A single amino acid change in protein synthesis initiation factor 4G renders cap-dependent translation resistant to picornaviral 2A proteases

Infection of cells with picornaviruses of the rhino-, aphtho-, and enterovirus groups causes a shut-off in cap-dependent translation of cellular mRNAs but permits cap-independent viral RNA translation to proceed. This shut-off is thought to be mediated in part by the proteolytic cleavage of eukaryotic initiation factor 4G (eIF4G), although there is evidence to the contrary. Cleavage of eIF4G results in the separation of the eIF4E-binding domain from the ribosome- and helicase-binding domains of the factor, thereby limiting the ability of eIF4G to function in cap-dependent recruitment of mRNAs. Previously we determined the cleavage site within eIF4G targeted by the 2A proteases from human coxsackievirus serotype B4 and human rhinovirus serotype 2 using highly purified eIF4F and recombinant proteases. To examine further the role proteolysis of eIF4G plays in shut-off of translation, we altered the 2A cleavage site in human eIF4G by site-directed mutagenesis. Strikingly, the replacement of one amino acid at the 2A cleavage site resulted in a protein that is approximately 100-fold resistant to cleavage by coxsackievirus 2A protease and 10-50-fold for rhinovirus 2A. Alteration of the cleavage site had no effect on factor activity since the variant was just as active as wild-type eIF4G in restoring cap-dependent translation to an in vitro translation system depleted of endogenous eIF4G. Furthermore, the presence of the variant form of eIF4G rendered in vitro translation reactions resistant to the 2A protease-mediated inhibition of cap-dependent translation initiation. These results support the model that 2A proteases inhibit cap-dependent translation through direct proteolysis of eIF4G.

Stimulation of protein synthesis, eukaryotic translation initiation factor 4E phosphorylation, and PHAS-I phosphorylation by insulin requires insulin receptor substrate 1 and phosphatidylinositol 3-kinase

Insulin rapidly stimulates protein synthesis in a wide variety of tissues. This stimulation is associated with phosphorylation of several translational initiation and elongation factors, but little is known about the signaling pathways to these events. To study these pathways, we have used a myeloid progenitor cell line (32D) which is dependent on interleukin 3 but insensitive to insulin because of the very low levels of insulin receptor (IR) and the complete lack of insulin receptor substrate (IRS)-signaling proteins (IRS-1 and IRS-2). Expression of more IR permits partial stimulation of mitogen-activated protein kinase by insulin, and expression of IRS-1 alone mediates insulin stimulation of the 70-kDa S6 kinase (pp70S6K) by the endogenous IR. However, expression of both IR and IRS-1 is required for stimulation of protein synthesis. Moreover, this effect requires activation of phosphatidylinositol 3-kinase (PI3K), as determined by wortmannin inhibition and the use of an IRS-1 variant lacking all Tyr residues except those which activate PI3K. Stimulation of general protein synthesis does not involve activation by IRS-1 of GRB-2-SOS-p21ras or SH-PTP2, since IRS-1 variants lacking the SH2-binding Tyr residues for these proteins are fully active. Nor does it involve pp70S6K, since rapamycin, while strongly inhibiting the synthesis of a small subset of growth-regulated proteins, only slightly inhibits total protein synthesis. Recruitment of mRNAs to the ribosome is enhanced by phosphorylation of eIF4E, the cap-binding protein, and PHAS-I, a protein that specifically binds eIF4E. The behavior of cell lines containing IRS-1 variants and inhibition by wortmannin and rapamycin indicate that the phosphorylation of both proteins requires IRS-1-mediated stimulation of PI3K and pp70S6K but not mitogen-activated protein kinase or SH-PTP2.

Internal initiation of translation directed by the 5'-untranslated region of the mRNA for eIF4G, a factor involved in the picornavirus-induced switch from cap-dependent to internal initiation

The eIF4 group initiation factors carry out recognition of the mRNA cap, unwinding of mRNA secondary structure, and binding of mRNA to the 43 S preinitiation complex. Infection by picornaviruses results in proteolytic cleavage of one of these factors, eIF4G, an event that severely restricts cap-dependent translation but permits cap-independent initiation to proceed from internal ribosome entry sequences in picornaviral RNAs. The 5'-untranslated region (5'-UTR) of eIF4G mRNA resembles such picornaviral sequences in being unusually long and containing multiple open reading frames and a polypyrimidine tract. When inserted upstream of a luciferase reporter gene, this 5'-UTR served as a translational enhancer in four different cell lines. Mutation of all four upstream ATG codons to AAG did not alter the translational enhancement. The presence of the eIF4G 5'-UTR between an RNA hairpin and the luciferase cistron stimulated expression 119-fold. Similarly, the presence of the 5'-UTR between the two cistrons of a bicistronic mRNA stimulated expression of the downstream cistron 42-fold. These results indicate that the eIF4G 5'-UTR directs internal initiation. The ability to continue synthesis of eIF4G when the cell is unable to carry out normal cap-dependent translation may represent an autoregulatory mechanism or be part of the cellular response to stresses that interrupt cap-dependent translation.

Mapping of functional domains in eukaryotic protein synthesis initiation factor 4G (eIF4G) with picornaviral proteases. Implications for cap-dependent and cap-independent translational initiation

Cap-dependent binding of mRNA to the 40 S ribosomal subunit during translational initiation requires the association of eukaryotic initiation factor 4G (eIF4G; formerly eIF-4 gamma and p220) with other initiation factors, notably eIF4E, eIF4A, and eIF3. Infection of cells by picornaviruses results in proteolytic cleavage of eIF4G and generation of a cap-independent translational state. Rhinovirus 2A protease and foot-and-mouth-disease virus L protease were used to analyze the association of eIF4G with eIF4A, eIF4E, and eIF3. Both proteases bisect eIF4G into N- and C-terminal fragments termed cpN and cpC. cpN was shown to contain the eIF4E-binding site, as judged by retention on m7GTP-Sepharose, whereas cpC was bound to eIF3 and eIF4A, based on ultracentrifugal co-sedimentation. Further proteolysis of cpN by L protease produced an 18-kDa polypeptide termed cpN2 which retained eIF4E binding activity and corresponded to amino acid residues 319-479 of rabbit eIF4G. Further proteolysis of cpC yielded several smaller fragments. cpC2 (approximately 887-1402) contained the eIF4A binding site, whereas cpC3 (approximately 480-886) contained the eIF3 binding site. These results suggest that cleavage by picornaviral proteases at residues 479-486 separates eIF4G into two domains, one required for recruiting capped mRNAs and one for attaching mRNA to the ribosome and directing helicase activity. Only the latter would appear to be necessary for internal initiation of picornaviral RNAs.

Phosphorylation of eukaryotic protein synthesis initiation factor 4E at Ser-209

Initiation factor 4E (eIF-4E) binds to the m7GTP-containing cap of eukaryotic mRNA and facilitates the entry of mRNA into the initiation cycle of protein synthesis. eIF-4E is a phosphoprotein, and the phosphorylated form binds to mRNA caps 3-4-fold more tightly than the nonphosphorylated form. A previous study indicated that the major phosphorylation site was Ser-53 (Rychlik, W., Russ, M. A., and Rhoads, R. E. (1987) J. Biol. Chem. 262, 10434-10437). In the present study, we synthesized the phosphopeptide expected to result from tryptic digestion of eIF-4E, O-phosphoseryllysine. Surprisingly, the tryptic and synthetic phosphopeptides did not comigrate electrophoretically. Accordingly, we redetermined the phosphorylation site by isolating a chymotryptic phosphopeptide on reverse phase high performance liquid chromatography. The peptide was sequenced by Edman degradation and corresponded to 198QSHADTATKSGSTTKNRF215. The site of phosphorylation was determined to be Ser-209 by four methods: the increase in the ratio of dehydroalanine to serine derivatives during Edman degradation, the release of 32P, the further digestion of the chymotryptic phosphopeptide with trypsin, Glu-C, and Asp-N, and site-directed mutagenesis of eIF-4E cDNA. The S209A variant was not phosphorylated in a rabbit reticulocyte lysate system, whereas the wild-type, S53A, and S207A variants were. This site falls within the consensus sequence for phosphorylation by protein kinase C.

Human protein synthesis initiation factor eIF-4 gamma is encoded by a single gene (EIF4G) that maps to chromosome 3q27-qter

Messenger RNA binding to the ribosome, the rate-limiting step in eukaryotic protein synthesis, is catalyzed by the eIF-4 group of initiation factors. These factors collectively bind to the m7GTP-containing cap of mRNA and unwind mRNA secondary structure at the expense of ATP. One member of the group, eIF-4 gamma, is the target for proteolytic cleavage during picornavirus infection, an event that is thought to be responsible for the inhibition of host cellular mRNA translation. Human eIF-4 gamma migrates as a cluster of polypeptides in the range of 200-220 kDa during SDS-PAGE, raising the possibility that it is a family of proteins encoded by separate genes. In this study, we present genomic Southern blotting results which indicate that there is only a single gene, here designated EIF4G. Furthermore, we have employed a PCR approach to map EIF4G to chromosome 3q27-qter.

Inhibition of protein synthesis by the heme-controlled eIF-2 alpha kinase leads to the appearance of mRNA-containing 48S complexes that contain eIF-4E but lack methionyl-tRNA(f)

Phosphorylation of the initiation factor eIF-2 by the heme-regulated eIF-2 alpha kinase (HCR) results in pronounced inhibition of protein synthesis and of binding of Met-tRNA(f) to 40S subunits in reticulocyte lysates. This inhibition is associated with the appearance of a more rapidly sedimenting 48S complex; this contains mRNA detectable by poly(U) hybridization, but not Met-tRNA(f). In contrast, 48S complexes that accumulate in the presence of the initiation inhibitor edeine contain both Met-tRNA(f) and mRNA. We have compared the composition of the particles that accumulate in the presence of HCR with those seen in the presence of edeine and find that both particles contain the cap binding protein, eIF-4E. Moreover, both particles exhibit a buoyant density of 1.40 g/ml in CsCl equilibrium density gradients. This is consistent with the presence of 500-700 kDa of protein additional to ribosomal structural protein, and suggests the presence of eIF-3 on both types of 48S complex. Lysates pre-treated with HCR and then treated with edeine show the ability to accumulate 48S complexes containing Met-tRNA(f), though at a slower rate than control lysates. These observations are discussed in the light of mechanisms previously suggested for the appearance of 48S particles following HCR treatment. In addition, we have observed association of eIF-4E with polysomes and 80S monosomes in reticulocyte lysates, suggesting that this factor may not be released immediately following the binding of the 40S ribosomal subunit to the 5' end of the mRNA.

Foot-and-mouth disease virus leader proteinase: purification of the Lb form and determination of its cleavage site on eIF-4 gamma

Many picornaviruses cause a dramatic decrease in the translation of cellular mRNAs in the infected cell, without affecting the translation of their own RNA. Specific proteolysis of protein synthesis initiation factor eIF-4 gamma occurs during infection with rhinoviruses, enteroviruses, and aphthoviruses, apparently leading to an inability of the ribosomes to bind capped mRNAs. Cleavage of eIF-4 gamma in human rhinoviruses and enteroviruses is carried out by the viral 2A proteinase; in aphthoviruses (i.e., foot-and-mouth disease viruses), the leader proteinase is responsible for this reaction. We describe here the purification to homogeneity of the Lb form of the leader proteinase expressed in Escherichia coli. The primary cleavage products of eIF-4 gamma obtained in vitro with purified leader or 2A proteinase are electrophoretically indistinguishable from those found during infection in vivo. However, additional proteolysis products of eIF-4 gamma are observed with the leader proteinase and the human rhinovirus type 2 2A proteinase in vitro. The cleavage site of the leader proteinase in eIF-4 gamma from rabbit reticulocyte was determined by sequencing the purified C-terminal cleavage product by automated Edman degradation. The cleavage site is between Gly-479 and Arg-480 and thus differs from that of rhinovirus and enterovirus 2A proteinases, which cleave between Arg-486 and Gly-487.

Chromatographic resolution of in vivo phosphorylated and nonphosphorylated eukaryotic translation initiation factor eIF-4E: increased cap affinity of the phosphorylated form

Eukaryotic translation initiation factor eIF-4E plays a central role in the recognition of the 7-methylguanosine-containing cap structure of mRNA and the formation of initiation complexes during protein synthesis. eIF-4E exists in both phosphorylated and nonphosphorylated forms, and the primary site of phosphorylation has been identified. Previous studies have suggested that eIF-4E phosphorylation facilitates its participation in protein synthesis. However, the biochemical basis for the functional difference between the two forms of eIF-4E is unknown. To address this directly, we have developed a method for the separation of phosphorylated and nonphosphorylated eIF-4E from rabbit reticulocytes by chromatography on rRNA-Sepharose. Using the resultant purified forms, we have studied the protein's interaction with the cap analogs m7GTP and m7GpppG and with the cap of globin mRNA by fluorescence quenching of tryptophan residues. It was found that phosphorylated eIF-4E had 3- to 4-fold greater affinity for cap analogs and mRNA than nonphosphorylated eIF-4E. The equilibrium binding constants (x 10(5), expressed as M-1) for the interaction of phosphorylated eIF-4E with m7GTP, m7GpppG, and globin mRNA were 20.0 +/- 0.1, 16.4 +/- 0.1, and 31.0 +/- 0.1, respectively, whereas those for the nonphosphorylated form were 5.5 +/- 0.4, 4.3 +/- 0.4, and 10.0 +/- 0.1, respectively. Treatment with potato acid phosphatase converted the phosphorylated form to the nonphosphorylated form and decreased the binding constant for m7GTP by a factor of 3. The increased affinity for mRNA caps may account for the in vivo and in vitro correlations between eIF-4E phosphorylation and accelerated protein synthesis and cell growth.

Mechanism of differential regulation of IL-2 in murine Th1 and Th2 T cell subsets. 1. Induction of IL-2 transcription in Th2 cells by up-regulation of transcription factors with the protein synthesis initiation factor 4E

Regulation of IL-2 gene expression in response to receptor-mediated stimuli is known to be mediated primarily by the IL-2 transcriptional enhancer and multiple transcription factors. However, the mechanism that controls the differential expression of the IL-2 gene in both human and murine CD4+ Th cell subsets (Th1-IL-2+ and Th2-IL-2-) is not clearly understood. Differential IL-2 gene expression was assessed in murine Th1 and Th2 subsets by analyzing the expression of a Escherichia coli lacZ reporter gene under control of the human IL-2 enhancer (IL2ZH) transfected in both T cell subsets. Stimulation of transfected T cells with the mitogen Con A, anti-CD3 Ab, or PMA plus ionomycin activated the IL2ZH construct in Th1 but not Th2 cells. However, IL2ZH was activated in stimulated Th2 cells that were co-transfected with a vector that overexpressed the eukaryotic initiation factor 4E (eIF-4E). It has been shown that eIF-4E is rate limiting for protein synthesis and its overexpression leads to increased rates of protein synthesis. Hence, eIF-4E overexpression could have overcome a deficiency in transcriptionally active levels of IL-2 regulatory factors in Th2 cells leading to IL-2 enhancer activation. This possibility was supported by demonstrating that transcriptionally active levels of the critical IL-2 transcription factor, nuclear factor of activated T cells (NF-AT), occurred only in Th2 cells overexpressing eIF-4E but not in normal Th2 cells, thus indicating that the inability of Th2 cells to express IL-2 was associated with inadequate levels of at least one transcription factor, NF-AT. Moreover, these results were confirmed by the observation that eIF-4E overexpression augmented NF-AT binding activity in Th2 cells. These data suggest that concentrations of inducible transcription factors are a major component of the regulatory mechanisms dictating IL-2 expression and may be under translational control in Th1/Th2 T cell subsets.

Mechanism of differential regulation of IL-2 in murine Th1 and Th2 T cell subsets. 1. Induction of IL-2 transcription in Th2 cells by up-regulation of transcription factors with the protein synthesis initiation factor 4E

Regulation of IL-2 gene expression in response to receptor-mediated stimuli is known to be mediated primarily by the IL-2 transcriptional enhancer and multiple transcription factors. However, the mechanism that controls the differential expression of the IL-2 gene in both human and murine CD4+ Th cell subsets (Th1-IL-2+ and Th2-IL-2-) is not clearly understood. Differential IL-2 gene expression was assessed in murine Th1 and Th2 subsets by analyzing the expression of a Escherichia coli lacZ reporter gene under control of the human IL-2 enhancer (IL2ZH) transfected in both T cell subsets. Stimulation of transfected T cells with the mitogen Con A, anti-CD3 Ab, or PMA plus ionomycin activated the IL2ZH construct in Th1 but not Th2 cells. However, IL2ZH was activated in stimulated Th2 cells that were co-transfected with a vector that overexpressed the eukaryotic initiation factor 4E (eIF-4E). It has been shown that eIF-4E is rate limiting for protein synthesis and its overexpression leads to increased rates of protein synthesis. Hence, eIF-4E overexpression could have overcome a deficiency in transcriptionally active levels of IL-2 regulatory factors in Th2 cells leading to IL-2 enhancer activation. This possibility was supported by demonstrating that transcriptionally active levels of the critical IL-2 transcription factor, nuclear factor of activated T cells (NF-AT), occurred only in Th2 cells overexpressing eIF-4E but not in normal Th2 cells, thus indicating that the inability of Th2 cells to express IL-2 was associated with inadequate levels of at least one transcription factor, NF-AT. Moreover, these results were confirmed by the observation that eIF-4E overexpression augmented NF-AT binding activity in Th2 cells. These data suggest that concentrations of inducible transcription factors are a major component of the regulatory mechanisms dictating IL-2 expression and may be under translational control in Th1/Th2 T cell subsets.

In vitro synthesis of human protein synthesis initiation factor 4 gamma and its localization on 43 and 48 S initiation complexes

The rate of protein synthesis is controlled in a large number of physiological situations at the stage of 48 S initiation complex formation, a phase that involves the recruitment of mRNA to the 40 S ribosomal subunit. This process is mediated by the eukaryotic initiation factor-4 (eIF-4) group of translation initiation factors consisting of eIF-4E, eIF-4A, eIF-4B, and eIF-4 gamma. In order to develop a new tool to study this process, we have produced radiolabeled eIF-4 gamma by in vitro transcription and translation. Despite the fact that eIF-4 gamma is predicted from the cDNA sequence to be 154 kDa, the major synthetic product migrated on SDS-polyacrylamide gel electrophoresis at 205 kDa. Although this is similar to the migration of the fastest polypeptide of authentic eIF-4 gamma (approximately 206 kDa), no products were found to co-migrate with the slowest forms of authentic eIF-4 gamma (210-220 kDa), suggesting that these forms derive from extensive modification of the initial polypeptide. The in vitro product also formed a complex with eIF-4E, as judged by its ability to bind to m7GTP-Sepharose. Sucrose gradient sedimentation studies demonstrated that eIF-4 gamma was present on both 43 and 48 S initiation complexes but not 80 S complexes. This supports a model in which free eIF-4E binds to mRNA followed by binding of the eIF-4E.mRNA complex to a 43 S initiation complex already containing eIF-4 gamma.

Participation of initiation factors in the recruitment of mRNA to ribosomes

The step of protein synthesis which is normally rate limiting, formation of the 48S initiation complex, is catalyzed by the group 4 initiation factors. Collectively they recognize the 7-methylguanosine-containing cap of mRNA, unwind mRNA secondary structure, and allow scanning for the initiation codon by the small ribosomal subunit. The activities of the eIF-4 polypeptides are modulated by phosphorylation. Recent studies shed new light on the mechanism of assembly of the 48S initiation complex and the effect of phosphorylation of one of the eIF-4 polypeptides, the cap-binding protein eIF-4E.

Decreasing the level of translation initiation factor 4E with antisense RNA causes reversal of ras-mediated transformation and tumorigenesis of cloned rat embryo fibroblasts

Transformation of cloned rat embryo fibroblasts (CREF) with the T24-ras oncogene results in loss of contact inhibition, growth in soft agar and tumor formation in nude mice. Previously we showed that in such cells (CREF T24), the phosphorylation rate of protein synthesis initiation factor 4E (eIF-4E) is increased, correlating with an increase in the general rate of protein synthesis. In the present study, we have expressed antisense RNA complementary to eIF-4E mRNA in CREF T24 cells using a stably integrated vector. Cells expressing antisense RNA (CREF T24/AS) contained 30-50% of the normal level of eIF-4E and exhibited many of the properties of untransformed cells. CREF T24 had a spindle-shaped, refractile appearance, whereas CREF T24/AS grew in ordered, parallel patterns and exhibited contact inhibition similar to untransformed CREF. The rates of growth and protein synthesis in CREF T24/AS were decreased compared to CREF T24 but were not as low as in CREF. The efficiency of growth in soft agar was 11-fold lower for CREF T24/AS compared with CREF T24. The latency period for tumor formation in nude mice was increased from 8 days for CREF T24 to 17-27 days for CREF T24/AS and various clonal lines derived from them. Cell lines established from these CREF T24/AS-derived tumors were shown to have partially regained the eIF-4E levels characteristic of CREF T24. These results demonstrate that many of the phenotypic alterations associated with ras-induced malignant transformation can be reversed by a moderate reduction of the translational initiation capacity and therefore may be mediated through a translational mechanism.

Mapping the cleavage site in protein synthesis initiation factor eIF-4 gamma of the 2A proteases from human Coxsackievirus and rhinovirus

The rate-limiting step of eukaryotic protein synthesis is the binding of mRNA to the 40 S ribosomal subunit, a step which is catalyzed by initiation factors of the eIF-4 (eukaryotic initiation factor 4) group: eIF-4A, eIF-4B, eIF-4E, and eIF-4 gamma. Infection of cells with picornaviruses of the rhino- and enterovirus groups causes a shut-off in translation of cellular mRNAs but permits viral RNA translation to proceed. This change in translational specificity is thought to be mediated by proteolytic cleavage of eIF-4 gamma, which is catalyzed, directly or indirectly, by the picornaviral 2A protease. In this report we have used highly purified recombinant 2A protease from either human Coxsackievirus serotype B4 or rhinovirus serotype 2 to cleave eIF-4 gamma in vitro in the eIF-4 complex purified from rabbit reticulocytes. Neither the rate of cleavage nor fragment sizes were affected by addition of eIF-3. The NH2- and COOH-terminal fragments of eIF-4 gamma were separated by reverse phase HPLC and identified with specific antibodies, and the NH2-terminal sequence of the COOH-terminal fragment was determined by automated Edman degradation. The cleavage site for both proteases is 479GRPALSSR decreases GPPRGGPG494 in rabbit eIF-4 gamma, corresponding to 478GRTTLSTR decreases GPPRGGPG493 in human eIF-4 gamma.

Mechanism of action of developmentally regulated sea urchin inhibitor of eIF-4

The developmentally regulated inhibitor of eIF-4 function found in unfertilized sea urchin eggs has been partially purified and its mechanism of action studied in vitro using purified recombinant eIF-4 alpha and cell-free translation systems. The results demonstrate that although the phosphorylation of eIF-4 alpha is necessary to promote protein synthesis, it is not sufficient to maintain all aspects of eIF-4 function. The egg inhibitor does not change eIF-4 alpha phosphorylation state. During the blockage of initiation caused by the egg inhibitor, eIF-4 alpha remains phosphorylated but accumulates in a 48S initiation intermediate. This suggests that the egg inhibitor functions by preventing the release of eIF-4 alpha from the small ribosomal subunit. The characteristics of the inhibitor in a reticulocyte translation system demonstrate that eIF-4 activity is inhibited within 3-6 min. However, the inhibitor's characteristics in a mRNA-dependent translation system contrast with this. Preincubation with the inhibitor for 5-25 min prior to the addition of mRNA does not prevent endogenous eIF-4 from participating in translation but diminishes its ability to be reutilized, consistent with the accumulation of eIF-4 alpha on the small ribosomal subunit. The ribosomal localization of the inhibitor suggests that it could prevent eIF-4 alpha release by direct binding. The gradual inactivation of the inhibitor following fertilization indicates that it represents a component of a novel regulatory cascade that modulates eIF-4 activity.

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

The nucleotide sequences of the cDNAs for the two subunits, p82 and p28, of the isozyme form of wheat germ eukaryotic initiation factor 4F (eIF-(iso)4F) were determined. The cDNA for the p82 subunit encodes a polypeptide of 86,514 Da. The deduced amino acid sequence of p82 contains possible motifs for ATP binding, metal binding, and phosphorylation. The cDNA sequence for the small subunit, p28, which is a m7G cap-binding protein, encodes a polypeptide of 23,524 Da. The deduced amino acid sequence of p28 is similar (approximately 38%) to cap-binding proteins from yeast and mammals. The p28 of wheat eIF-(iso)4F does not contain a serine or threonine in the vicinity of the serine (Ser53) of mammalian cap-binding protein which is phosphorylated and shown to affect activity in mammalian cells.

Amino acid sequence of the human protein synthesis initiation factor eIF-4 gamma

Eukaryotic protein synthesis initiation factor (eIF) 4 gamma, also known as p220, is a component of the protein complex eIF-4, which is involved in the recognition of the mRNA cap, ATP-dependent unwinding of 5'-terminal secondary structure and recruitment of mRNA to the ribosome. Peptide sequence data from rabbit reticulocyte eIF-4 gamma was used to synthesize oligonucleotide probes and polymerase chain reaction primers. These were used to screen lambda-cDNA libraries from rabbit and human brain, yielding a partial rabbit and a complete human cDNA sequence of 5.1 kilobases. Northern blot and primer extension analysis indicated that the cDNA sequence was complete. To confirm that the cDNA represented that of eIF-4 gamma, three peptides were synthesized based on cDNA sequences and used to produce anti-peptide antibodies. The antibodies specifically recognized intact eIF-4 gamma and its cleavage products following poliovirus infection. The eIF-4 gamma mRNA contains AUG codons at nucleotides 6, 67, 90, 165, and 369, but only the last is followed by a long open reading frame. The eIF-4 gamma polypeptide is 154 kDa (1396 amino acid residues) and contains sequence motifs of potential interest: a sequence (AGLGPR) that is similar to the substrate recognition sequence of protease 2A from rhinovirus serotype 14, five PEST regions with scores greater than 10, which are characteristic of rapidly degraded proteins, stretches of polyglutamic acid, and numerous potential phosphorylation sites.

Preferential translation of heat shock mRNAs in HeLa cells deficient in protein synthesis initiation factors eIF-4E and eIF-4 gamma

Expression of antisense RNA against eukaryotic translation initiation factor 4E (eIF-4E) in HeLa cells causes a reduction in the levels of both eIF-4E and eIF-4 gamma (p220) and a concomitant decrease in the rates of both cell growth and protein synthesis (De Benedetti, A., Joshi-Barve, S., Rinker-Schaffer, C., and Rhoads, R. E. (1991) Mol. Cell Biol. 11, 5435-5445). The synthesis of most proteins in the antisense RNA-expressing cells (AS cells) is decreased, but certain proteins continue to be synthesized. In the present study, we identified many of these as stress-inducible or heat shock proteins (HSPs). By mobilities on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by reactivity with monoclonal antibodies generated against human HSPs, four of these were shown to be HSP 90, HSP 70, HSP 65, and HSP 27. The steady-state levels of HSP 90, 70, and 27 were elevated in relation to total protein in AS cells. Pulse labeling and immunoprecipitation indicated that HSP 90 and HSP 70 were synthesized more rapidly in AS cells than in control cells. The accelerated synthesis of HSPs in the AS cells was not due, however, to increased mRNA levels; the levels of HSP 90 and 70 mRNAs either remained the same or decreased after induction of antisense RNA expression. Actin mRNA, a typical cellular mRNA, was found on high polysomes in control cells but shifted to smaller polysomes in AS cells, as expected from the general decrease in translational initiation caused by eIF-4E and eIF-4 gamma depletion. HSP 90 and 70 mRNAs showed the opposite behavior; they were associated with small polysomes in control cells but shifted to higher polysomes in AS cells. These results demonstrate that HSP mRNAs have little or no requirement in vivo for the cap-recognition machinery and suggest that these mRNAs may utilize an alternative, cap-independent mechanism of translational initiation.