Globin mRNA translation on Artemia salina ribosomes with components from Friend leukemia cells

Shiga toxin attacks bacterial ribosomes as effectively as eucaryotic ribosomes

Several pathogenic bacteria, including Shigelladysenteriae and certain strains of Escherichia coli, produce potent class 2 ribosome inhibiting proteins (RIPs) termed Shiga toxins (Stx). The toxins are bipartite molecules composed of a single A chain (StxA) noncovalently associated with a pentamer of receptor-binding B subunits (StxB). StxA and Stx1A from E. coli are protoxins. Proteolysis generates an A1 enzyme (28 kDa) and an A2 fragment (3 kDa), which remain bound, inactivating the enzyme, until a disulfide bond linking them is reduced. Efforts to express active recombinant Stx1A1 in the cytoplasm of E. coli were very difficult and led to the hypothesis that Stx1A1 is toxic to E. coli. We created the gene for a His-tagged Stx1A1 (cStx1A1) and expressed it in E. coli from a tightly controlled expression vector. About 1-2 mg of protein can be purified in a one-step isolation from 1 L of culture. cStx1A1, RTA, and PAP exhibited similar high toxicity against the Artemia ribosomes with IC50 values near 1 nM. Surprisingly, Stx1A1 had an IC50 of 0.8 nM against E. coli ribosomes, about the same as it had for Artemia ribosomes. This is about 250 times more active than PAP against bacterial targets, making Stx1A1 the most powerful RIP toxin presently known against E. coli ribosomes.

Structure and activity of an active site substitution of ricin A chain

The A chain of ricin (RTA) is an N-glycosidase which inactivates ribosomes by removing a single adenine base from a conserved region of rRNA. X-ray structures and site-directed mutagenesis revealed that Arg 180 interacts with the target adenine hydrogen bonding with N3. It may fully or partially protonate that atom as part of the hydrolysis mechanism. Arg 180 was previously converted to His (R180H) and shown to greatly reduce activity. Here R180H is shown to reduce overall activity 500-fold against Artemia salina ribosomes. A 2.2 A crystal structure reveals the mutation causes a rearrangement of the active site cleft, with Tyr 80 moving to block access to the adenine recognition site. His 180 forms a strong aromatic interaction with Trp 211, Tyr 80, and Tyr 123. A complex is formed with 250 mM AMP. The nucleotide binds in the active site region, but in an apparently nonproductive orientation. His 180 cannot bond to N3 and is screened from the substrate analog by the intervening Tyr 80. It may be that natural polynucleotide substrates, using additional interactions, can displace Tyr 80 and effect a productive binding.

The synthesis of polyphenylalanine on ribosomes to which erythromycin is bound

Erythromycin binds to the large subunit of Escherichia coli ribosomes at a specific site that is very close to the amino acid of aminoacyl-tRNA bound into the peptidyltransferase center, and to the site to which puromycin is bound, the P and A sites, respectively, of the classical two-site model of ribosome function. Both erythromycin and puromycin affect fluorescence from fluorescent derivatives of aminoacyl-tRNAs, while both puromycin and aminoacyl-tRNAs affect fluorescence of fluorescent derivatives of erythromycylamine. The results demonstrate unequivocally that erythromycin, deacylated tRNA, a peptidyl-tRNA analogue and puromycin can be bound simultaneously to the same ribosome. Nascent peptides of more than a few amino acids in length block binding of erythromycin to the ribosomes but, unlike most other peptides, long polyphenylalanine chains can be synthesized on ribosomes to which erythromycin is bound. It is suggested that this refractory synthesis in the presence of erythromycin reflects the atypical physical and structural properties of polyphenylalanine.

Movement of tRNA but not the nascent peptide during peptide bond formation on ribosomes

The results from experiments involving nonradiative energy transfer indicate that a fluorescent probe on the 5'-end of tRNA(Phe) moves more than 20 A towards probes on ribosomal protein L1 as a peptide bond is formed during the peptidyl transferase reaction on Escherichia coli ribosomes. The peptide itself moves no more than a few angstroms during peptide bond formation, as judged by the movement of fluorescent probes attached to the phenylalanine amino group of phenylalanyl-tRNA. Other results demonstrate that an analogue of peptidyl-tRNA, deacylated tRNA, and puromycin can be bound simultaneously to the same ribosome, indicating that there are three physically distinct sites to which tRNA is bound during the reaction steps by which peptides are elongated. The results appear to be consistent with the displacement model of peptide elongation.

Requirements for antiribosomal activity of pokeweed antiviral protein

It has been known for some time that pokeweed antiviral protein acts by enzymatically inhibiting protein synthesis on eucaryotic ribosome systems. The site of this action is known to be the ribosome itself. In this paper we show that the pokeweed antiviral protein reaction against ribosomes is a strong function of salt concentrations, where 160 mM K+ and 3 mM Mg2+ retards the reaction, while 20 mM K+ and 2 mM Mg2+ allows maximum reaction rate. It is also shown, however, that an unidentified protein in the postribosomal supernatant solution, together with ATP, allows the ribosome to be attacked even in the presence of high salt. Kinetic analysis of the antiviral protein reaction has been carried out under both sets of conditions, and reveals that the turnover number for the enzyme is about 300-400 mol/mol per min. in each case. The Km for ribosomes is 1 microM in the presence of low salt and 0.2 microM at higher salt in the presence of postribosomal supernatant factors plus ATP. The antiviral protein reaction is also shown to be pH dependent and is controlled by a residue with pKa value of approx. 7.0, apparently a histidine. Stoichiometric reaction of the enzyme with iodoacetamide results in a significant loss of antiribosomal activity.

Hemin-independent control of globin synthesis in Friend erythroleukemia cells induced to differentiate

A hemin-independent translational inhibitor that prevents synthesis of rabbit globin when uninduced Friend leukemia (FL) cell and rabbit reticulocyte lysates are mixed [Cimadevilla, J. M. & Hardesty, B. (1975) Biochem. Biophys. Res. Commun. 63, 931-937] cannot be detected in FL cells induced to differentiate. Mixing of lysates of FL cells induced with hexamethylene bisacetamide or aminonucleoside of puromycin and rabbit reticulocytes does not cause inhibition of rabbit globin synthesis. Induction also results in the cells acquiring sensitivity to the inhibitor from uninduced FL cells. A reduction in total protein synthesis is observed when uninduced and induced FL cell lysates are mixed. Inhibition does not result from competition by an excess of uninduced FL cell mRNA species for the translational machinery because uninduced FL cell lysates retain their inhibitory activity after treatment with micrococcal nuclease. Rabbit globin mRNA recovered from rabbit reticulocyte lysates that have been incubated with lysates of uninduced FL cells can still be translated effectively, indicating that inhibition does not result from modification of other species of mRNA by uninduced FL cell lysates. A switch to hemin-dependent translational control does not follow induction of differentiation. The rate of amino acid incorporation in induced FL cell lysates--like that in uninduced FL cell lysates--is unaffected by omission of exogenous hemin from the system. Its presence is not required to prevent activation of heme-regulated inhibitor. From these data, it is clear that the control of protein synthesis in FL cells--whether or not they are induced--is different from that regulated by hemin in normal erythroid cells.

In vitro translation of hypoxanthine/guanine phosphoribosyltransferase mRNA: characterization of a mouse neuroblastoma cell line that has elevated levels of hypoxanthine/guanine phosphoribosyltransferase protein

Antibody specific for the native form of Chinese hamster hypoxanthine/guanine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) was used to detect the synthesis of HPRT protein in a rabbit reticulocyte lysate translation system primed with mRNA from Chinese hamster tissues and cultured cells. Electrophoretic analysis of the immunopurified products from the translation of mRNA from wild-type and a series of mutant Chinese hamster cells indicated that HPRT synthesis in vitro qualitatively and quantitatively corresponded to synthesis in vivo. The translation system was used to identify two mRNA sources producing high levels of HPRT protein: Chinese hamster brain and a mouse neuroblastoma HPRT revertant cell line, NBR4. Translation of NBR4 mRNA generated 25-50 times more HPRT protein than mRNA from wild-type cells. The basis for HPRT overproduction is considered in view of an X chromosome alteration found in NBR4 cells.

Subcellular distribution of aminoacyl-tRNA synthetases in various eukaryotic cells

The total amount, size distribution and binding of aminoacyl-tRNA synthetases to ribosomes in a variety of mammalian and avian cells was studied under standard conditions of sample preparation and assay. Aminoacyl-tRNA synthetases appear to exist in three general forms; 'free' enzyme of about 4-9 S, one or more 'enzyme complexes' of about 18-25 S, and in association with ribosomes. The aminoacyl-tRNA synthetase activity for many individual amino acids was surprisingly similar in cell types chosen to be diverse with respect to differentiation state, transformation, and growth rate. Total activity for all amino acids varied about 4-fold, based on a constant volume of cells. Embryonic tissues had a comparatively high proportion of total synthetase activity associated with ribosomes, whereas this value was relatively low for mouse liver. Distinctive distribution patterns with common and variable features were observed for individual enzymes. The only aminoacyl-tRNA synthetases found not to be associated in significant amounts with either 18-25 S enzyme complexes or ribosomes in any of the cell types examined were the enzymes for alanine, histidine, and serine. All cell types evidenced 18-25-S synthetase activity for arginine, aspartic acid, glutamine, glutamic acid, isoleucine, leucine, lysine, methionine, proline, and valine, although in quite variable porportions of the total activity observed for these amino acids. For example, of the valyl-tRNA synthetase activity not associated with ribosomes, 35% and 100% were found to sediment at 18-25 S in Friend leukemia cells and mouse liver respectively. All cells had two easily distinguishable peaks of arginyl tRNA synthetase activity at 4-9S and 18-25S respectively; however, the relative proportion of enzyme activity in the peaks differed between cell types. Phenylalanyl-tRNA synthetase was not observed to occur in an 18-25-S complex in any of the cell types examined but was bound to ribosomes in variable but generally relatively high proportions. Numerous other specific differences are described. No underlying physiological or biochemical principle has been recognized to account for the specific distribution patterns observed. However, they may reflect variations in cellular architecture that may be related to regulation of protein synthesis.

A simple method for the purification of reticulocyte globin messenger ribonucleic acid

A relatively simple and inexpensive method has been developed for the preparation of highly purified rabbit reticulocyte globin mRNA. After phenol extraction, polysomal RNA was chromatographed on Sigmacell type 38 cellulose and Sepharose 4B. The resulting mRNA preparation has a purity in excess of 90%. No selective loss of either alpha or beta globin mRNA is observed.