mammalian eukaryotic initiation factor 4GI (eIF4GI) may be divided into three roughly equal regions; an amino-terminal one-third (amino acids [aa] 1 to 634), which contains the poly(A) binding protein (PABP) and eIF4E binding sites; a middle third (aa 635 to 1039), which binds eIF4A and eIF3; and a carboxy-terminal third (aa 1040 to 1560), which harbors a second eIF4A binding site and a docking sequence for the Ser/Thr kinase Mnk1. Previous reports demonstrated that the middle one-third of eIF4GI is sufficient for cap-independent translation. To delineate the eIF4GI core sequence required for cap-dependent translation, various truncated versions of eIF4GI were examined in an in vitro ribosome binding assay with beta-globin mRNA. A sequence of 540 aa encompassing aa 550 to 1090, which contains the eIF4E binding site and the middle region of eIF4GI, is the minimal sequence required for cap-dependent translation. In agreement with this, a point mutation in eIF4GI which abolished eIF4A binding in the middle region completely inhibited ribosomal binding. However, the eIF4GI C-terminal third region, which does not have a counterpart in yeast, modulates the activity of the core sequence. When the eIF4A binding site in the C-terminal region of eIF4GI was mutated, ribosome binding was decreased three- to fourfold. These data indicate that the interaction of eIF4A with the middle region of eIF4GI is necessary for translation, whereas the interaction of eIF4A with the C-terminal region plays a modulatory role.

Eukaryotic initiation factor 4B from wheat and Arabidopsis thaliana is a member of a multigene family

Clones of eukaryotic initiation factor (eIF) 4B from wheat and Arabidopsis thaliana were obtained from cDNA and genomic libraries. The exon/intron organization of the genes from wheat and A. thaliana is very similar. The deduced amino acid sequences for the wheat and Arabidopsis eIF4B proteins showed overall similarity to each other, but very little similarity to eIF4B from other eukaryotes. The recombinant form of eIF4B supports polypeptide synthesis in an in vitro translation system and reacts with antibodies to native wheat eIF4B. In contrast to mammalian eIF4B and eIF4A, the combination of wheat eIF4B and eIF4A does not stimulate RNA-dependent ATP hydrolysis activity; however, wheat eIF4B does stimulate eIF4F and eIF4A RNA-dependent ATP hydrolysis activity. Interestingly, eIF4B does not stimulate eIF(iso)4F and eIF4A hydrolysis activity. Gel filtration experiments indicate wheat eIF4B, like its mammalian counterpart, self-associates to form a homodimer.

Eukaryotic translation initiation factor 4AIII (eIF4AIII) is functionally distinct from eIF4AI and eIF4AII

Eukaryotic initiation factor 4A (eIF4A) is an RNA-dependent ATPase and ATP-dependent RNA helicase that is thought to melt the 5' proximal secondary structure of eukaryotic mRNAs to facilitate attachment of the 40S ribosomal subunit. eIF4A functions in a complex termed eIF4F with two other initiation factors (eIF4E and eIF4G). Two isoforms of eIF4A, eIF4AI and eIF4AII, which are encoded by two different genes, are functionally indistinguishable. A third member of the eIF4A family, eIF4AIII, whose human homolog exhibits 65% amino acid identity to human eIF4AI, has also been cloned from Xenopus and tobacco, but its function in translation has not been characterized. In this study, human eIF4AIII was characterized biochemically. While eIF4AIII, like eIF4AI, exhibits RNA-dependent ATPase activity and ATP-dependent RNA helicase activity, it fails to substitute for eIF4AI in an in vitro-reconstituted 40S ribosome binding assay. Instead, eIF4AIII inhibits translation in a reticulocyte lysate system. In addition, whereas eIF4AI binds independently to the middle and carboxy-terminal fragments of eIF4G, eIF4AIII binds to the middle fragment only. These functional differences between eIF4AI and eIF4AIII suggest that eIF4AIII might play an inhibitory role in translation under physiological conditions.

Oxygen deprivation stimulates Ca2+-mediated phosphorylation of mRNA cap-binding protein eIF4E in maize roots

Flooding of maize seedlings causes O2 deprivation that leads to a global reduction in protein synthesis and selective translation of cytoplasmic mRNAs. Since selective translation in animal cells can involve the cap-binding protein eIF4E, we characterized the distinct mRNA cap-binding proteins eIF4E and eIFiso4E of maize. These proteins have 45% deduced amino acid sequence identity and are highly conserved at residues of eIF4E that function in intermolecular interactions in animals. Maize eIF4E is a phosphoprotein. O2 deprivation resulted in a decrease in the isoelectric point of eIF4E, consistent with additional phosphorylation. Modification of eIF4E was mimicked by treatment with caffeine under aerobic conditions and blocked by treatment with ruthenium red under O2 deprivation, implicating Ca2+ as a second messenger in eIF4E modification. In contrast, no isoelectric variants of eIFiso4E were detected. The possible role of cytosolic Ca2+ and pH in regulation of mRNA cap-binding protein activity under O2 deprivation is discussed.

Cap-dependent translation initiation in eukaryotes is regulated by a molecular mimic of eIF4G

eIF4G uses a conserved Tyr-X-X-X-X-Leu-phi segment (where X is variable and phi is hydrophobic) to recognize eIF4E during cap-dependent translation initiation in eukaryotes. High-resolution X-ray crystallography and complementary biophysical methods have revealed that this eIF4E recognition motif undergoes a disorder-to-order transition, adopting an L-shaped, extended chain/alpha-helical conformation when it interacts with a phylogenetically invariant portion of the convex surface of eIF4E. Inhibitors of translation initiation known as eIF4E-binding proteins (4E-BPs) contain similar eIF4E recognition motifs. These molecules are molecular mimics of eIF4G, which act by occupying the same binding site on the convex dorsum of eIF4E and blocking assembly of the translation machinery. The implications of our results for translation initiation are discussed in detail, and a molecular mechanism for relief of translation inhibition following phosphorylation of the 4E-BPs is proposed.

Identification of a nucleic acid binding domain in eukaryotic initiation factor eIFiso4G from wheat

Higher plants have two complexes that bind the m7G-cap structure of mRNA and mediate interactions between mRNA and ribosomal subunits, designated eIF4F and eIFiso4F. Both complexes contain a small subunit that binds the 5'-cap structure of mRNA, and a large subunit, eIF4G or eIFiso4G, that binds other translation factors and RNA. Sequence-specific proteases were used to cleave native cap-binding complexes into structural domains, which were purified by affinity chromatography. We show here that eIFiso4G contains a central protease-resistant domain that binds specifically to nucleic acids. This domain spans Gln170 to Glu443 and includes four of the six homology blocks shared by eIFiso4G and eIF4G. A slightly shorter overlapping sequence, from Gly202 to Lys445, had no nucleic acid binding activity, indicating that the N-terminal end of the nucleic acid binding site lies within Gln170 to Arg201. The binding of the central domain and native eIFiso4F to RNA homopolymers and double- and single-stranded DNAs was studied. Both molecules had highest affinity for poly(G) and recognized single- and double-stranded sequences.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Phosphorylation of eIF4E at a conserved serine in Aplysia

We have cloned eIF4E from the marine mollusk, Aplysia californica. The sequence of eIF4E from Aplysia is more similar to vertebrate eIF4Es than to other invertebrate sequences. Aplysia eIF4E is encoded by two tissue-specific RNAs. Antibodies raised to the carboxyl terminus of eIF4E recognize a 29-kDa protein that can bind to 7-methyl-GTP caps. The phosphorylation site identified in mammalian eIF4E is conserved in the Aplysia homologue, and an Aplysia eIF4E fusion protein is phosphorylated well by both Aplysia protein kinase C isoforms. However, protein kinase C phosphorylates both Ser-207 and Thr-208 in vitro, while only Ser-207 is phosphorylated in vivo. We have confirmed that Ser-207 is phosphorylated in vivo by raising a phosphopeptide antibody to this site. This antibody will be useful in determining the signal transduction pathways leading to eIF4E phosphorylation in Aplysia.

Multiple isoforms of eukaryotic protein synthesis initiation factor 4E in Caenorhabditis elegans can distinguish between mono- and trimethylated mRNA cap structures

The rate-limiting step for cap-dependent translation initiation in eukaryotes is recruitment of mRNA to the ribosome. An early event in this process is recognition of the m7GTP-containing cap structure at the 5'-end of the mRNA by initiation factor eIF4E. In the nematode Caenorhabditis elegans, mRNAs from 70% of the genes contain a different cap structure, m32,2,7GTP. This cap structure is poorly recognized by mammalian elF4E, suggesting that C. elegans may possess a specialized form of elF4E that can recognize m32,2,7GTP. Analysis of the C. elegans genomic sequence data base revealed the presence of three elF4E-like genes, here named ife-1, ife-2, and ife-3. cDNAs for these three eIF4E isoforms were cloned and sequenced. Isoform-specific antibodies were prepared from synthetic peptides based on nonhomologous regions of the three proteins. All three eIF4E isoforms were detected in extracts of C. elegans and were retained on m7GTP-Sepharose. One eIF4E isoform, IFE-1, was also retained on m32,2,7GTP-Sepharose. Furthermore, binding of IFE-1 and IFE-2 to m7GTP-Sepharose was inhibited by m32,2,7GTP. These results suggest that IFE-1 and IFE-2 bind both m7GTP- and m32,2, 7GTP-containing mRNA cap structures, although with different affinities. In conjunction with IFE-3, these eIF4E isoforms would permit cap-dependent recruitment of all C. elegans mRNAs to the ribosome.

The Arabidopsis thaliana cDNAs coding for eIF4E and eIF(iso)4E are not functionally equivalent for yeast complementation and are differentially expressed during plant development

Two cDNAs (At.EIF4E1 and At.EIF4E2) encoding, respectively, the eukaryotic initiation factors eIF4E and eIF(iso)4E of Arabidopsis thaliana were isolated by complementation of a Saccharomyces cerevisiae conditional mutant. The deduced amino acid sequences of the proteins are homologous to those from monocotyledonous plants, yeast and mammals. The corresponding genes were identified in YAC clones mapping to chromosome IV (At.EIF4E1) and to chromosome V (At.EIF4E2). The yeast strain complemented by At.EIF4E2 grew poorly compared with an isogenic strain expressing At.EIF4E1. Northern and in situ hybridization analysis show that both Arabidopsis At.EIF4E1 and At.EIF4E2 mRNAs are differentially accumulated in plant tissues. The At.EIF4E1 mRNA is expressed in all tissues except in the cells of the specialization zone of the roots; the At.EIF4E2 mRNA is particularly abundant in floral organs and in young developing tissues. This work further demonstrates an association between a high level of EIF4E mRNAs and cell proliferation and suggests that the plant eIF4E isoforms may have distinct functions in cell development and metabolism.

A novel functional human eukaryotic translation initiation factor 4G

Mammalian eukaryotic translation initiation factor 4F (eIF4F) is a cap-binding protein complex consisting of three subunits: eIF4E, eIF4A, and eIF4G. In yeast and plants, two related eIF4G species are encoded by two different genes. To date, however, only one functional eIF4G polypeptide, referred to here as eIF4GI, has been identified in mammals. Here we describe the discovery and functional characterization of a closely related homolog, referred to as eIF4GII. eIF4GI and eIF4GII share 46% identity at the amino acid level and possess an overall similarity of 56%. The homology is particularly high in certain regions of the central and carboxy portions, while the amino-terminal regions are more divergent. Far-Western analysis and coimmunoprecipitation experiments were used to demonstrate that eIF4GII directly interacts with eIF4E, eIF4A, and eIF3. eIF4GII, like eIF4GI, is also cleaved upon picornavirus infection. eIF4GII restores cap-dependent translation in a reticulocyte lysate which had been pretreated with rhinovirus 2A to cleave endogenous eIF4G. Finally, eIF4GII exists as a complex with eIF4E in HeLa cells, because eIF4GII and eIF4E can be purified together by cap affinity chromatography. Taken together, our findings indicate that eIF4GII is a functional homolog of eIF4GI. These results may have important implications for the understanding of the mechanism of shutoff of host protein synthesis following picornavirus infection.

Interaction of the viral protein genome linked of turnip mosaic potyvirus with the translational eukaryotic initiation factor (iso) 4E of Arabidopsis thaliana using the yeast two-hybrid system

The yeast LexA interaction trap was used to screen a cDNA library from Arabidopsis thaliana in order to identify proteins that interact with the viral protein genome linked (VPg)-proteinase of turnip mosaic potyvirus. The screen allowed the isolation of four candidate cDNA clones. Clones pHC4, pHC21, and pHC40 were partially sequenced but no homologies to known proteins were found. However, the amino acid sequence deduced from the complete nucleotide sequence of pSW56 revealed that it was the eukaryotic initiation factor (iso) 4E [eIF(iso)4E]. Deletion analysis indicated that the VPg domain was involved in the interaction with the plant protein. Interaction between the viral protein and the cellular protein was confirmed by ELISA-based binding experiments. eIF(iso)4E plays an essential role in the initiation of the translation of capped mRNAs and its association with VPg would point to a role of the viral protein in the translation of the virus.

Translation initiation factors eIF-iso4G and eIF-4B interact with the poly(A)-binding protein and increase its RNA binding activity

The 5'-cap and the poly(A) tail act synergistically to increase the translational efficiency of eukaryotic mRNAs, which suggests that these two mRNA elements communicate during translation. We report here that the cap-associated eukaryotic initiation factors (eIFs), i. e. the two isoforms of the cap-binding complex (eIF-4F and eIF-iso4F) and eIF-4B, bind to the poly(A)-binding protein (PABP) both in the presence and absence of poly(A) RNA. The interactions between PABP and eIF-4F, eIF-iso4F, and eIF-4B were measured in the absence of poly(A) RNA using far Western analysis and confirmed by direct fluorescence titration studies. The functional consequence of the interaction between these initiation factors and PABP was examined using RNA binding assays and RNA mobility shift analysis. eIF-4F, eIF-iso4F, and eIF-4B promoted PABP activity through a shift in its equilibrium affinity for poly(A). eIF-iso4G, the large subunit of eIF-iso4F, was the subunit responsible for the interaction between eIF-iso4F and PABP and was the subunit that promoted PABP RNA binding activity. Truncation analysis of eIF-iso4G indicated that a domain close to its N-terminal end appeared to be involved in binding PABP. These results suggest that the interaction between PABP and eIF-4B and eIF-iso4G may be involved in mediating the functional co-dependence observed between the cap and the poly(A) tail during translation.

Disulfide bond formation is not involved in cap-binding activity of Xenopus translation initiation factor eIF-4E

The eukaryotic initiation factor eIF-4E from Xenopus laevis was expressed in Escherichia coli and refolded in an active form. To define the cysteine residues forming a disulfide bond in Xenopus eIF-4E, each of the 3 cysteine residues was changed to serine by site-directed mutagenesis. Cap-binding activities of the mutant proteins were evaluated by 7-methyl-GTP(m7GTP)-affinity column chromatography. Even the mutant protein containing no cysteine showed an affinity for m7GTP. From the above results and the estimation of the sulfhydryl groups by Ellman's assay method, we concluded that a disulfide bond is not involved in the active Xenopus eIF-4E.

Assignment of the beta-subunit of wheat eIF2 by protein and DNA sequence analysis and immunoanalysis

Wheat germ initiation factor 2 (eIF2), like mammalian and yeast eIF2, contains three nonidentical subunits. The estimated molecular weights for the wheat subunits are 38,000 (p38), 42,000 (p42), and 50,000 (p50). Peptide sequence was obtained for the p38 subunit of wheat eIF2 and the resulting amino acid sequence suggested that it was actually the equivalent of the mammalian beta-subunit. A wheat sprout cDNA expression library was screened with antibody affinity purified to the p38 subunit. The DNA sequence of the clones obtained also indicated that the p38 subunit was the equivalent to the mammalian beta-subunit. The wheat p38 subunit was then expressed in Escherichia coli and antibodies raised to the purified recombinant protein. Only the p38 subunit of purified wheat germ eIF2 reacted with the antisera. The p38 subunit of wheat eIF2 is therefore the equivalent of mammalian eIF2beta.

A novel translational repressor mRNA is edited extensively in livers containing tumors caused by the transgene expression of the apoB mRNA-editing enzyme

Transgene expression of the apolipoprotein B mRNA-editing enzyme (APOBEC-1) causes dysplasia and carcinoma in mouse and rabbit livers. Using a modified differential display technique, we identified a novel mRNA (NAT1 for novel APOBEC-1 target no. 1) that is extensively edited at multiple sites in these livers. The aberrant editing alters encoded amino acids, creates stop codons, and results in markedly reduced levels of the NAT1 protein in transgenic mouse livers. NAT1 is expressed ubiquitously and is extraordinarily conserved among species. It has homology to the carboxy-terminal portion of the eukaryotic translation initiation factor (eIF) 4G that binds eIF4A and eIF4E to form eIF4F. NAT1 binds eIF4A but not eIF4E and inhibits both cap-dependent and cap-independent translation. NAT1 is likely to be a fundamental translational repressor, and its aberrant editing could contribute to the potent oncogenesis induced by overexpression of APOBEC-1.

The wheat poly(A)-binding protein functionally complements pab1 in yeast

Poly(A)-binding protein (PAB) binds to the poly(A) tail of most eukaryotic mRNAs and influences its translational efficiency as well as its stability. Although the primary structure of PAB is well conserved in eukaryotes, its functional conservation across species has not been extensively investigated. In order to determine whether PAB from a monocot plant species could function in yeast, a protein characterized as having PAB activity was purified from wheat and a cDNA encoding for PAB was isolated from a wheat seedling expression library. Wheat PAB (72 kDa as estimated by SDS/PAGE and a theoretical mass of 70 823 Da as determined from the cDNA) was present in multiple isoforms and exhibited binding characteristics similar to that determined for yeast PAB. Comparison of the wheat PAB protein sequence with PABs from yeast and other species revealed that wheat PAB contained the characteristic features of all PABs, including four RNA binding domains each of which contained the conserved RNP1 and RNP2 sequence motifs. The wheat PAB cDNA functionally complemented a pab1 mutant in yeast suggesting that, although the amino acid sequence of wheat PAB is only 47% conserved from that of yeast PAB, this monocot protein can function in yeast.

The phosphorylation state of translation initiation factors is regulated developmentally and following heat shock in wheat

Several translation initiation factors in mammals and yeast are regulated by phosphorylation. The phosphorylation state of these factors is subject to alteration during development, environmental stress (heat shock, starvation, or heme deprivation), or viral infection. The phosphorylation state and the effect of changes in phosphorylation of the translation initiation factors of higher plants have not been previously investigated. We have determined the isoelectric states for the wheat translation initiation factors eIF-4A, eIF-4B, eIF-4F, eIF-iso4F, and eIF-2 and the poly(A)-binding protein in the seed, during germination, and following heat shock of wheat seedlings using two-dimensional gel electrophoresis and Western analysis. We found that the developmentally induced changes in isoelectric state observed during germination or the stress-induced changes were consistent with changes in phosphorylation. Treatment of the phosphorylated forms of the factors with phosphatases confirmed that the nature of the modification was due to phosphorylation. The isoelectric states of eIF-4B, eIF-4F (eIF-4E, p26), eIF-iso4F (eIF-iso4E, p28), and eIF-2alpha (p42) were altered during germination, suggesting that phosphorylation of these factors is developmentally regulated and correlates with the resumption of protein synthesis that occurs during germination. The phosphorylation of eIF-2beta (p38) or poly(A)-binding protein did not change either during germination or following a thermal stress. Only the phosphorylation state of two factors, eIF-4A and eIF-4B, changed following a heat shock, suggesting that plants may differ significantly from animals in the way in which their translational machinery is modified in response to a thermal stress.

Mutational analysis of the functional domains of the large subunit of the isozyme form of wheat initiation factor eIF4F

The isozyme form of plant eukaryotic initiation factor 4F (eIF(iso)4F) contains two subunits: p28, a cap-binding protein, and p86. To identify the functional domains of p86, truncations of the p86 cDNA were made, and the protein was expressed in Escherichia coli and purified. The deletion mutants were tested for the ability to bind the p28 subunit by two methods. In addition, these deletion mutants were evaluated in vitro by the ability to catalyze eIF4A and RNA-dependent ATP hydrolysis and to support polypeptide synthesis. The loss of the ability to bind p28 occurs within the first 90 amino acids of the N terminus and abrogates the ability of p86 to participate in translation initiation and bind to eIF4A, but does not affect ATP hydrolysis. Up to 299 amino acid residues from the C terminus of p86 must be deleted before an effect is observed on the ATP hydrolysis activity. Thus, the p28 binding and ATP hydrolysis activities appear to lie on two separate domains and are functionally uncoupled. In addition, at least a portion of the eIF4A binding domain appears to be in close proximity to the p28 binding domain and is also uncoupled from the ATP hydrolysis activity.

Sequences of two expressed sequence tags (EST) from rice encoding different cap-binding proteins

Wheat has been shown to have two forms of the cap-binding protein that participate in the initiation of translation. To identify cap-binding proteins from other higher plant species, the expressed sequence tag (EST) database was searched. Several rice ESTs were identified with similarity to both forms of the wheat cap-binding proteins. Two of the rice ESTs were obtained and the cDNA sequences completed. The deduced amino acid sequences of the rice cap-binding proteins are compared to the wheat cap-binding proteins and cap-binding proteins from Saccharomyces cerevisiae, Drosophila melanogaster, Xenopus laevis and human.

pH-dependent and ligand induced conformational changes of eucaryotic protein synthesis initiation factor eIF-(iso)4F: a circular dichroism study

The structural features of wheat germ protein synthesis initiation factor eIF-(iso)4F, which has a cap binding protein as one of its two subunits, are unknown. In this study, circular dichroism (CD) spectra and secondary structure prediction were obtained for eIF-(iso)4F and its two subunits, p28 and p86. The alpha-helix content of eIF-(iso)4F changed from 42% at pH 6.3 to 15% at pH 7.6, the optimum pH for cap binding. The beta-sheet content increased from 14% (pH 6.3) to 38% at pH 7.6. The CD spectra of the two subunits, p28 and p86 were also measured and analyzed. The separated subunits both had a higher alpha-helix content at pH 7.6 than the native protein, giving values of 60% and 34% alpha-helix for p28 and p86, respectively. Binding of the dinucleotide cap analog to p28 reduced the alpha-helix content to approximately 8% with an increase in the beta sheet content from 10% to 37%. The conformational changes in eIF-(iso)4F upon binding with mRNA are dependent on cap or oligonucleotide structure. A conformation consisting of approximately the same alpha-helix and beta-sheet content can be induced by ligands even at non-optimal pH values. This large conformational transition suggests eIF-(iso)4F binds nucleic acids by interaction of a beta-sheet motif and that this conformational transition may have a regulatory role.

The plant translational apparatus

Protein synthesis in both eukaryotic and prokaryotic cells is a complex process requiring a large number of macromolecules: initiation factors, elongation factors, termination factors, ribosomes, mRNA, amino-acylsynthetases and tRNAs. This review focuses on our current knowledge of protein synthesis in higher plants.

Characterization of multiple mRNAs that encode mammalian translation initiation factor 5 (eIF-5)

Eukaryotic translation initiation factor 5 (eIF-5) interacts with the 40 S initiation complex (40S.mRNA.MettRNAf.eIF-2.GTP) to promote the hydrolysis of bound GTP with the concomitant joining of the 60 S ribosomal subunit to the 40 S initiation complex to form a functional 80 S initiation complex. In this paper, the multiple mRNAs that encode mammalian eIF-5 have been characterized. In rat tissues, three major eIF-5 mRNAs of 3.5, 2.8, and 2.2 kilobases in length are detected. All major eIF-5 mRNAs are initiated from a single transcription initiation site, contain identical 5'-untranslated and coding regions, but differ from one another only in the length of their 3'-untranslated regions. The different lengths of the 3'-untranslated region of eIF-5 mRNAs are generated by the use of alternative polyadenylation signals. Additionally, we demonstrate tissue-specific variations in eIF-5 mRNA expression as well as preference for polyadenylation sites. These results should lead to increased understanding of the regulation of eIF-5 gene expression.

Alternatively spliced transcripts from the Drosophila eIF4E gene produce two different Cap-binding proteins

Eukaryotic initiation factor 4E (eIF4E) is the subunit of eIF4F that binds to the cap structure at the 5' end of messenger RNA and is a critical component for the regulation of translation initiation. Using 7-methyl-GTP-Sepharose affinity chromatography, two distinct cap-binding proteins that migrate on SDS-polyacrylamide gel electrophoresis at approximately 35 kDa were purified from Drosophila adults. Peptide microsequence analysis indicated that these two proteins differ at their amino termini. Analysis of a set of cDNA clones encoding eIF4E led to the conclusion that the two different protein isoforms, which we term eIF4EI and eIF4EII, result from three alternatively spliced transcripts from a single eIF4E gene, which maps to region 67A8-B2 on polytene chromosomes. The three eIF4E transcripts also vary greatly in the lengths of their 5'-UTRs, suggesting the possibility of complex translational control of expression of the two eIF4E isoforms.

Hybrid proteins between Pseudomonas aeruginosa exotoxin A and poliovirus 2Apro cleave p220 in HeLa cells

Cleavage of p220, a component of the initiation factor eIF-4F, has been correlated with the inhibition of host translation during poliovirus infection. To obtain p220 cleavage in the absence of any other poliovirus gene products, hybrid proteins containing Pseudomonas aeruginosa exotoxin A and poliovirus protease 2Apro have been constructed. The addition of the hybrid molecules to cultured cells did not lead to substantial p220 cleavage. However, the simultaneous presence of the hybrid toxin with replicationally inactive chicken adenovirus particles results in efficient cleavage of p220 in the intact cells. Under these conditions, cellular translation continues unabated for several hours, arguing against a direct requirement for intact p220 in each round of the initiation of translation of cellular mRNAs.