Differential inactivation of soluble reticulocyte transfer factors with N-ethylmaleimide

Binding of periodate-oxidized guanine nucleotides to eukaryotic elongation factor 2

Periodate-oxidized guanine nucleotides (GTPox and GDPox) were shown to bind stoichiometrically to rat liver elongation factor 2 (EF-2). This binding was quantitatively inhibited in the presence of GTP. After binding, oxidized nucleotides remained on EF-2 despite extensive dialysis. They exchanged, however, with free quanine nucleotides in the course of prolonged (greater than 1 h) incubations. The prior reduction EF-2.GTPox with NaBH4 abolished, to a large extent, this slow exchange. Thus, a Schiff's base was implicated to be formed between EF-2 and oxidized guanine nucleotides. Mg2+ increased the GTPox concentration necessary for a stoichiometric binding to EF-2. EF-2-oxidized nucleotide conjugates bound in the presence of ribosomes a second molecule of GTP (or GTPox). GTPox bound to EF-2 in the presence of ribosomes appeared to exchange readily with free GTP. Moreover, GTPox proved to be active as substrate in EF-2 and ribosome-dependent GTPase reaction: Km values found for GTPox and GTP were 7.7 and 3.4 microM, respectively. The binding of GTPox to EF-2 inhibited only partially the subsequent ribosome-dependent GTP binding, and GTPase reaction or polyphenylalanine (polyPhe) synthesis. On the other hand, the binding of GuoPP[CH2]Pox to EF-2 inhibited all of these reactions strongly. The nature of the binding site involved in the direct interactions of EF-2 with guanine nucleotides is discussed in the light of these results.

Functional groups of elongation factor 2 involved in interactions with guanosine nucleotides and ribosomes

Treatment of rat liver EF-2 with N-ethylmaleimide (MalNEt) did not affect the direct interactions of the factor with guanine nucleotides or with ribosomes, but inhibited the binding of guanosine 5'-(beta, gamma-methylene)triphosphate (GuoPP(CH2)P) to the EF-2-ribosome complex. The amino group reactive reagent 2,4,6-trinitrobenzenesulfonate (TNBS), however, inhibited specifically the direct interactions of EF-2 with guanine nucleotides, but not the binding of GuoPP(CH2)P to the EF-2-ribosome complex. The different sensitivities of EF-2 to MalNEt and to TNBS suggested that the binding sites involved in the binary vs. ternary complex might correspond to different conformational states or might even be distinct physical entities.

Investigation of sesquiterpene lactones as protein synthesis inhibitors of P-388 lymphocytic leukemia cells

The mode of action of helenalin and bis(helenalinyl) malonate as protein synthesis inhibitors of P-388 lymphocytic leukemia cells was investigated. The initial characterizations were carried out in crude lysates of the P-388 cells. In the lysate, there was a 4 min lag after the addition of drug before inhibition of protein synthesis occurred. Both drugs allowed run-off of preformed polysomes, but did significantly inhibit the formation of the 80 S initiation complex suggesting a preferential inhibition of one or more initiation reactions. The effect of these drugs on inhibition of both elongation and initiation reactions was further investigated using more fractionated systems prepared from P-388 cells. Poly(U)-directed polyphenylalanine synthesis was marginally inhibited by both drugs, but the degree of inhibition was not sufficient to explain the inhibition observed in either the lysate or in whole cell preparations of P-388. The formation of the ternary initiation complex was not significantly inhibited by either drug, but the conversion of this complex to the 48 and 80 S initiation complexes was inhibited. The inhibition of 48 S initiation complex formation by both drugs was sufficient to explain their inhibition of protein synthesis in whole cells.

Role of elongation factor 1 in the translational control of rodent brain protein synthesis

The translational control of protein synthesis during early postnatal neural development and aging was examined in the mouse and the rat. The activity of brain elongation factor 1 (bEF-1) was found to decrease exponentially with age and to decline parallel to the age-dependent decrease in total protein synthesis in both rodents. This decrement in bEF-1 activity fell within the range of reported age-related decreases in protein synthesis in in vitro systems. The factor was present in multiple forms; the lighter species predominated in older animals, whereas the young light form apparently disappeared with increasing age, and was replaced by other arising from the heavy form. Elongation factor 1 derived from young brains functioned as a rate-limiting component in polypeptide synthesis in previously saturated adult systems. The data suggest that bEF-1 has an important modulatory effect on total brain protein synthesis.

The nature of the differential inhibition of polyphenylalanine synthesis in extracts from Artemia salina and rabbit reticulocytes by the Phytolacca americana protein

The difference in sensitivity of polyphenylalanine synthesis in extracts from Artemia salina and rabbit reticulocytes to inhibition by the Phytolacca americana protein (PAP) has been found to be linked to the source of the supernatant enzyme fraction and not the ribosomes. In the presence of reticulocyte supernatant enzyme fraction polyphenylalanine synthesis in less sensitive to inhibition by PAP than that observed in the presence of A. salina supernatant enzyme fraction. The results suggest that reticulocyte elongation factor 2 is responsible for this effect.

Protein synthesis and loss of viability in rye embryos. The lability of transferase enzymes during senscence

Poor germination and eventual loss of viability of rye grains on storage reflects a decreasing ability of embryos to synthesize protein in vivo. Cell-free protein-synthesizing systems from low viability embryo stocks exhibit lesions in the soluble components of the post-ribosomal supernatant fractions. tRNA and aminoacyl-tRNA synthetases are not impaired. The activity of enzymes concerned with transfer of phenylalanyl-tRNA from the aminoacyl to the peptidyl site on the ribosome is slightly decreased. The transfer enzymes (transferase I) involved in the binding of phenylalanyl-tRNA to the aminoacyl site show a loss of activity that closely reflects the loss of viability and the decline of protein synthesis of the embryo in vivo. The inactivation of labile transferase I components may be a major factor leading to senescence and loss of viability in rye grains.

The mechanism of messenger RNA translocation through ribosomes

The two recognized enzymatic steps involved in the extension of peptides on ribosomes of the 80S type have been studied in a highly purified transfer system derived from rabbit reticulocytes. Data presented are interpreted to reflect three ribosomal binding sites through which transfer RNA is moved in two independent enzymatic reactions each of which requires guanosine 5'-triphosphate hydrolysis. The binding enzyme facilitates translocation between the entry and acceptor ribosomal sites. Transferase II is involved in translocation between the acceptor and donor ribosomal sites. A model is proposed to account for movement of transfer RNA and messenger RNA between the ribosomal sites.

Deacylated tRNA-phe binding to a reticulocyte ribosomal site for the initiation of polyphenylalanine synthesis

The initiation of polyphenylalanine synthesis at low MgCl(2) concentration in the reticulocyte transfer system has been found to have a stringent requirement for deacylated tRNA(Phe). An initial poly-U-directed complex between deacylated tRNA(Phe) and ribosomes is formed. The onset of polyphenylalanine synthesis causes the rapid release of tRNA(Phe) from the complex. Thermal dissociation studies indicate that deacylated tRNA(Phe) and phenylalanyl-tRNA are bound to the same ribosomes. A total of three ribosomal tRNA binding sites is indicated.