Effect of polyamines on isoleucyl-tRNA formation by rat-liver isoleucyl-tRNA synthetase

The functional role of polyamines in eukaryotic cells

Polyamines, consisting of putrescine, spermidine and spermine are essential for normal cell growth and viability in eukaryotic cells. Since polyamines are cations, they interact with DNA, ATP, phospholipids, specific kinds of proteins, and especially with RNA. Consequently, the functions of these acidic compounds and some proteins are modified by polyamines. In this review, the functional modifications of these molecules by polyamines are presented. Structural change of specific mRNAs by polyamines causes the stimulation of the synthesis of several different proteins, which are important for cell growth and viability. eIF5 A, the only known protein containing a spermidine derivative, i.e. hypusine, also functions at the level of translation. Experimental results thus far obtained strongly suggest that the most important function of polyamines is at the level of translation.

Modulation of cellular function by polyamines

Polyamines (putrescine, spermidine and spermine) are essential for normal cell growth. The polyamine levels in cells are regulated by biosynthesis, degradation, and transport. Polyamines can modulate the functions of DNA, nucleotide triphosphates, proteins, and especially RNA because most polyamines exist in a polyamine-RNA complex in cells. Thus, the major focus on this review is on the role of polyamines in protein synthesis. In addition, effects of polyamines on B to Z conversion of DNA, transcription, phosphorylation of proteins, cell cycle progression, apoptosis and ion channels, especially NMDA receptors, are outlined. The function of eIF5A is also briefly discussed. Finally, a correlation between acrolein, produced from polyamines by polyamine oxidases, and chronic renal failure or brain stroke is summarized. Increased levels of polyamine oxidases and acrolein are good markers of chronic renal failure and brain stroke.

Polyamines: mysterious modulators of cellular functions

In recent years the functions of polyamines (putrescine, spermidine, and spermine) have been studied at the molecular level. Polyamines can modulate the functions of RNA, DNA, nucleotide triphosphates, proteins, and other acidic substances. A major part of the cellular functions of polyamines can be explained through a structural change of RNA which occurs at physiological concentrations of Mg(2+) and K(+) because most polyamines exist in a polyamine-RNA complex within cells. Polyamines were found to modulate protein synthesis at several different levels including stimulation of special kinds of protein synthesis, stimulation of the assembly of 30 S ribosomal subunits and stimulation of Ile-tRNA formation. Effects of polyamines on ion channels have also been reported and are gradually being clarified at the molecular level.

Polyamine stimulation of the synthesis of oligopeptide-binding protein (OppA). Involvement of a structural change of the Shine-Dalgarno sequence and the initiation codon aug in oppa mRNA

We previously suggested that the degree of polyamine stimulation of oligopeptide-binding protein (OppA) synthesis is dependent on the secondary structure and position of the Shine-Dalgarno (SD) sequence of OppA mRNA. To study the structural change of OppA mRNA induced by polyamines and polyamine stimulation of initiation complex formation, four different 130-mer OppA mRNAs containing the initiation region were synthesized in vitro. The structural change of these mRNAs induced by polyamines was examined by measuring their sensitivity to RNase T(1), specific for single-stranded RNA, and RNase V(1), which recognizes double-stranded or stacked RNA. In parallel, the effect of spermidine on mRNA-dependent fMet-tRNA binding to ribosomes was examined. Our results indicate that the secondary structure of the SD sequence and initiation codon AUG is important for the efficiency of initiation complex formation and that spermidine relaxes the structure of the SD sequence and the initiation codon AUG. The existence of a GC-rich double-stranded region close to the SD sequence is important for spermidine stimulation of fMet-tRNA binding to ribosomes. Spermidine apparently binds to this GC-rich stem and causes a structural change of the SD sequence and the initiation codon, facilitating an interaction with 30 S ribosomal subunits.

Increase in fidelity of rat liver Ile-tRNA formation by both spermine and the aminoacyl-tRNA synthetase complex

To examine the polyamine effects on the fidelity at the aminoacylation level and the physiological significance of the existence of the aminoacyl-tRNA synthetase complex (ARSC) in animal cells, a single-chain Ile-tRNA synthetase (IRSS) was isolated from the complex by treatment with trypsin. Ile-tRNA formation by IRSS was strongly stimulated by spermine, similar to the results with ARSC. Two misacylations (Val-tRNAIle and Ile-tRNAiMet formation) by IRSS were measured. The error frequency was higher in Ile-tRNAiMet formation (tRNA misacylation) than in Val-tRNAIle formation (amino acid misacylation). Spermine did not influence significantly Ile-tRNAiMet formation, but it stimulated Val-tRNAIle formation by IRSS. Accordingly, spermine decreased the error frequency of tRNA misacylation, but not amino acid misacylation. These results suggest that the conformational changes of individual tRNA by spermine differ from each other, meaning that spermine influences the interaction between individual tRNA and aminoacyl-tRNA synthetase variously. When the aminoacylations of tRNAIle from rat liver, yeast, and Escherichia coli were compared with ARSC and IRSS, the relative speed of Ile-tRNA formation with tRNAIle from other species was faster with IRSS than with ARSC. This indicates that ARSC can recognize tRNAIle from the same species more specifically than IRSS. These results show that both spermine and ARSC are involved in the increase of fidelity of rat liver Ile-tRNA formation.

Correlation between spermine stimulation of rat liver Ile-tRNA formation and structural change of the acceptor stem by spermine

We have recently reported that the interaction of spermine with the acceptor and anticodon stems may be important for spermine stimulation of rat liver Ile-tRNA formation [Peng, Z. et al. (1990) Arch. Biochem. Biophys. 279, 138-145]. To pinpoint which interaction of spermine is more important for spermine stimulation of Ile-tRNA formation, Ile-tRNA formation and ribonuclease V1 sensitivity of tRNA(Ile) were studied using purified tRNAs(Ile) from rat liver, wheat germ, brewer's yeast, torula yeast and Escherichia coli. The results indicate that spermine stimulation of rat liver Ile-tRNA formation correlated with the structural change of the acceptor stem by spermine. The nucleotide sequence of wheat germ tRNA(Ile) was also determined.

Concentration-dependent effects of natural polyamines on peptide chain initiation and elongation in a cell-free system of protein synthesis

Spermidine and spermine at submillimolar concentrations stimulate the rate of incorporation of amino acid into protein in a cell-free system, directed either by endogenous or exogenous mRNA (TMV, globin). The stimulatory effects of these polyamines are exerted at both the stages of initiation and elogation and are more pronounced in the case of TMV or globin mRNA, amounting to approximately 2.3-fold stimulation over the polyamine-free system. The number of polysomes and the polysome-associated radioactivity increase approximately 2-fold in the presence of spermine. Synthesis of large polypeptides is a characteristic feature of the stimulatory event. However, elevated concentrations of spermidine and spermine strongly inhibit amino acid incorporation into protein. Inhibition is manifest at the stage of peptide elongation. In the case of endogenous mRNA the addition of an excess of polyamines results in a non uniform inhibition of amino acid incorporation. A most interesting finding is that, with increasing concentrations of polyamines, the intensity of four bands with Mr values of 63000, 44000, 15500 and 12500 respectively, increases or leastwise remains constant while others fade, indicating differential translation of proteins in the presence of polyamines.

Responsibility of tRNA(Ile) for spermine stimulation of rat liver Ile-tRNA formation

To determine whether tRNA or aminoacyl-tRNA synthetase is responsible for spermine stimulation of rat liver Ile-tRNA formation, homologous and heterologous Ile-tRNA formations were carried out with Escherichia coli and rat liver tRNA(Ile) and their respective purified Ile-tRNA synthetases. Spermine stimulation was observed only when tRNA from the rat liver was used. Spermine bound to rat liver tRNA(Ile) but not to the purified aminoacyl-tRNA synthetase complex. Kinetic analysis of Ile-tRNA formation revealed that spermine increased the Vmax and Km values for rat liver tRNA(Ile). The Km value for ATP and isoleucine did not change significantly in the presence of spermine. Furthermore, higher concentrations of rat liver tRNA(Ile) tended to inhibit Ile-tRNA formation if spermine was absent. Spermine restored isoleucine-dependent PPi-ATP exchange in the presence of rat liver tRNA(Ile), an inhibitor of this exchange. The nucleotide sequence of rat liver tRNA(Ile) was determined and compared with that of E. coli tRNA(Ile). Differences in nucleotide sequences of the two tRNAs(Ile) were observed mainly in the acceptor and anticodon stems. Limited ribonuclease V1 digestion of the 3'-32P-labeled rat liver tRNA(Ile) showed that both the anticodon and acceptor stems were structurally changed by spermine, and that the structural change by spermine was different from that by Mg2+. The influence of spermine on the ribonuclease V1 digestion of E. coli tRNA(Ile) was different from that of rat liver tRNA(Ile). The results suggest that the interaction of spermine with the acceptor and anticodon stems may be important for spermine stimulation of rat liver Ile-tRNA formation.

Comparison of the enzymatic behavior of high molecular weight and free lysyl-tRNA synthetase from rat liver: kinetic analysis of lysylation of tRNA

Lysyl-tRNA synthetase occurs in the high molecular weight form in rat liver. The high molecular weight lysyl-tRNA synthetase has been previously demonstrated to exist as multienzyme complexes of aminoacyl-tRNA synthetases. The multienzyme complexes can be dissociated by hydrophobic interaction chromatography and yield fully active, free lysyl-tRNA synthetase. The free form is found to be twice as active as the complexed form in lysylation. Bisubstrate and product inhibition kinetics of lysylation are systematically carried out for highly purified free lysyl-tRNA synthetase and the 18 S synthetase complex. Surprisingly, the two enzyme forms exhibit distinctly different kinetic patterns in bisubstrate and product inhibition kinetics under identical conditions. The 18 S synthetase complex shows kinetic patterns consistent with an ordered bi uni uni bi ping pong mechanism, while the results of free lysyl-tRNA synthetase do not. We conclude that structural organization of lysyl-tRNA synthetase beyond quaternary structure of proteins may alter the enzyme behavior.

Polyamine depletion results in impairment of polyribosome formation and protein synthesis before onset of DNA synthesis in mitogen-activated human lymphocytes

Lymphocytes, exposed to mitogens in culture, show enhanced protein and RNA synthesis before the onset of DNA synthesis. Inhibition by DL-alpha-difluoromethylornithine of polyamine synthesis in phytohaemagglutinin-activated human lymphocytes resulted in a suppression of protein synthesis, which was evident before the initiation of DNA synthesis. The mitogen-induced increase in the incorporation of [3H]thymidine into DNA was subsequently inhibited in parallel with the activity of thymidine kinase in the polyamine-depleted cells. Ultraviolet absorbance measurement of the ribosomes after sucrose gradient centrifugation revealed a suppression of polyribosome formation that coincided with the decrease in the rate of protein synthesis. The disturbance in the polysomal profiles did not appear to be due to a shortage of mRNA, since the synthesis of poly(A)-rich mRNA was reduced less than that of rRNA after inhibiting polyamine synthesis. Entry of both the pre-existing and newly synthesized ribosomal subunits into polysomal structures was found to be impaired. These results thus suggest an important role for polyamines in the initiation step of protein synthesis.

Differential stimulation by polyamines of phage DNA-directed in vitro synthesis of proteins

The effect of polyamines on T7- and lambda rifd18 DNA-directed synthesis of proteins in an Escherichia coli cell-free system has been studied. When T7 DNA was used as a template, the degree of stimulation by spermidine of protein synthesis was larger with T7 RNA polymerase than with Mr 42 K protein, while the synthesis of Mr 13.5 K protein was not stimulated significantly by spermidine. The synthesis of T7 RNA polymerase was stimulated approx. 10-fold by 1 mM spermidine. When lambda rifd18 DNA was used as a template, the synthesis of beta beta' subunits of RNA polymerase was stimulated greatly by spermidine, while the synthesis of elongation factor Tu and ribosomal proteins was not stimulated significantly by spermidine. Spermidine stimulation of T7 DNA-directed synthesis of T7 RNA polymerase was at the level of both translation and transcription. The degree of stimulation by spermidine was greater at the level of translation. Putrescine stimulated the synthesis of T7 RNA polymerase and Mr 42 K protein to a small degree at the level of translation.

Ornithine decarboxylase may be a multifunctional protein

Ornithine decarboxylase may undergo posttranslational modifications which alter its function. Both transamidation of glutamine residues in the enzyme catalyzed by TGase and phosphorylation of serine and threonine residues catalyzed by a polyamine-stimulated protein kinase have been demonstrated. Data are presented which suggest that these modifications result in translocation of the modified protein to the nucleolus where it regulates the activity of RNA polymerase I to transcribe rDNA, the only active nucleolar genes. Transamidation of specific proteins with primary amines catalyzed by intracellular TGase may be an important posttranslational modification, capable of altering genetic transcription. The rapid half-life of ODC (10-15 min) may be related to rapid posttranslational modification with loss of enzymatic activity rather than to protein degradation.

Mechanism of aminoacylation of tRNA. Influence of spermine on the kinetics of aminoacyl-tRNA synthetases by isoleucyl- and valyl-tRNA synthetases from Mycobacterium smegmatis

Isoleucyl-tRNA synthetase has been purified to homogeneity from Mycobacterium smegmatis. The influence of spermine on the kinetics of valyl-tRNA and isoleucyl-tRNA formation has been investigated by Cleland's method (Cleland, W.W. (1963) Biochim. Biophys. Acta 67, 104-137, 173-187, 188-196). The results suggest that in the presence of spermine and suboptimal concentration of Mg2+, the formation of valyl-tRNA and isoleucyl-tRNA follows a sequential mechanism. In the presence of an optimal concentration of Mg2+, both valyl-tRNA and isoleucyl-tRNA formation proceeds by a ping-pong mechanism. However, in the presence of spermine and optimal concentrations of Mg2+, valyl-tRNA formation follows the ping-pong mechanism while isoleucyl-tRNA formation follows the sequential mechanism.

Antagonistic action between antibodies directed against 7-methylguanosine and polyamines on translation in vitro of RNA induced by measles virus

Antibodies directed against N7-methylguanosine (m7Guo) were prepared and added to a wheat germ cell-free protein-synthesizing system programmed with RNA extracted from monkey cells persistently infected with measles virus. A dose-dependent inhibition of [35S]methionine incorporation was observed when RNA was preincubated with anti-m7Guo immunoglobulins. Antibodies preincubated with m7Guo did not show any inhibiting activity. The inhibitory effect of antibodies was abolished when RNA was preincubated with immunoglobulins in the presence of spermine and spermidine. When polyamines were added to the assay programmed with the IgG-RNA complex, no inhibition was observed.

Effect of polyamines on cell-free protein synthesizing systems from rat cerebral cortex, cerebellum and liver

The naturally occurring polyamines spermine and spermidine stimulate in low concentrations the amino acid-incorporating activity of cell-free systems from cerebral cortex, cerebellum and liver from the rat to a significant degree, but in high concentrations they have strong inhibiting effects on these systems. Maximal polyamine stimulation is observed with suboptimal Mg2+ concentrations in the reaction mixtures; increasing Mg2+ to 15 mM results in an inhibition of cell-free protein synthesis by polyamines even at low concentrations. The stimulating effect of spermine and spermidine involves a facilitated binding of template RNA and aminoacylated tRNA to the ribosomes. In contrast, the aminoacylation of tRNA is not influenced. These data indicate that spermine as well as spermidine have regulatory functions during the translational process in cerebral cortex, cerebellum and liver. They seem to interact with ribosomal functions and to cause a shift in the optimal Mg2+ concentration required for the synthesis of brain and liver proteins.

Mechanism of spermine stimulation of rat-liver isoleucyl-tRNA formation

The mechanism of spermine stimulation of rat-liver isoleucyl-tRNA formation has been studied by Cleland's method [Biochim. Biophys. Acta, 67, 104--137 & 173--197 (1963)]. It was demonstrated that isoleucyl-tRNA formation proceeded through a ping-pong mechanism in the presence of spermine and magnesium. From a study of the effect of spermine on both the formation of isoleucyl-AMP . enzyme complex and on the subsequent formation of isoleucyl-tRNA from the isoleucyl-AMP . enzyme complex, evidence is presented that the stimulatory effect of spermine functions mainly through the spermine preventing the tRNA inhibition of isoleucyl-AMP . enzyme complex formation.