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• W2012843513 abstract "Cap-binding proteins specifically bind to the 7-methyl guanosine (m7G) functional group at the 5′ end of eukaryotic mRNAs. A novel Arabidopsis thalianaprotein has been identified that has sequence similarity to cap-binding proteins but is clearly a different form of the protein. The most obvious primary sequence difference is the substitution of two of the eight conserved tryptophan residues with other aromatic amino acids in the novel protein. Analogous forms of this novel protein appear to be present in other higher eukaryotes but not in yeast. Analysis of the native and recombinant forms of the novel protein by retention on m7GTP-Sepharose indicate that it is a functional cap-binding protein. Measurements of the dissociation constant for this protein indicate that it binds m7GTP 5–20-fold tighter than eukaryotic initiation factor (eIF)(iso)4E. The novel protein also supports the initiation of translation of capped mRNA in vitro. Biochemical analysis and yeast two-hybrid data indicate that it interacts with eIF(iso)4G to form a complex. Based on these observations, this protein appears to be able to function as a cap-binding protein and is given the designation of novel cap-binding protein (nCBP). Cap-binding proteins specifically bind to the 7-methyl guanosine (m7G) functional group at the 5′ end of eukaryotic mRNAs. A novel Arabidopsis thalianaprotein has been identified that has sequence similarity to cap-binding proteins but is clearly a different form of the protein. The most obvious primary sequence difference is the substitution of two of the eight conserved tryptophan residues with other aromatic amino acids in the novel protein. Analogous forms of this novel protein appear to be present in other higher eukaryotes but not in yeast. Analysis of the native and recombinant forms of the novel protein by retention on m7GTP-Sepharose indicate that it is a functional cap-binding protein. Measurements of the dissociation constant for this protein indicate that it binds m7GTP 5–20-fold tighter than eukaryotic initiation factor (eIF)(iso)4E. The novel protein also supports the initiation of translation of capped mRNA in vitro. Biochemical analysis and yeast two-hybrid data indicate that it interacts with eIF(iso)4G to form a complex. Based on these observations, this protein appears to be able to function as a cap-binding protein and is given the designation of novel cap-binding protein (nCBP). Cap-binding proteins (eIF4E) 1The abbreviations used are: eIF, eukaryotic initiation factor; DTT, dithiothreitol; EST, expressed sequence tag; IPTG, isopropylβ-d-thiogalactopyranoside; m7G, 7-methyl guanosine; PAGE, polyacrylamide gel electrophoresis; X-gal, 5-bromo-4-chloro-3-indolyl-β-dgalactopyranoside. 1The abbreviations used are: eIF, eukaryotic initiation factor; DTT, dithiothreitol; EST, expressed sequence tag; IPTG, isopropylβ-d-thiogalactopyranoside; m7G, 7-methyl guanosine; PAGE, polyacrylamide gel electrophoresis; X-gal, 5-bromo-4-chloro-3-indolyl-β-dgalactopyranoside. specifically recognize and bind the m7G functional group found at the 5′ end of most eukaryotic cellular mRNAs (for recent reviews, see Refs. 1Pain V.M. Eur. J. Biochem. 1996; 236: 747-771Crossref PubMed Scopus (636) Google Scholar and 2Merrick W.C. Hershey J.W.B. Hershey J.W.B. Mathews M.B. Sonenberg N. Translational Control. Cold Spring Harbor Laboratory Press, Plainview, NY1996: 31-70Google Scholar). The binding of eIF4E is thought to be the first step in the assembly of several initiation factors on the mRNA prior to the binding of the 40 S ribosome (1Pain V.M. Eur. J. Biochem. 1996; 236: 747-771Crossref PubMed Scopus (636) Google Scholar, 2Merrick W.C. Hershey J.W.B. Hershey J.W.B. Mathews M.B. Sonenberg N. Translational Control. Cold Spring Harbor Laboratory Press, Plainview, NY1996: 31-70Google Scholar, 3Sachs A.B. Sarnow P. Hentze M.W. Cell. 1997; 89: 831-838Abstract Full Text Full Text PDF PubMed Scopus (587) Google Scholar). One of the structural features of all eIF4E proteins is the presence of eight tryptophan residues in the same relative positions. Recently, the cocrystal structures of the mouse eIF4E (4Marcotrigiano J. Gingras A.C. Sonenberg N. Burley S.K. Cell. 1997; 89: 951-961Abstract Full Text Full Text PDF PubMed Scopus (557) Google Scholar) and yeast eIF4E (5Matsuo H. Li H.J. McGuire A.M. Fletcher C.M. Gingras A.C. Sonenberg N. Wagner G. Nat. Struct. Biol. 1997; 4: 717-724Crossref PubMed Scopus (327) Google Scholar) with m7GDP were solved. Tryptophan residues 3, 5, and 8 were found to be involved in binding the m7G functional group (4Marcotrigiano J. Gingras A.C. Sonenberg N. Burley S.K. Cell. 1997; 89: 951-961Abstract Full Text Full Text PDF PubMed Scopus (557) Google Scholar, 5Matsuo H. Li H.J. McGuire A.M. Fletcher C.M. Gingras A.C. Sonenberg N. Wagner G. Nat. Struct. Biol. 1997; 4: 717-724Crossref PubMed Scopus (327) Google Scholar). eIF4E is usually associated with a larger subunit (eIF4G) in a complex named eIF4F (1Pain V.M. Eur. J. Biochem. 1996; 236: 747-771Crossref PubMed Scopus (636) Google Scholar,6Morley S.J. Cusack S. Pain V.M. RNA. 1997; 3: 1085-1104PubMed Google Scholar). eIF4A may also be present in the eIF4E·eIF4G complex depending upon the method of purification (7Etchison D. Milburn S. Mol. Cell. Biol. 1987; 76: 15-25Google Scholar, 8Webster C. Gaut R.L. Browning K.S. Ravel J.M. Roberts J.K.M. J. Biol. Chem. 1991; 266: 23341-23346Abstract Full Text PDF PubMed Google Scholar). Higher plants possess a unique second form of eIF4F designated eIF(iso)4F (9Browning K.S. Plant Mol. Biol. 1996; 32: 107-144Crossref PubMed Scopus (186) Google Scholar). eIF(iso)4F contains distinct forms of the cap-binding protein (eIF(iso)4E) and the larger subunit (eIF(iso)4G) (10Browning K.S. Webster C. Roberts J.K.M. Ravel J.M. J. Biol. Chem. 1992; 267: 10096-10100Abstract Full Text PDF PubMed Google Scholar). The two isoenzyme forms of plant eIF4F have the same activities in vitro (9Browning K.S. Plant Mol. Biol. 1996; 32: 107-144Crossref PubMed Scopus (186) Google Scholar); however, the eIF(iso)4E prefers hypermethylated caps and mRNAs with less secondary structure (11Carberry S.E. Darzynkiewicz E. Goss D.J. Biochemistry. 1991; 30: 1624-1627Crossref PubMed Scopus (52) Google Scholar, 12Carberry S.E. Goss D.J. Biochemistry. 1991; 30: 4542-4545Crossref PubMed Scopus (33) Google Scholar). Recently, a second form of mammalian eIF4G was reported (13Gradi A. Imataka H. Svitkin Y.V. Rom E. Raught B. Morino S. Sonenberg N. Mol. Cell. Biol. 1998; 18: 334-342Crossref PubMed Scopus (247) Google Scholar). However, this form of eIF4G was not reported to have a distinct form of eIF4E associated with it and does not appear to be the functional equivalent of eIF(iso)4G. The mammalian eIF4E is known to be phosphorylated at Ser-209, and the phosphorylation state appears to correlate with activity of the protein (reviewed in Refs. 14Sonenberg N. Hershey J.W.B. Mathews M.B. Sonenberg N. Translational Control. Cold Spring Harbor Laboratory Press, Plainview, NY1996: 245-270Google Scholar and 15Rhoads R.E. Joshi-Barve S. Rinker-Schaeffer C. Prog. Nucleic Acid Res. 1993; 46: 183-219Crossref PubMed Scopus (73) Google Scholar). Certain stimuli, including insulin and several growth factors, induce phosphorylation of eIF4E (14Sonenberg N. Hershey J.W.B. Mathews M.B. Sonenberg N. Translational Control. Cold Spring Harbor Laboratory Press, Plainview, NY1996: 245-270Google Scholar). Overexpression of the mammalian eIF4E results in cell transformation, suggesting that eIF4E levels play a critical role in normal growth and/or development (16De Benedetti A. Rhoads R.E. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 8212-8216Crossref PubMed Scopus (255) Google Scholar, 17Lazaris-Karatzas A. Montine K.S. Sonenberg N. Nature. 1990; 345: 544-547Crossref PubMed Scopus (801) Google Scholar). The recent discovery of proteins that specifically bind mammalian eIF4E and sequester it from interacting with eIF4G have linked the insulin signaling pathway directly to translation (reviewed in Refs. 14Sonenberg N. Hershey J.W.B. Mathews M.B. Sonenberg N. Translational Control. Cold Spring Harbor Laboratory Press, Plainview, NY1996: 245-270Google Scholar and 18Lawrence Jr., J.C. Abraham R.T. Trends Biochem. Sci. 1997; 22: 345-349Abstract Full Text PDF PubMed Scopus (186) Google Scholar). These eIF4E binding proteins (4E-BP) are a major target for phosphorylation following insulin or growth factor treatment, and the phosphorylated form of 4E-BP no longer binds eIF4E (18Lawrence Jr., J.C. Abraham R.T. Trends Biochem. Sci. 1997; 22: 345-349Abstract Full Text PDF PubMed Scopus (186) Google Scholar). Thus the availability and phosphorylation state of eIF4E are crucial regulatory mechanisms for translational control in mammals. It is not known if similar types of regulation by phosphorylation or sequestration occur in plants. In this report, we have identified a novel cap-binding protein (nCBP) from Arabidopsis thaliana that is distinct in both amino acid sequence and m7GTP binding properties from eIF4E or eIF(iso)4E and appears to be present only in higher eukaryotes. The cDNA (19Newman T. De Bruijn F.J. Green P. Keegstra K. Kende H. McIntosh L. Ohlrogge J. Raikhel N. Somerville S. Thomashow M. Retzel E. Somerville C. Plant Physiol. 1994; 106: 1241-1255Crossref PubMed Scopus (559) Google Scholar) encoding the expressed sequence tag (EST) for the nCBP (77G6T7) was obtained from the Arabidopsis Biological Resource Center (ABRC, Ohio State University). DNA sequencing of the cDNA (both strands) was performed at the DNA sequencing facility of the Institute for Cellular and Molecular Biology (University of Texas at Austin). Expression vector pET15b and Escherichia coli DE3(HMS174) were obtained from Novagen (Madison, WI). Restriction enzymes and DNA modifying enzymes were obtained from Life Technologies Inc. IPTG and X-gal were from Ambion (Austin, TX) or Research Products International (Mt. Prospect, IL). The A. thaliana suspension culture was generously provided by A. N. S. Reddy (Colorado State University). Preparation of the Arabidopsis extracts and chromatography on m7GTP-Sepharose (Pharmacia Biotech Inc.) will be described elsewhere but were similar to the preparation and fractionation of wheat germ extracts (20Lax S.R. Lauer S.J. Browning K.S. Ravel J.M. Methods Enzymol. 1986; 118: 109-128Crossref PubMed Scopus (91) Google Scholar). Wheat eIF(iso)4E and eIF(iso)4G were prepared as described previously (21Van Heerden A. Browning K.S. J. Biol. Chem. 1994; 269: 17454-17457Abstract Full Text PDF PubMed Google Scholar). Protein determinations were by the method of Bradford (22Bradford M.M. Anal. Biochem. 1976; 72: 248-254Crossref PubMed Scopus (214455) Google Scholar). SDS-PAGE (12.5 or 20% acrylamide) and Western blots were carried out as described previously (23Browning K.S. Humphreys J. Hobbs W. Smith G.B. Ravel J.M. J. Biol. Chem. 1990; 265: 17967-17973Abstract Full Text PDF PubMed Google Scholar). The silver stain kit was from Novex (San Diego, CA). Antibodies to recombinant nCBP were raised in rabbits at the University of Texas M. D. Anderson Cancer Center Veterinary Services (Bastrop, TX). The Matchmaker Two-hybrid kit was obtained fromCLONTECH. The coding region for Arabidopsis nCBP was amplified with the appropriate primers containing XhoI and BamHI sites. The amplified DNA was cloned into pET15b restricted with XhoI and BamHI and in frame with the 6-histidine tag. The plasmid construct was confirmed by DNA sequencing. The plasmid containing the nCBP coding region was transformed into E. coli DE3(HMS174). Expression of the recombinant protein was carried out as described for eIF(iso)4E, except the incubation temperature of the culture after induction was at 25 °C (21Van Heerden A. Browning K.S. J. Biol. Chem. 1994; 269: 17454-17457Abstract Full Text PDF PubMed Google Scholar). The cell pellet from a 1.5-liter culture was resuspended in 15 ml of buffer C (20 mmHepes-KOH, pH 7.6, 10% glycerol) containing 0.4 m KCl. The cells were sonicated and centrifuged for 30 min at 30,000 rpm in a Ti-60 rotor, and the supernatant was applied to a Ni-NTA column (Qiagen, Valencia, CA) equilibrated with the same buffer. The column was washed with buffer C containing 0.1 m KCl and 40 mm imidazole and then eluted with buffer C containing 0.1 mm KCl and 400 mm imidazole. The fractions containing the highest amount of protein were pooled and dialyzed against buffer B (20 mm Hepes-KOH, pH 7.6, 10% glycerol, 1 mm DTT, 0.1 mm EDTA) containing 0.1m KCl. nCBP prepared by Ni-NTA chromatography was used to raise rabbit antibodies. Alternatively for purification on m7GTP-Sepharose, the cell pellet was resuspended in 10 ml of 20 mm Hepes-KOH, pH 7.6, 20% glycerol, 1 mm DTT, 0.1 mm EDTA containing 0.4 m KCl. The cells were sonicated and treated as described above, and the supernatant was applied to a 2-ml m7GTP-Sepharose column (capacity ∼1.5 mg). The column was washed with 20 mm Hepes-KOH, pH 7.6, 20% glycerol, 1 mm DTT, 0.1 mm EDTA, 0.1 m KCl. The nCBP was eluted with the same buffer solution containing 400 μm m7GTP. The fractions containing the highest amount of protein were pooled and dialyzed against 20 mm Hepes-KOH, pH 7.6, 20% glycerol, 1 mm DTT, 0.1 mm EDTA, 0.1 m KCl. The activity of nCBP was measured in a wheat germ translation system (100 μl) dependent upon the addition of eIF(iso)4G and eIF(iso)4E (21Van Heerden A. Browning K.S. J. Biol. Chem. 1994; 269: 17454-17457Abstract Full Text PDF PubMed Google Scholar). The indicated amounts of either nCBP or eIF(iso)4E were added to the reaction mixture containing 20 pmol of eIF(iso)4G. The amount of [14C]leucine incorporated was determined as described previously (20Lax S.R. Lauer S.J. Browning K.S. Ravel J.M. Methods Enzymol. 1986; 118: 109-128Crossref PubMed Scopus (91) Google Scholar). Capped β-globin mRNA was prepared as described (24Fletcher L. Corbin S.D. Browning K.S. Ravel J.M. J. Biol. Chem. 1990; 265: 19582-19587Abstract Full Text PDF PubMed Google Scholar). Fluorescence measurements were carried out at 23 °C in 20 mm HEPES, pH 7.6, 1 mm MgCl2. The concentration of protein was 0.25 μm for both nCBP and eIF(iso)4F. The data were collected and analyzed as described previously (25Sha M. Wang Y. Xiang T. van Heerden A. Browning K.S. Goss D.J. J. Biol. Chem. 1995; 270: 29904-29909Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar). Excitation was at 280 nm and emission monitored at 330 nm. Background fluorescence was subtracted. A mixture containing 130 μg of recombinant nCBP and 130 μg of recombinant wheat eIF(iso)4G was applied to a 0.5 ml column of m7GTP-Sepharose equilibrated in buffer B containing 0.1 m KCl, and the flow-through fractions (400 μl) were collected. The matrix was washed with an additional 5 ml of buffer. The retained protein was eluted with buffer B containing 0.3 m KCl and 400 μmm7GTP, and fractions (400 μl) were collected. The flow-through and eluate fractions were analyzed by SDS-PAGE. The coding regions of nCBP and Arabidopsis eIF(iso)4E were placed into the binding domain plasmid, pGBT9 (26Metz A.M. Browning K.S. J. Biol. Chem. 1996; 271: 31033-31036Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar), after amplification of the coding region with the appropriate primers. The constructs were confirmed by DNA sequencing. The wheat eIF(iso)4E in pGBT9/N and wheat eIF(iso)4G in pGAD424 were prepared as described previously (26Metz A.M. Browning K.S. J. Biol. Chem. 1996; 271: 31033-31036Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar). The β-galactosidase color assay with X-gal was performed according to the protocol of the manufacturer. Analysis of the EST data base for A. thalianacap-binding proteins, eIF4E and eIF(iso)4E, revealed a sequence that appeared to be a novel form of eIF4E. This novel cap-binding protein, termed nCBP, differed significantly in sequence from other known eIF4E or eIF(iso)4E sequences. One of the obvious differences was the substitution of two of the conserved tryptophan residues (Trp-1 and -3) with other aromatic residues (Fig. 1 A). Further inspection of the EST data base revealed many mouse and human ESTs that were more similar to the Arabidopsis nCBP than to mammalian eIF4E. The number of independent mouse and human nCBP ESTs that appear in the EST data base suggest that the mammalian nCBP mRNAs are highly expressed. This prediction was confirmed by Northern blot analysis of RNA from various mouse tissues that showed high levels of nCBP mRNA expression in all tissues. 2J. Schaunessy, University of Arkansas Medical Center, personal communication. Temeles et al. (27Temeles G.L. Ram P.T. Rothstein J.L. Schutz R.M. Mol. Reprod. Dev. 1994; 37: 121-129Crossref PubMed Scopus (139) Google Scholar) reported several mouse cDNA sequences for transcripts that were highly expressed in preimplantation embryonic mouse tissue, but the protein products were not all identified. One of these unidentified cDNAs (GenBank™ DataBank accession number U01137) appears to encode the mouse nCBP protein (Fig. 1 B). Sequence comparison of the nCBP from Arabidopsis and mouse shows that these two proteins are more similar to each other (note Trp-1 and -3 are altered also in mouse nCBP) than to their cognate normal cap-binding proteins (compare Fig. 1, panels A and B). Two of the non-conservative amino acid changes in the Arabidopsis nCBP are also present in the mouse nCBP, e.g. L136→E and T171→N (numbering for Arabidopsis nCBP). These two amino acids are absolutely conserved among eIF4E from plants, yeast, Drosophila, Xenopus, and mammals, suggesting that the mouse and Arabidopsis nCBP are more related to each other than to their cognate eIF4E. Three of the conserved tryptophans (Trp-3, -5, and -8 in Fig. 1 A) were recently shown to be involved in the binding of m7GDP by solving the structures of mouse eIF4E and yeast eIF4E with bound m7GDP (4Marcotrigiano J. Gingras A.C. Sonenberg N. Burley S.K. Cell. 1997; 89: 951-961Abstract Full Text Full Text PDF PubMed Scopus (557) Google Scholar, 5Matsuo H. Li H.J. McGuire A.M. Fletcher C.M. Gingras A.C. Sonenberg N. Wagner G. Nat. Struct. Biol. 1997; 4: 717-724Crossref PubMed Scopus (327) Google Scholar). Mutagenesis studies with yeast (28Altmann M. Edery I. Trachsel H. Sonenberg N. J. Biol. Chem. 1988; 263: 17229-17232Abstract Full Text PDF PubMed Google Scholar) and human (29Morino S. Hazama H. Ozaki M. Teraoka Y. Shibata S. Doi M. Ueda H. Ishida T. Uesugi S. Eur. J. Biochem. 1996; 239: 597-601Crossref PubMed Scopus (51) Google Scholar) eIF4E indicated that mutation of the conserved Trp residues (with the exception of Trp 4 in yeast) either eliminated or reduced the ability of eIF4E to bind m7G affinity columns. It was, therefore, of great interest to determine if the nCBP, which is lacking Trp-1 and -3, is able to bind m7G functional groups. An E. coli extract containing the expressed recombinant nCBP was applied to a m7GTP-Sepharose column to test its ability to interact with m7G functional groups. The m7GTP-Sepharose column (2.0 ml) has an estimated capacity of 1.5 mg. We find that fewer nonspecific proteins are retained on the column when the capacity of the column is slightly exceeded. Consequently, only 70% of the soluble recombinant nCBP remaining after ultracentrifugation was retained on the column as judged by densitometry (Fig. 2, lanes 1and 2). The nCBP was eluted with m7GTP (Fig. 2,lanes 3–8) and approximately 1.2 mg of protein was recovered. It was observed that the nCBP appeared to elute less well from the m7GTP-Sepharose column than either eIF(iso)4E or eIF4E. Increasing the concentration of m7GTP from 100 μm to 400 μm appeared to enhance the elution and the recovery. This observation suggested that the nCBP may have a more avid interaction with m7G functional groups. Initial measurements using fluorescence spectroscopy indicate at least a 5–20-fold difference in the binding affinity of nCBP and eIF(iso)4E for m7GTP (Fig. 3). The high affinity of m7GTP for nCBP precludes accurate determination of the binding constant by fluorescence due to limiting signal at low protein concentrations. Nonetheless, the evident difference in binding affinities provides an explanation for the difficulty in elution of nCBP from m7GTP-Sepharose. Further analysis with other cap analogs and capped oligonucleotides will be essential to elucidate the basis for the differences in observed binding constants. To obtain an estimate of the relative amount of nCBP present in Arabidopsis, an extract was prepared from Arabidopsis suspension cultures and applied to a m7GTP-Sepharose column. Fig. 4 A shows a silver-stained gel of the m7GTP eluate (lane 1) and purified recombinant nCBP (lanes 2 and 3). The Arabidopsis eIF4E and eIF(iso)4E in the eluate were identified by Western blot analysis using mouse antiserum raised to recombinant wheat eIF(iso)4E (Fig. 4 B, lane 1) and by mass spectrometry identification of peptides (data not shown). The amount of nCBP present in the m7GTP eluate (Fig. 4 A, lane 1) appears to be at least 10-fold lower than that of either eIF4E or eIF(iso)4E, suggesting that this protein is present in very low amounts in Arabidopsis. nCBP in the Arabidopsis m7GTP eluate was identified with antiserum to recombinant nCBP (Fig. 4 B, lane 2). Antibody raised to recombinant wheat eIF(iso)4E cross-reacts with both Arabidopsis eIF4E and eIF(iso)4E (Fig. 4 B,lane 1), indicating that these proteins are closely related. However, note that there is no cross-reaction between the nCBP and other Arabidopsis cap-binding proteins with either the antiserum to nCBP or the antiserum to wheat eIF(iso)4F (Fig. 4 B, lanes 1 and 2). The lack of cross-reaction between nCBP and eIF4E or eIF(iso)4E, and the amino acid sequence data suggest that nCBP is a more distantly related form of a cap-binding protein. To further test the interaction of the nCBP with m7G functional groups, the ability of nCBP to translate a capped mRNA was determined. As shown in Fig. 5 A, the nCBP is able to substitute for wheat eIF(iso)4E, in the presence of wheat eIF(iso)4G. The nCBP is able to support initiation of translation at about 30% of the level of eIF(iso)4E. This result implies that nCBP interacts with a capped mRNA but also forms a functional complex with eIF(iso)4G. We have also obtained data that show nCBP is also able to interact with eIF4G (data not shown). It is not clear at this time whether the lower protein synthesis activity is an innate characteristic of nCBP itself or is due to some other unknown reason (e.g. not having the correct binding partner). The inhibition of translation initiation by m7GTP in the presence of eIF(iso)4E or nCBP was also determined. As shown in Fig. 5 B, the nCBP appears to be much less sensitive to the presence of m7GTP in the translation assay than eIF(iso)4E. At least 5–6-fold more m7GTP was required to obtain 50% inhibition. The order of addition of the m7GTP did not appear to have any affect on the level of inhibition. 3K. A. Ruud, S. R. Lax, and K. S. Browning, unpublished data. This result implies that the interaction of capped β-globin mRNA is much stronger with the nCBP than with eIF(iso)4E and therefore requires higher amounts of m7GTP to dissociate the complex. Additional fluorescence binding experiments with capped RNAs will be necessary to determine the dissociation constants relative to that of m7GTP. The ability of nCBP to support polypeptide synthesis in the presence of eIF(iso)4G implies the ability to form a complex. If the two proteins form a stable complex, the complex will be retained on m7GTP-Sepharose. We have shown previously (21Van Heerden A. Browning K.S. J. Biol. Chem. 1994; 269: 17454-17457Abstract Full Text PDF PubMed Google Scholar) that eIF(iso)4G is not retained on m7GTP-Sepharose except in the presence of eIF(iso)4E. Recombinant wheat eIF(iso)4G and recombinant nCBP were incubated together and applied to a m7GTP-Sepharose column. Elution of the column with m7GTP shows that both the nCBP and eIF(iso)4G were bound (Fig. 6). These results confirm that the recombinant nCBP interacts with eIF(iso)4G to form a functional complex. Further evidence of the interaction of the nCBP with eIF(iso)4G was obtained in the yeast two-hybrid system. We have previously shown that wheat eIF(iso)4G and eIF(iso)4E interact strongly in the yeast two-hybrid system (26Metz A.M. Browning K.S. J. Biol. Chem. 1996; 271: 31033-31036Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar). The nCBP and Arabidopsis eIF(iso)4E were placed into a binding domain vector (pGBT9/N) and co-transformed into yeast with wheat eIF(iso)4G in an activation domain vector (pGAD424). Specific interaction with the nCBP and wheat eIF(iso)4G was observed (TableI) as well as interaction of the control wheat or Arabidopsis eIF(iso)4E with wheat eIF(iso)4G. No interaction of nCBP with eIF4A or eIF4B was observed in this system (data not shown). These results further confirm the potential for nCBP to specifically interact with eIF(iso)4G or a similar protein.Table ISummary of yeast two-hybrid protein interaction assayActivation domain, pGADBinding domain, pGBTInteraction1-aScored by the intensity of blue color formation.Wheat eIF(iso)4GpGBT (no insert)−Wheat eIF(iso)4GArabidopsis nCBP++++Wheat eIF(iso)4GArabidopsis eIF(iso)4E++++Wheat eIF(iso)4GWheat eIF(iso)4E++++pGAD (no insert)Arabidopsis nCBP−pGAD (no insert)Arabidopsis eIF(iso)4E−pGAD (no insert)Wheat eIF(iso)4E−1-a Scored by the intensity of blue color formation. Open table in a new tab We have identified a new and novel protein that functions as a cap-binding protein and can interact with eIF(iso)4G to form a complex that supports the protein synthesis initiation of a capped mRNA. This protein appears to be ubiquitous in higher eukaryotes. ESTs for the nCBP are present for maize, Brassica napus, mouse, and human; also, a Caenorhabditis elegans predicted gene product appears to be very similar to the nCBP. Interestingly, there does not appear to be a homolog of this protein in yeast. Careful inspection of the yeast genome did not reveal a hidden gene that is similar to the nCBP. Therefore nCBP may be necessary for a function specific to multicellular organisms (i.e. differentiation). The mouse nCBP cDNA was among a group of cDNAs found in a screen for mRNAs that were highly expressed in preimplantation embryos (27Temeles G.L. Ram P.T. Rothstein J.L. Schutz R.M. Mol. Reprod. Dev. 1994; 37: 121-129Crossref PubMed Scopus (139) Google Scholar), a tissue undergoing rapid differentiation. The precise biological role of the nCBP is not obvious. The higher binding affinity of nCBP for m7GTP would imply a discriminatory role for the nCBP protein during initiation of translation. However, the lower activity of nCBP in translation suggests that a possible function may be to sequester mRNAs and either slow down or prevent their translation. We have been unable to demonstrate any competition in vitro of the nCBP with eIF4E or eIF(iso)4E (data not shown). However, we have preliminary evidence that the translation of some mRNAs is not supported by the nCBP (data not shown), suggesting that discrimination and/or sequestering may be the functional role of nCBP. We are currently determining the expression levels of nCBP mRNA and protein in various Arabidopsis tissues as well as identifying additional proteins that interact with nCBP. This additional data may provide more information about the function of this novel protein in the initiation of translation or regulation of gene expression in higher eukaryotes. We thank Sandra Lax for technical assistance and Karen Wong for providing the Arabidopsis extracts. Mass spectrometry peptide mapping of the Arabidopsis eIF4E and eIF(iso)4E was performed by Dr. John Leszyk (Worchester Institute, Worchester, MA)." @default.
• W2012843513 created "2016-06-24" @default.
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• W2012843513 creator A5032626074 @default.
• W2012843513 creator A5070388182 @default.
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• W2012843513 date "1998-04-01" @default.
• W2012843513 modified "2023-10-16" @default.
• W2012843513 title "Identification and Characterization of a Novel Cap-binding Protein from Arabidopsis thaliana" @default.
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