Non-toxic type 2 ribosome-inactivating proteins (RIPs) from Sambucus: occurrence, cellular and molecular activities and potential uses

Ribosome-inactivating proteins (RIPs) are a family of enzymes that trigger the catalytic inactivation of ribosomes. The most known member of the family is the highly poisonous two-chain ricin isolated from Ricinus communis L. Sambucus species contain a number of two-chain RIPs structurally and enzymatically related to ricin which have the noteworthy feature that, having an enzymatic activity on ribosomes, leading to the inhibition of protein synthesis, higher than ricin, they are lacking of the tremendous unspecific toxicity of ricin. Therefore, they have been called non-toxic type 2 RIPs. The most representative and studied members are nigrin b present in the bark of the common (black) elder Sambucus nigra L. and ebulin 1 present in the leaves of the dwarf elder Sambucus ebulus L. The molecular basis for the low unspecific activities of nigrin b and ebulin 1 as compared with ricin seems to be related with single changes of amino acids in the high affinity sugar binding sites of the B chains. These changes determine the intracellular traffic of these proteins and thus the cellular toxicity. Conjugation ofnigrin b or ebulin 1 to either transferrin or monoclonal antibodies provided highly active conjugates targeting cancer. Thus these non-toxic type 2 RIPs are promising tools for cancer therapy.

Effects of chronic administration of either ethanol or pentanol on rat duodenum morphology

The morphology of the rat duodenum after chronic treatment with 15% (v/v) ethanol and 4% (v/v) pentanol was studied. Male Wistar rats of experimental groups were given ethanol and pentanol for 15 weeks with food and fluid freely available. Ethanol-15% and 4% pentanol-fed rats showed a significantly reduced fluid and food intake as compared with control rats. The study of the mucosa indicated that the number of chronic inflammatory infiltrating (mononuclear cells) and goblet cells was higher in the groups of the ethanol- and pentanol-fed rats than in the control group. There was an increase in the thickness of the brush border in pentanol-fed rats. Intervillus adhesion was concurrently observed in the pentanol-fed rats but not in the control or ethanol-fed rats. After ethanol feeding many of the villi developed blebs at the apex of the villus or laterally on its upper half. These blebs generally remained intact. In contrast, after pentanol feeding no bleb formation was appreciated. The intake of ethanol and other short chain alcohols present in alcoholic beverages leads to mainfold disturbances on the rat duodenum. These findings suggest that the chronic ingestion of pentanol seems to promote cellular changes but less important than those observed after chronic ethanol ingestion.

Differences in cytotoxicity of native and engineered RIPs can be used to assess their ability to reach the cytoplasm

Ricin is a heterodimeric cytotoxin composed of RTB, a galactose binding lectin, and RTA, an enzymatic N-glycosidase. The toxin is endocytosed, and after intracellular routing, RTA is translocated to the cytoplasm where it inactivates ribosomes resulting in a loss of host cell protein synthesis and cell death. We show for the first time that the cytotoxicity against cultured T cells by several RTA mutants is directly proportional to the enzyme activity of RTA, suggesting this is a reliable system to measure translocation effects. Large discrepancies between cytotoxicity and enzyme action for a given pair of toxins are therefore attributable to differences in cell binding, uptake, or membrane translocation. Fluid phase uptake and cytotoxicity of isolated RTA are essentially identical to that of the single chain toxin PAP. This important finding suggests that RTA, and the A chain of class 2 RIPs in general, has not evolved special translocation signals to complement the increased target cell binding facilitated by RTB. Experiments with the lectin RCA and with ebulin suggest those toxins have diminished cytotoxicity probably mediated by comparative deficiencies in B chain binding. Addition of a KDEL sequence to RTA increases fluid phase uptake, consistent with the notion that transport to the ER is important for cytotoxicity. Fusion of MBP or GST to the amino terminus of RTA has little effect on enzyme action or cytotoxicity. This result is not altered by protease inhibitors, suggesting the fusion proteins are probably not cleaved prior to translocation of the toxic A chain and implying that the toxins can carry large passenger proteins into the cytoplasm, an observation with interesting potential for analytical and therapeutic chemistry.

Differential sensitivity of HELA cells to the type 2 ribosome-inactivating proteins ebulin l, nigrin b and nigrin f as compared with ricin

The new type 2 RIPs ebulin l, nigrin b and nigrin f present in Sambucus display toxicity to HELA cells several orders of magnitude lower than that displayed by ricin. Despite N-terminal amino acid homology between the three RIPs in both the A and the B chains, these compounds display very different degrees of toxicity to HELA cells that does not seem to be paralleled by immunologic correlations. It is suggested that small changes in the protein structure are most probably responsible for the different degrees of toxicity.

Effects and molecular action of ribosome-inactivating proteins on ribosomes from Streptomyces lividans

The effects of 29 type 1 and 2 type 2 ribosome-inactivating proteins (RIPs) from plants on polyuridylic acid-directed polyphenylalanine synthesis carried out by purified ribosomes from Streptomyces lividans were studied. Only dianthin 32, saporins R1 and R3, momordin I, trichokirin, Hura crepitans RIP 5 from latex, crotins 2 and 3, and PAPs C, R, and S, inhibited polyphenylalanine synthesis. Both the type 2 RIPs ricin and volkensin were ineffective on translation. The magnesium concentration affected the inhibition of translation to a considerable extent. Upon treatment with inhibitory RIPs, extraction of rRNA and further treatment with acid aniline, S. lividans ribosomes released an RNA fragment of about 130 nucleotides. The 5' terminal sequence of this rRNA fragment was 5'-GAGGACCGGGACGGACGAACCUCUGGUGUGCCAGUUGU-3', similar to the sequence obtained in Escherichia coli. This indicates that the most probable molecular action of these RIPs on S. lividans and E. coli ribosomes is the same: depurination of the rRNA at a site relevant to the translation mechanism and that has been highly conserved throughout evolution.

Sensitivity of translation by Brevibacterium lactofermentum ribosomes to type 1 and type 2 ribosome-inactivating proteins

An active cell-free translation system was prepared from Brevibacterium lactofermentum, a Gram-positive bacteria used in molecular cloning and protein expression. The system contained high speed postribosomal supernatant (S 370), purified ribosomes and a tRNA mixture from Escherichia coli, and synthesized polyuridylic acid-directed polyphenylalanine once optimized for mono and divalent ions, time, and temperature. The translation system was evaluated for sensitivity to several translational inhibitors including several N-glycosidase ribosome-inactivating proteins (RIPs) isolated from plants. The pattern of inhibition by RIPs resembled that observed recently for Gram-negative bacteria such as Escherichia coli and Agrobacterium tumefaciens [Girbés et al., J. Bacteriol., 175, 6721-6724 (1993)]. A typical inhibitory type 1 RIP such as crotin 2 promoted depurination of the rRNA, which upon treatment with acid aniline released a fragment of approximately 230 nucleotides. On these grounds, we propose that bacterial ribosome sensitivity to plant RIPs depends on the bacterial ribosome-specific presence of protein recognition domains in the RIP present only in some RIP but not in others.

Intraocular irrigating solutions and vitrectomy-related changes (in protein, lactic and ascorbic acid) in rabbit vitreous

Vitrectomy and the subsequent vitreous fluid exchange alter the chemical and physical properties of the vitreous. This study assessed the time course alterations of total protein, ascorbic acid and lactic acid in the rabbit vitreous cavity following vitrectomy. Normal values were 2.2 +/- 1.2 mg/ml, 8.5 +/- 1.0 mg/100 ml and 16.8 +/- 3.5 mumol/ml, respectively. Nine groups of 5-10 adult albino rabbits underwent closed vitrectomy using physiologic saline, Ringer's lactate and BSS plus solutions. Samples were withdrawn at 3 and 24 h and 7 days after vitrectomy. Total protein levels increased in all groups at 3 and 24 h (20-30 mg/ml). Ascorbic acid measurements at 24 h rose only with saline and BSS plus. At 24 h the BSS plus group was the only one demonstrating an increased lactic acid concentration.

Inhibition of protein synthesis by (aminooxy)acetate in rat liver

(Aminooxy)acetate and D-cycloserine, two inhibitors of hepatic transamination reactions, inhibited also protein synthesis in isolated cells and postmitochondrial supernatants from rat liver. Both inhibitors acted in extracts only in concentrations higher than 1 mM. However, while D-cycloserine acted in isolated cells, as in extracts, (aminooxy)acetate inhibits protein synthesis in isolated cells by 50% of the control in the range 0.03-3 mM. NH+4 and H2O2, two by-products of (aminooxy)acetate degradation, inhibited protein synthesis in isolated liver cells, but at such a high concentration that the inhibition of protein synthesis carried out by (aminooxy)acetate cannot be explained by generation of these species. The results point out that the inhibitory action of (aminooxy)acetate on protein synthesis appears to require the integrity of the molecule.

Acute effects of ethanol in the control of protein synthesis in isolated rat liver cells

The acute effect of ethanol on hepatic protein synthesis is a rather controversial issue. In view of the conflicting reports on this subject, the effect of ethanol on protein labeling from L-[3H]valine in isolated liver cells was studied under a variety of experimental conditions. When tracer doses of the isotope were utilized, ethanol consistently decreased the rate of protein labeling, regardless of the metabolic conditions of the cells. This inhibition was not prevented by doses of 4-methylpyrazole large enough to abolish all the characteristic metabolic effects of ethanol, and it was not related to perturbations on the rates of L-valine transport and/or proteolysis. When ethanol was tested in the presence of saturating doses of L-[3H]valine no effect on protein labeling was observed. These observations suggest that the ethanol effect in decreasing protein labelling from tracer doses of the radioactive precursor does not reflect variations in the rate of protein synthesis but reflects changes in the specific activity of the precursor. These changes probably are secondary to variations in the dimensions of the amino acid pool utilized for protein synthesis. Even though it showed a lack of effect when tested alone, in the presence of saturating doses of the radioactive precursor ethanol inhibited the stimulatory effects on protein synthesis mediated by glucose and several gluconeogenic substrates. This effect of ethanol was not prevented by inhibitors of alcohol dehydrogenase, indicating that a shift of the NAD system to a more reduced state is not the mediator of its action. It is suggested that ethanol probably acted by changing the steady-state levels of some common effector(s) generated from the metabolism of all these fuels or else by preventing the inactivation of a translational repressor.

Polypeptide-chain elongation promoted by guanyl-5'-yl imidodiphosphate

In a purified system from Escherichia coli containing ribosomes complexed with poly(uridylic acid) and N-acetyl-phenylalanyl-tRNA, the nonhydrolyzable analog of GTP, guanyl-5'-yl imidodiphosphate (Guo-5'-P2-NH-P), promotes polypeptide synthesis at a rate several times slower than GTP. The activity is completely dependent on elongation factors EF-T (i.e, EF-Ts + EF-Tu) and EF-G. Examination of individual steps of the elongation cycle in partial reactions shows that Guo-5'-P2-NH-P is as efficient as GTP in promoting the EF-T-dependent binding of phenylalanyl-tRNA to the ribosomal A site. In contrast, Guo-5'-P2-NH-P promotes the translocation-dependent binding of phenylalanyl-tRNA to a ribosome complexed with A-site-bound N-acetyl-phenylalanyl-tRNA much more slowly than GTP. This slow rate of binding is due to the presence of EF-G on the ribosome, and not to sluggish translocation, since (a) the rate remains slow even after translocation of N-acetylphenylalanyl-tRNA is completed, (b) it is greatly speeded up by removal of EF-G from the reaction mixture (after translocation has occurred), and (c) it is slowed down again by readdition of the factor. Moreover, with post-translocated ribosomes and in the absence of EF-G, formation of dipeptide subsequent to the EF-T-dependent binding of phenylalanyl-tRNA is much slower when binding of this substrate has been promoted by Guo-5'-P2-NH-P than it is when promoted by GTP. The results suggest that, during polymerization with Guo-5'-P2-NH-P, EF-G and EF-Tu are slowly released from the ribosome and, consequently, the steps of the elongation cycle subsequent to translocation and aminoacyl-tRNA binding (aminoacyl-tRNA binding and peptide bond formation, respectively) are delayed. Thus, durong elongation cycle, GTP hydrolysis is probably essential for fast release of the factors from the ribosome.

A form of elongation factor G insensitive to N-ethyl-maleimide

Treatment of elongation factor G (EF-G) with the thiol reagent N-ethylmaleimide only partially inhibits (10 to 70%) the activity of the factor in (a) guanosine nucleotide-EF-G-ribosome complex formation, (b) uncoupled ribosome-dependent GTP hydrolysis, and (c) polypeptide synthesis. Moreover, a similar treatment of the factor with N-[3H]ethylmaleimide does not lead to 3H-label being associated with a GDP-EF-G-ribosome-fusidic acid complex. Thus, the results indicate the presence in EF-G preparations of a form of the factor that does not react with N-ethylmaleimide.