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R E Rhoads

Publications and source records attributed to R E Rhoads.

At least 55 records · Page 3Linked to original sources
PubMed1994

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.

Amino Acid Sequence↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Amino Acid Sequence↗
PubMed1993

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.

Animals↗
PubMed1992

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↗
PubMed1992

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.

Amino Acid Sequence↗
PubMed1992

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.

Blotting, Northern↗
PubMed1992

Ras transformation of cloned rat embryo fibroblasts results in increased rates of protein synthesis and phosphorylation of eukaryotic initiation factor 4E.

Eukaryotic initiation factor 4E (eIF-4E) is a 25-kDa phosphoprotein that binds to the 7-methylguanosine cap of mRNA and acts, along with other eIF-4 polypeptides, to unwind mRNA secondary structure at the 5' terminus. Recent studies have indicated that eIF-4E acts as a protooncogene, but only in its phosphorylated state. In order to determine the role of eIF-4E in oncogenesis, we examined its regulation and expression in cloned rat embryo fibroblasts transformed with the Harvey ras (Ha-ras) oncogene. The expression of Ha-ras increased the rate of protein synthesis but did not increase the levels of eIF-4E mRNA or protein. However, a dramatic increase (7-fold) in phosphate incorporation into eIF-4E was observed. The percentage of eIF-4E in the phosphorylated state was the same in transfected and control cells, indicating that both phosphorylation and dephosphorylation of eIF-4E were increased. Phosphopeptide mapping of eIF-4E from transformed cells indicated a single site of phosphorylation at Ser-53, which is the same as that identified previously in eIF-4E from reticulocytes and HeLa cells. These results indicate that p21ras is part of the signal transduction pathway leading to phosphorylation of eIF-4E. These findings also provide a potential mechanism for cell transformation by p21ras which involves the preferential stimulation of translation of certain mRNAs.

Animals↗
PubMed1992

Analysis of the tissue-specific distribution of mRNAs encoding the plasma membrane calcium-pumping ATPases and characterization of an alternately spliced form of PMCA4 at the cDNA and genomic levels.

The plasma membrane Ca(2+)-pumping ATPase (Ca(2+)-ATPase) mRNAs are encoded on four different genes designated PMCA1-PMCA4. The primary transcripts from some of these genes are known to be alternately spliced in the region encoding the regulatory domains of the enzymes. The known alternately spliced forms of these Ca(2+)-ATPase mRNAs and a new spliced variant of PMCA4 (PMCA4b), presented here, represent at least nine different mRNAs encoding the Ca(2+)-ATPases. In this report, the examination of the tissue-specific distribution of these alternately spliced mRNAs using polymerase chain reaction amplification of cDNA coupled with Southern blotting revealed that each spliced variant had a unique tissue distribution. PMCA1b and PMCA4a were present in all tissues examined. PMCA1a, PMCA1b, and PMCA4b were expressed in excitable tissues, whereas PMCA1d was expressed only in muscle tissues. PMCA2 was found in liver, adrenal gland, spinal cord, and brain. PMCA3a was present in spinal cord, and PMCA3b in thymus, adrenal gland, spinal cord, and brain. The mRNA for a new spliced variant of PMCA4 (PMCA4b) was detected in this study. Complementary DNAs for this isoform were isolated and characterized from human and bovine brain. This alternately spliced form of the PMCA4 mRNA contained an exon inserted at the splice junction immediately following the sequence encoding the calmodulin-binding domain. As has also been shown for PMCA1a, this insertion produced a shift in the reading frame at the 3'-end of the PMCA4 mRNA that yielded a sequence encoding a Ca(2+)-ATPase lacking a large portion of the C-terminal regulatory domain. When the human PMCA4 gene spanning this region of variable exon splicing was sequenced, it confirmed the intron-exon boundaries where alternate splicing occurs to produce PMCA4a and PMCA4b.

Amino Acid Sequence↗
PubMed1992

Binding of protein synthesis initiation factor 4E to oligoribonucleotides: effects of cap accessibility and secondary structure.

The binding of rabbit globin mRNA to the 25-kDa cap binding protein eIF-4E from human erythrocytes was found to be 5.3-fold stronger than the binding of the cap analogue m7GpppG to eIF-4E [Gross et al. (1990) Biochemistry 29, 5008-5012]. In order to investigate whether this effect is due to the longer sequence of nucleotides in globin mRNA or to other features such as cap accessibility or secondary structure, oligoribonucleotide analogues of rabbit alpha-globin mRNA were synthesized by T7 RNA polymerase from a synthetic oligodeoxynucleotide template in the presence of m7GpppG; these oligoribonucleotide analogues possess varying degrees of cap accessibility and secondary structure. Equilibrium association constants for the interaction of these oligoribonucleotides and purified human erythrocyte eIF-4E were obtained from direct fluorescence titration experiments. The data indicate that while the presence of the m7G cap is required for efficient recognition by eIF-4E, the cap need not be completely sterically accessible, since other structural features within the mRNA also influence binding.

Animals↗
PubMed1991

A novel BK virus-based episomal vector for expression of foreign genes in mammalian cells.

A composite mammalian cell-E. coli shuttle vector was developed based on the human papova virus BK and pSV-neo. The vector contains a dioxin-responsive enhancer (DRE) controlling a mouse mammary tumor virus (MMTV) promoter for the inducible expression of inserted genes. In human cells the vector replicates episomally, presumably utilizing the BKV rather than the SV40 origin, and expresses the BK T/t antigens. A deletion in the late BK region precludes the expression of the core/capsid proteins VP1, VP2, and VP3, thereby preventing the infectious lytic cycle. HeLa cells which were transfected with this vector and selected for resistance to the antibiotic G418 maintained the construct primarily in episomal form during more than one year of continuous culture, with little or no integration into the host genome. Transformed cells cultured in higher concentrations of G418 contained higher copy numbers of the vector. This permits one to vary the dosage of an inserted gene easily and reversibly without the need of conventional amplification techniques and clonal analysis. Using a chloramphenicol acetyl transferase (CAT) reporter gene inserted downstream of the MMTV promoter, we found that CAT expression was greater in clones with higher vector copy number. CAT expression was inducible with 2,3,7,8-tetrachlorodibenzo-p-dioxin, but inducibility was found to be inversely proportional to the copy number. Transformation of bacteria with plasmid molecules retrieved from the mammalian host was efficient, making this vector well adapted for the screening of cDNA libraries for the ability to express a phenotype in mammalian cells. Moreover, DNA sequences were stable during long-term passage in mammalian cells; vector passaged continuously for more than one year retained fully functional bacterial genes for resistance to chloramphenicol and ampicillin.

BK Virus↗
PubMed1991

Insulin induction of ornithine decarboxylase. Importance of mRNA secondary structure and phosphorylation of eucaryotic initiation factors eIF-4B and eIF-4E.

We investigated the possibility that insulin could stimulate translation of ornithine decarboxylase (ODC) mRNA in a murine fibroblast cell line that expresses large numbers of human insulin receptors (HIR 3.5 cells). Within 3 h after exposure to 70 nM insulin, ODC enzyme activity increased approximately 50-fold and mRNA accumulation 3-fold in the HIR 3.5 cells but not in normal fibroblasts. Pretreatment of cells with cycloheximide completely inhibited insulin-stimulated ODC expression; actinomycin D partially inhibited this effect. To determine the influence of the 5' untranslated region (5'UTR) of ODC mRNA on insulin-regulated ODC expression, plasmids were constructed which contained sequences from the 5'UTR of a rat ODC mRNA interposed between the ferritin promoter and the coding region of the human growth hormone gene. These constructions were then expressed transiently in HIR 3.5 cells. Insulin stimulated a 2-4-fold change in growth hormone accumulation in the medium of cells transiently expressing plasmids containing the entire 5'UTR of ODC mRNA or just the 5'-most 115 bases, a G/C-rich conserved sequence predicted to form a stem-loop structure and shown previously to be responsible for constitutive inhibition of translation. There was a direct correlation between the extent of insulin stimulation and the predicted secondary structure of the added 5'UTR sequences. To determine whether this effect might be due to insulin activation of initiation factors responsible for melting mRNA secondary structure, we examined the effect of insulin on the phosphorylation states of two such factors, eucaryotic initiation factors eIF-4B and eIF-4E. Insulin stimulated the phosphorylation of both initiation factors; this stimulation was evident at 15 min and maximal by 60 min. These results suggest a potential general mechanism by which insulin could preferentially stimulate translation of mRNAs whose 5'UTRs exhibit significant secondary structure by activating initiation factors involved in melting such secondary structures.

Animals↗
PubMed1991

In vitro cleavage at or near the N-terminus of the helper component protein in the tobacco vein mottling virus polyprotein.

Translation of tobacco vein mottling virus (TVMV) RNA in a wheat germ system resulted in two products that were not observed in a rabbit reticulocyte system. One of these was the N-terminal protein, based on its being the most abundant product and its migration on SDS-PAGE at about 34 kDa. The second product was similar or identical to helper component (HC) isolated from TVMV-infected plants, based on co-migration with HC on SDS-PAGE and immunoprecipitation with anti-HC antibodies. The N-terminus of this product was determined by radiochemical Edman degradation to be Ser-257 of the polyprotein. This assignment was supported by peptide mapping with a tryptophan-specific reagent. A similar cleavage was observed when tobacco etch virus was translated in a wheat germ system. Comparison with homologous regions in five other potyviruses indicated conservation of amino acid residues on both sides of the proposed cleavage site. Conversion of Phe-256 to Met, Pro, Arg, His, or Trp by site-directed mutagenesis of a TVMV RNA transcription template inhibited cleavage in the wheat germ system. These results suggest that in vitro cleavage occurs between Phe-256 and Ser-257 and that this cleavage is the same as the in vivo cleavage which liberates the N-terminus of HC.

Amino Acid Sequence↗
PubMed1991

Protein synthesis, cell growth and oncogenesis.

Two lines of investigation support a new hypothesis concerning the role of protein synthesis in the mitogenic pathway. The first is that a variety of mitogens and oncogene products increase phosphorylation and thereby activate eIF-4E, which is involved in the rate-limiting transfer of mRNA to ribosomes. The second is that overexpression or microinjection of eIF-4E induce rapid cell proliferation and oncogenic transformation.

Animals↗