Roles of methionine transfer RNA's in protein synthesis in rabbit reticulocytes

Mutant methionyl-tRNA synthetase from bacteria enables site-selective N-terminal labeling of proteins expressed in mammalian cells

Newly synthesized cellular proteins can be tagged with a variety of metabolic labels that distinguish them from preexisting proteins and allow them to be identified and tracked. Many such labels are incorporated into proteins via the endogenous cellular machinery and can be used in numerous cell types and organisms. Though broad applicability has advantages, we aimed to develop a strategy to restrict protein labeling to specified mammalian cells that express a transgene. Here we report that heterologous expression of a mutant methionyl-tRNA synthetase from Escherichia coli permits incorporation of azidonorleucine (Anl) into proteins made in mammalian (HEK293) cells. Anl is incorporated site-selectively at N-terminal positions (in competition with initiator methionines) and is not found at internal sites. Site selectivity is enabled by the fact that the bacterial synthetase aminoacylates mammalian initiator tRNA, but not elongator tRNA. N-terminally labeled proteins can be selectively conjugated to a variety of useful probes; here we demonstrate use of this system in enrichment and visualization of proteins made during various stages of the cell cycle. N-terminal incorporation of Anl may also be used to engineer modified proteins for therapeutic and other applications.

Protein synthesis in brine shrimp embryos. Dormant and developing embryos of Artemia salina contain equivalent amounts of chain initiation factors 2

Dormant and developing embryos of Artemia salina contain equivalent amounts of eIF-2, the eukaryotic initiation factor which forms a ternary complex with GTP and Met-tRNAf. The factor was purified from 0.5 M NH4Cl ribosomal washes by (NH4)2SO4 fractionation, followed by chromatography on heparin-Sepharose, DEAE-cellulose, hydroxyapatite and phosphocellulose. Purified preparations from dormant and developing embryos have similar specific activities and nucleotide requirements. The mobility of both proteins in dodecylsulfate gel electrophoresis is indistinguishable, and each contains three major polypeptide chains of molecular weight 52 000, 45 000 and 42 000. Both proteins are also immunologically identical, and each stimulates amino acid incorporation in a cell-free system of protein synthesis. The binding of [35S]Met-tRNAf to 40-S ribosomal subunits is catalyzed by eIF-2 isolated from dormant or developing embryos and is dependent upon GPT and AUG. Binding of [35S]Met-tRNAf to 40-S ribosomal subunits, and ternary complex formation with eIF-2, GTP, and [35S]Met-tRNAf is stimulated 2--3-fold by a factor present in the 0.5 M NH4Cl ribosomal wash and which elutes from DEAE-cellulose at 50 mM KCl. This protein does not exhibit GTP-dependent binding of [35S]Met-tRNAf. Binding of GDP and GTP was investigated with purified eIF-2 from developing embryos. The factor forms a binary complex with GDP or GTP, and eIF-2-bound [3H]GDP exchanges very slowly with free nucleotides. Our results suggest that eIF-2 does not limit resumption of embryo development following encystment, nor does it limit mRNA translation in extracts from dormant embryos.

The efficiency of methionine incorporation from isoaccepting species of tRNAMet into rabbit globin in an homologous reticulocyte lysate system

Rabbit globin alpha and beta chains were labeled with [3H]leucine, and with [35S] -methionine from reticulocyte tRNAMet isoacceptors using a rabbit reticulocyte cell-free synthesis system. [35S]Methionine from the three tRNAMet species isolated by RPC-5 chromatography was incorporated into internal positions of both alpha and beta globin. The initiator tRNA, tRNAIMet, exhibited very low efficiency for incorporating methionine internally, while tRNAIIMet was four times more efficient than tRNAIIIMet. Amino acid analysis of the tryptic peptides of the labeled globins revealed that all three isoacceptors incorporated methionine into the normal methionine peptides. Similar studies with Escherichia coli [35S]Met-tRNAfMet showed a 3-fold increase over the reticulocyte initiator tRNA in its capacity to incorporate methionine into the internal positions of rabbit globin.

Role of GTP in eukaryotic polypeptide-chain initiation. Purification and properties of a factor from Artemia salina embryos which interacts with initiator transfer RNA and guanine nucleotides

1. A factor, which makes a ternary complex with GTP and eukaryotic initiator tRNA (Met-tRNAi), has been purified 100-fold from developed cysts of Artemia salina. Some of the properties of the purified factor have been studied. 2. Mg2+ appears to inhibit ternary complex formation. 3. Little or no ternary complex is formed when 5 muM GTP is replaced by an identical concentration of UTP, CTP or ATP. The analog, guanosine 5'-(beta, gamma-imino)triphosphate [GMP-P(NHP)] seems to be a much better substitute for GTP than guanosine 5'-(beta, gamma-methylene)triphosphate [GMP-P(CH2)P]. Since GMP-P(NH)P is as effective as GTP in ternary complex formation, it would appear that GTP plays the role of an allosteric effector in this step of eukaryotic polypeptide chain initiation. 4. GDP inhibits both the rate and extent of ternary complex formation. The inhibition is largely reversed by adding a 5-fold molar excess of GTP over GDP. DGDP is slightly less inhibitory than GDP. UDP and CDP are much less inhibitory than GDP and very little inhibition is obtained with ADP. 5. The preformed ternary complex is rapidly and completely destroyed in the presence of N-ethylmaleimide. The results suggest that free--SH groups of the factor may be essential for maintaining the integrity of the ternary complex.

Regulation of ternary (Met-tRNAf - GTP - eukaryotic initiation factor 2) protein synthesis initiation complex formation by the adenylate energy charge

Formation of the ternary [Met-tRNAf - GTP - eukaryotic initiation factor 2] protein synthesis initiation complex in rabbit reticulocyte ribosomal eluates is dependent on the GTP: GDP ratio and on the adenylate energy charge. The elements controlling ternary initiation complex formation have been studied in a reconstituted system contianing eukaryotic initiation factor 2 and nucleoside diphosphate kinase purified from the ribosomal eluate. The concentration of GTP required for half maximal formation of the ternary initiation complex is 2.5 - 10(6) M; GDP is a potent competitive inhibitor with Ki equals 3.4 - 10(7) M. Sensitive control of ternary initiation complex formation by the adenylate energy charge occurs through nucleoside diphosphate kinase regulation of the GTP : GDP ratio. Over a wide range of GTP : GDP ratios, 50% of maximal ternary initiation complex formation is observed at an adenylate energy charge of 0.85-0.90 resembling that seen in the unfractionated system. Small changes in adenylate energy charge near this value result in significant changes in the extent of ternary initiation complex formation. Since GTP is continually hydrolyzed to GDP during protein synthesis and since GDP is a competitive inhibitor of GTP binding to several of the protein factors necessary for mRNA translation, the synthetic process provides sensitive control by product inhibition. Ribosome-associated nucleoside diphosphate kinase control of GTP regeneration in response to the adenylate energy charge provides one mechanism for linking protein synthesis to the nutrient state and energy charge of the cell.

Nucleotide regulation of a eukaryotic protein synthesis initiation complex;

Formation of a ternary initiation complex containing Met-tRNAf, GTP and eukaryotic initiation factor 2, is the first step in sequential assembly of the initiation complex. The concentration of GTP required for half maximal formation of the ternary complex is 2.5 with 10(-6) M. GDP is a potent competitive inhibitor of ternary complex formation with Ki = 3.4 with 10(-7) M. The nucleotide binding site on eukaryotic initiation factor 2 demonstrates relative specificity for GDP with KD(GDP) = 3.0 with 10(-8) M; 100-fold higher concentrations of GTP than GDP are required for displacement of either [(3)H]GDP or [(3)h]gtp from the necleotide binding site. An ATP-dependent stimulation of ternary complex formation observed in partially purified initiation factor preparations is due to nucleoside diphosphate kinase (EC 2.7.4.6) which serves to remove inhibitory levels of GDP by phosphorylation with ATP. Since GTP is hydrolyzed to GDP during protein synthesis, this provides a mechanism by which the ATP:ADP ratio may regulate the rate of initiation of protein synthesis.

Binding of Met-tRNA-f to native and derived 40S ribosomal subunits

Our previous work has shown that the native 40S ribosomal subunits (those found free in the cell sap) but not polyribosomal 40S subunits have additional associated proteins that are removed by 0.5 M KCl. In this communication we present evidence that in the Ehrlich cell one of the native subunit associated proteins is the mammalian initiation factor that forms a Met-tRNA-f-factor-GTP complex, and is required for the binding of Met-tRNA-f to the 40S subunit. Initial examination of the KCl wash of the Ehrlich cell total ribosomal pellet revealed a factor which (1) shifted the elution of Met-tRNA-f and of GTP from the included to the excluded volume on Sephadex G-100 chromatography, (2) stimulated the binding of Met-tRNA-f to Millipore filters, and (3) stimulated the binding of Met-tRNA-f to salt-washed 40S subunits. These activities were dependent upon or enhanced by GTP; were inhibited by GDP; were much greater for Met-tRNA-f than for Met-tRNA-m or for lysyl-tRNA; and were concentrated in the KCl ribosomal wash and were not detected in the cell soluble fraction. Met-tRNA-f bound in conjunction with a specific amount of KCl wash protein, to form a distinctive particle of bouyant density 1.40 g cm minus 3 in CsCl, identical in density to one form of the native 40S subunit. Native 40S subunits, but no other subunits, contained a factor which was eluted by 0.5 M KCl and which (1) stimulated the binding of Met-tRNA-f to Millipore filters, and (2) stimulated the binding of Met-tRNA-f to salt-washed 40S subunits. The factor appeared to be localized on the native 40S subunit of density 1.40 g cm minus 3.

A very rapid effect of androgen on initiation of protein synthesis in prostate

The initiation of protein synthesis by ribosomal particles of rat ventral prostate was studied by measuring ribosomal binding of an initiator (35-S)methionyl-tRNAf. The binding activity is dependent on ribosomes, GTP, and a prostate cytosol protein fraction. The 40S but not the 60S ribosomal subunit particles are active. The cytosol activity decreases rapidly within one hour after the rat is castrated. This loss is prevented by an intraperitoneal injection of 17beta-hydroxy-5-alpha-androstan-3-one (5-alpha-dihydrotestosterone). The cytosol activity can be stimulated almost immediately (within 10 min) after an intravenous injection of low dose (15 mug per rat) of 5-alpha-dihydrotestosterone into the castrated rat.

Reversal of pactamycin inhibition of methionyl-puromycin synthesis and 80S initiation complex formation by a ribosomal joining factor

A crude mixture of polypeptide chain initiation factors (0.5 M KCl ribosomal wash) from reticulocyte ribosomes was fractionated by DEAE-cellulose column chromatography. Among several initiation factors obtained from the column, one factor eluting at 0.22-0.25 M KCl showed a remarkable ability to overcome the inhibition of Met-puromycin and 80S initiation complex formation caused by the antibiotic, pactamycin. Earlier experiments had shown that pactamycin does not prevent the binding of Met-tRNA(f) to the small ribosomal subunit but does interfere with the joining of the 60S ribosomal subunit to form the 80S initiation complex. A Lineweaver-Burk plot of initial rates of Met-puromycin formation showed that the interaction of the factor and pactamycin was of a competitive type. In the absence of the factor, [(35)S]Met-puromycin was not synthesized and [(35)S]Met-tRNA(f) bound only to the small ribosomal subunit. The amount of [(35)S]Met-tRNA(f) bound to 80S ribosomes bearing endogenous mRNA and the amount of [(35)S]Met-puromycin formed were directly related to the amount of factor added. Thus, this factor can be termed a "joining factor," and a simple assay of its activity can be devised based on its ability to overcome the pactamycin inhibition of the puromycin reaction.

Inhibition of Protein Synthesis in Cell-Free Systems from Interferon-Treated, Infected Cells: Further Characterization and Effect of Formylmethionyl-tRNA(F)

The translation of encephalomyocarditis virus (EMC) RNA is markedly inhibited in cell-free systems from interferon-treated, vaccinia virus-infected L-cells (10, 11). The polypeptide products synthesized in response to EMC RNA in cell-free systems from these and untreated infected cells have been analyzed by electrophoresis on polyacrylamide gels. Qualitatively, the same EMC-specific polypeptides were synthesized throughout. In experiments using preincubated microsomes from normal Krebs cells to assay cell sap from L-cells which had been exposed to interferon prior to infection, only the amount of the EMC-specific polypeptide products was reduced. This result suggests that there is an inhibition very early in translation in interferon-treated, infected cells. Initiation seems a priori the more attractive site for this inhibition, but an effect shortly after initiation cannot be excluded. With unfractionated cell-free systems from interferon-treated infected L-cells, however, there appeared to be an additional minor inhibitory effect on polypeptide chain elongation, in that the EMC-specific polypeptides synthesized showed not only a reduction in amount but also a bias towards lower molecular weight. The formylated methionyl initiator tRNA (Fmet-tRNA(F)) was used as a further probe into the apparent effect on intiation. With this reagent we have confirmed that there is one major initiation site for the translation of EMC RNA in these cell-free systems. In addition, the results have shown that EMC-specific polypeptide chains initiated with Fmet escape the major interferon-mediated inhibition at or shortly after initiation.

Evidence for the absence of the G-T-psi-C sequence from two mammalian initiator transfer RNAs

Analyses were performed on the purified initiator tRNA from rabbit liver to test for the presence of Psip and Tp in this molecule. Neither of these nucleotides could be detected after hydrolysis by piperidine, NaOH, or T2 RNase. Similarly digestion with venom phosphodiesterase plus phosphatase failed to release any pseudourdine or ribothymidine. Identical results were obtained with the initiator tRNA from sheep mammary gland. The absence of these nucleosides was confirmed by pancreatic and T1 RNase digestion of the rabbit-liver initiator tRNA. The classical G-T-Psi-C sequence was not detected in this molecule. An A-U-C-G sequence has been identified; it may possibly replace the G-T-Psi-C sequence.

Inhibition of reticulocyte peptide-chain initiation by pactamycin: accumulation of inactive ribosomal initiation complexes

Pactamycin does not inhibit the overall initiation factor- and GTP-dependent binding of [(35)S]Met-tRNA(f) to rabbit reticulocyte ribosomes but does prevent the formation of Met-puromycin, provided that the antibiotic is present during the course of the binding reaction. These data indicate that pactamycin blocks the synthesis of a functional peptide-chain initiation complex. Sucrose density gradient centrifugation analysis of binding reactions shows that pactamycin causes the accumulation of an initiation complex on the smaller ribosomal subunit (smaller initiation complex), to which the 60S ribosomal subunit either cannot join or with which it forms a larger inactive 80S initiation complex that falls apart under the conditions used for isolation. The smaller initiation complex formed in the presence of pactamycin differs from the normal intermediate in peptide-chain initiation in being much more resistant to degradation by pancreatic RNase. In the presence of pactamycin, the inactive smaller complex can also form on mRNA to which an unaffected ribosomal couple is already attached, forming an oligoribosome lacking a larger ribosomal subunit or a "1.5 mer." These effects of pactamycin can be overcome to a considerable degree by elevation of the Mg(2+) concentration.

Initiation of picornavirus protein synthesis in ascites cell extracts

The current model of picornavirus protein formation implies that initiation of protein synthesis occurs at a single site on the viral RNA, and that the large polypeptide formed is later cleaved. A direct test of this model was made in vitro by studying the incorporation of [(35)S]methionine from rabbit liver Met-tRNA(M) (Met) and fMet-tRNA(F) (Met) into encephalomyocarditis virus RNA-coded proteins in extracts of Ehrlich ascites cells. The incorporation of N-formylmethionine was complete within 5 min, while utilization of Met-tRNA(M) (Met) continued for 20 min. Tryptic digests of [(35)S]methionine-labeled products from Met-tRNA(M) (Met) analyzed by anion-exchange chromatography yielded more than 30 peptides, as compared to about 15 [(35)S]methionine-labeled peptides from purified encephalomyocarditis virus. In contrast, products labeled with fMet-tRNA(F) (Met) yielded one major (26)S-labeled tryptic peptide. The N-terminal location of methionine in this peptide was verified by Edman degradation. One predominant N-terminal tryptic peptide was also obtained with fMet-tRNA(F) (Met) when mouse Elberfeld and mengo-virus RNAs were used as messengers. On the basis of N-terminal compared with internal labeling of the products, no evidence for in vitro post-translational cleavage was found. The results are consistent with a single initiation site for synthesis of picornavirus proteins.

Initiation of protein synthesis in HeLa cells

Initiation of protein synthesis in HeLa cells has been synchronized by exposure of the cells to fluoride. Double-labeling of such cells for short pulses with [(35)S]methionine and a tritiated amino acid, followed by Edman degradation of the puromycin-released nascent peptides, has shown that the percent of N-terminal methionine incorporated compared to total incorporation is significantly higher than the value obtained with any of the other amino acids tested. The results suggest that the bulk of the nascent proteins synthesized in vivo by HeLa cells are initiated with methionine.

Formation of a mammalian initiation complex with reovirus messenger RNA, methionyl-tRNA F , and ribosomal subunits

Previous data demonstrated that reovirus mRNA, synthesized in vitro with the particulate RNA transcriptase of reovirus cores, efficiently directs the synthesis of polypeptides in vitro. The present studies indicate that all of the three size classes of reovirus mRNA produced in vitro can form protein initiation complexes with rat liver [(36)S]Met-tRNA(F) and incubated 40S and 60S ribosomal subunits, which had been washed in 0.5 M KCl of mouse fibroblast L-929 cells. Mild prior treatment of the mRNA with HCHO was required to expose the initiator region. The initiation complex reacted quantitatively with puromycin to form a puromycin peptide, whose electrophoretic properties were identical to methionyl-puromycin formed in response to poly(A,G,U) or the initiator codon AUG. The complex was relatively stable and specific for [(35)S]Met-tRNA(F); rat liver [(35)S]Met-tRNA(M) was unreactive unless the supernatant factors EF T(1) and EF T(2) were also present. However, the addition of fusidic acid, at a concentration that did not affect complex formation with [(35)S]Met-tRNA(F), completely inhibited Met-tRNA(M) utilization. Exogenous ribosomal factors and GTP were not required unless the separated 40S and 60S subunits were further treated with 1 M KCl. The data suggest that reovirus mRNA contains AUG initiator codons that form a complex with Met-tRNA(F) at a puromycin-reactive site on ribosomes.