Protein synthesis in rabbit reticulocytes. Characteristics of mRNA (AUG codon)-dependent binding of Met-tRNAfMet to 40 S and 80 S ribosomes

Natural mRNA is required for directing Met-tRNA(f) binding to 40S ribosomal subunits in animal cells: involvement of Co-eIF-2A in natural mRNA-directed initiation complex formation

Two protein factors, eIF-2 as well as a high molecular weight protein complex from reticulocyte ribosomal high-salt wash which we term Co-eIF-2, promote Met-tRNA(f) binding to 40S ribosomes. This binding is dependent on the presence of an AUG codon or natural mRNAs [Roy et al. (1984) Biochem. Biophys. Res. Commun. 122, 1418-1425]. Co-eIF-2 contains two component activities, Co-eIF-2A and Co-eIF-2C. Previously, we have purified an 80-kDa polypeptide containing Co-eIF-2A activity and showed that this polypeptide is a component of Co-eIF-2 and is responsible for Co-eIF-2A activity in Co-eIF-2 [Chakravarty et al. (1985) J. Biol. Chem. 260, 6945-6949]. We now report purification of a protein complex (subunits of Mr 180K, 110K, 65K, 63K, 53K, 50K, 43K, and 40K) containing Co-eIF-2C activity and devoid of Co-eIF-2A activity. In SDS-PAGE, the purified Co-eIF-2C preparation and an eIF-3 preparation (purified in Dr. A. Wahba's laboratory) separated into seven similar major polypeptides (Mr 110K, 65K, 63K, 53K, 50K, 43K, and 40K). The 50-kDa polypeptide in Co-eIF-2C was immunoreactive with a monoclonal antibody against eIF-4A (50 kDa). We have studied the roles of purified Co-eIF-2A and Co-eIF-2C activities in ternary and Met-tRNA(f).40S ribosome complex formation. The results are as follows: (1) At low and presumably physiological factor concentration (30 nM), eIF-2 did not form detectable levels of ternary complex. Moreover, such complex formation was totally dependent on the presence of Co-eIF-2A and/or Co-eIF-2C.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of fMet-tRNAf deacylase from Escherichia coli

A formylmethionyl-tRNAf deacylase has been purified about 330-fold from a crude initiation factor preparation (1 M NH4Cl ribosomal wash) from Escherichia coli Q13. The enzyme was nearly homogeneous and had an apparent molecular weight of 24 000. Rat liver methionyl-tRNAf and E. coli methionyl-tRNAm were not hydrolyzed significantly by the enzyme under standard conditions. Q beta RNA- and AUG(A)n-directed polypeptide synthesis was inhibited by the enzyme. The inhibition was at the level of initiation of polypeptide synthesis. The enzymatic activity was inhibited by various factors necessary for polypeptide synthesis. The activity was inhibited more by NH4Cl and spermidine than by Mg2+, GTP and ATP. The complex of formylmethionyl-tRNAf, initiation factor 2 and GTP was resistant to enzymatic hydrolysis, and the resistance was enhanced by the addition of AUG and ribosomes to the above reaction mixture.

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.

Methionyl-tRNA-Met-f deacylase. Purification, characterization, and effects on translational initiation complexes

A methionyl-tRNA-Met-f deacylase was found in ribosomal salt wash from cultured human cells of the HeLa line. This enzyme was purified by the use of DEAE-cellulose, ammonium sulfate precipitation, gel filtration and isoelectric focusing, and appears to be a protein with a native molecular weight of 80,000, which consists of two 40,000-Mr subunits. The mechanism of the Met-tRNA-Met-f deacylase is shown to involve end-product inhibition by the deacylated form of Met-tRNA-Met-f. The methionyl-tRNA-Met-f deacylase is rather specific for Met-tRNA-Met-f as opposed to Met-tRNA-Met-m, has a KCl optimum of 85 mM, is inhibited by MgCl2 and is inhibited by GTP and NAD+ at physiological concentration. 40-S and 60-S subunits inhibit the enzyme, possibly by binding to it. The stability of translational initiation complexes, containing methionyl-tRNA-Met-f, was investigated in the presence of the enzyme. Purified ternary complex was slowly broken down by the enzyme, while the 40-S-subunit . Met-tRNA-Met-f complex was stable in the presence of enzyme. The 80-S complex formed with A-U-G trinucleotide as the message molecule was broken down, whereas the 80-S complex formed with globin mRNA was stable in the presence of the enzyme. The physiological role of this enzyme is unclear, but it might act to regulate initiation by deacylating Met-tRNA-Met-f.