Fate and action of ricin in rat liver in vivo: translocation of endocytosed ricin into cytosol and induction of intrinsic apoptosis by ricin B‐chain

Cytotoxicity of many plant and bacterial toxins requires their endocytosis and retrograde transport from endosomes to the endoplasmic reticulum. Using cell fractionation and immunoblotting procedures, we have assessed the fate and action of the plant toxin ricin in rat liver in vivo, focusing on endosome‐associated events and induction of apoptosis. Injected ricin rapidly accumulated in endosomes as an intact A/B heterodimer (5–90 min) and was later (15–90 min) partially translocated to cytosol as A‐ and B‐chains. Unlike cholera and diphtheria toxins, which also undergo endocytosis in liver, neither in cell‐free endosomes loaded by ricin in vivo nor upon incubation with endosomal lysates did ricin undergo degradation in vitro. A time‐dependent translocation of ricin across the endosomal membrane occurred in cell‐free endosomes. Endosome‐located thioredoxin reductase‐1 was required for translocation as shown by its physical association with ricin chains and effects of its removal and inhibition. Ricin induced in vivo intrinsic apoptosis as judged by increased cytochrome c content, activation of caspase‐9 and caspase‐3, and enrichment of DNA fragments in cytosol. Furthermore, reduced ricin and ricin B‐chain caused cytochrome c release from mitochondria in vivo and in vitro, suggesting that the interaction of ricin B‐chain with mitochondria is involved in ricin‐induced apoptosis.     

An interaction between ricin and calreticulin that may have implications for toxin trafficking

Here we demonstrate that ricin is able to interact with the molecular chaperone calreticulin both in vitro and in vivo. The interaction occurred with ricin holotoxin, but not with free ricin A chain; and it was prevented in the presence of lactose, suggesting that it was mediated by the lectin activity of the ricin B chain. This lectin is galactose-specific, and metabolic labeling with [(3)H]galactose or treating galactose oxidase-modified calreticulin with sodium [(3)H]borohydride indicated that Vero cell calreticulin possesses a terminally galactosylated oligosaccharide. Brefeldin A treatment indicated that the intracellular interaction occurred initially in a post-Golgi stack compartment, possibly the trans-Golgi network, whereas the reductive separation of ricin subunits occurred in an earlier part of the secretory pathway, most probably the endoplasmic reticulum (ER). Intoxicating Vero cells with ricin whose A chain had been modified to include either a tyrosine sulfation site or the sulfation site plus available N-glycosylation sites, in the presence of Na(2)35SO(4), confirmed that calreticulin interacted with endocytosed ricin that had already undergone retrograde transport to both the Golgi and the ER. Although we cannot exclude the possibility that the interaction between ricin and calreticulin is an indirect one, the data presented are consistent with the idea that calreticulin may function as a recycling carrier for retrograde transport of ricin from the Golgi to the ER.     

Free Ricin A Chain Reaches an Early Compartment of the Secretory Pathway before It Enters the Cytosol

During the intoxication of mammalian cells by ricin, the catalytically active A chain must cross the membrane of an intracellular compartment in order to reach its ribosomal substrates in the cytosol. The actual site of ricin A chain translocation is unclear, and conflicting views hold that it enters the cytosol from endosomes or from an early compartment of the secretory pathway, possibly the lumen of the endoplasmic reticulum. Here we show that treating cells with brefeldin A, or transiently overexpressing mutant GTPases known to inhibit biochemical complexes mediating anterograde and retrograde transport between the endoplasmic reticulum and the Golgi complex, protected cells from intoxication by free ricin A chain. These data indicate that ricin A chain, either free or as part of intact ricin, reaches an early compartment of the secretory pathway before translocation into the cytosol occurs.     

Dislocation of ricin toxin A chains in human cells utilizes selective cellular factors

Ricin is a potent A-B toxin that is transported from the cell surface to the cytosol, where it inactivates ribosomes, leading to cell death. Ricin enters cells via endocytosis, where only a minute number of ricin molecules reach the endoplasmic reticulum (ER) lumen. Subsequently, the ricin A chain traverses the ER bilayer by a process referred to as dislocation or retrograde translocation to gain access to the cytosol. To study the molecular processes of ricin A chain dislocation, we have established, for the first time, a human cell system in which enzymatically attenuated ricin toxin A chains (RTA(E177D) and RTA(Δ177-181)) are expressed in the cell and directed to the ER. Using this human cell-based system, we found that ricin A chains underwent a rapid dislocation event that was quite distinct from the dislocation of a canonical ER soluble misfolded protein, null Hong Kong variant of α(1)-antitrypsin. Remarkably, ricin A chain dislocation occurred via a membrane-integrated intermediate and utilized the ER protein SEL1L for transport across the ER bilayer to inhibit protein synthesis. The data support a model in which ricin A chain dislocation occurs via a novel strategy of utilizing the hydrophobic nature of the ER membrane and selective ER components to gain access to the cytosol.     

Conformation-dependent antigenic determinants in the toxic lectin ricin.

The major part of the ricin-precipitable antibodies in sera produced by immunizing rabbits with formaldehyde-treated ricin is precipitated also by the isolated ricin A and B chains. In contrast, in antisera produced by immunizing with formaldehyde-treated ricinus agglutinin only a small part of the antibodies cross-reacting with ricin can be precipitated by the isolated A and B chains, or bound to immunoabsorbents containing the isolated ricin chains. In immunodiffusion studies with anti-ricinus agglutinin sera, a star-shaped precipitate was formed when isolated A and B chains recombined to form intact ricin. Both anti-ricin and anti-ricinus agglutinin sera neutralized effectively the ability of ricin to inhibit protein synthesis in HeLa cells. Anti-ricin serum also neutralized the inhibitory effect of the isolated A chain on protein synthesis in a cell-free system and the ability of the isolated B chain to induce indirect hemagglutination. In contrast, antiricinus agglutinin serum did not neutralize the biologic activities of the isolated ricin A and B chains. Anti-ricinus agglutinin serum formed a precipitate with the hybrid ricin A chain/abrin B chain, and protected against the toxic effect on HeLa cells of this hybrid, indicating conformational changes of ricin A chain upon binding to the B chain. It is concluded that the anti-ricinus agglutinin serum contains antibodies directed against conformational determinants present on intact ricin, but not present or exposed in the isolated A and B chains. At least part of these conformational determinants appears to be carried by the A chain.     

Cell surface and intracellular functions for ricin galactose binding.

The role of the two galactose binding sites of ricin B chain in ricin toxicity was evaluated by studying a series of ricin point mutants. Wild-type (WT) ricin and three ricin B chain point mutants having mutations in either 1) the first galactose binding domain (site 1 mutant, Met in place of Lys-40 and Gly in place of Asn-46), 2) the second galactose binding domain (site 2 mutant, Gly in place of Asn-255), or 3) both galactose binding domains (double site mutant containing all three amino acid replacements formerly stated) were expressed in Xenopus oocytes and then reassociated with recombinant ricin A chain. The different ricin B chains were mannosylated to the same extent. Cytotoxicity of these toxins was evaluated when cell entry was mediated either by galactose-containing receptors or through an alternate receptor, the mannose receptor of macrophages. WT ricin and each of the single domain mutants was able to kill Vero cells following uptake by galactose containing receptors. Lactose blocked the toxicity of each of these ricins. Site 1 and 2 mutants were 20-40 times less potent than WT ricin, and the double site mutant had no detectable cytotoxicity. WT ricin, the site 1 mutant, and the site 2 mutant also inhibited protein synthesis of mannose receptor-containing cells. Ricin can enter these cells through either a cell-surface galactose-containing receptor or through the mannose receptor. By including lactose in the cell medium, galactose-containing receptor-mediated uptake is blocked and cytotoxicity occurs solely via the mannose receptor. WT ricin, site 1, and site 2 mutants were cytotoxic to macrophages in the presence of lactose with the relative potency, WT greater than site 2 mutant greater than site 1 mutant. The double site mutant lacked cytotoxicity either in the absence or presence of lactose. Thus, even for mannose receptor-mediated toxicity of ricin, at least one galactose binding site remains necessary for cytotoxicity and two galactose binding sites further increases potency. These results are consistent with the model that the ricin B chain galactose binding activity plays a role not only in cell surface binding but also intracellularly for ricin cytotoxicity.     

Expression and functional properties of genetically engineered ricin B chain lacking galactose-binding activity.

Ricin is a potent plant toxin consisting of two disulfide-bonded subunits. The A chain of ricin is an N-glycosidase which inactivates 28 S RNA and inhibits protein synthesis. The B chain is a galactose-specific lectin with two galactose-binding sites. The genes encoding preproricin and its A and B chains have been cloned and expressed. In addition, X-ray crystallographic studies have identified the galactose-contact residues in both the high- and low-affinity galactose-binding sites of the B chain. In this study, the high-affinity galactose-contact residue of the B chain was changed from Asn-255 to Ala-255 by oligonucleotide-directed mutagenesis. The resulting mutant was sequenced to confirm the presence of a single mutation and was expressed in Cos-M6 cells. Both wild-type and mutant recombinant B chain could be immunoprecipitated with a heterologous anti-B chain antibody and both could form A-B heterodimers. However, as compared to the wild-type, the mutant B chain lacked more than 99% of its lectin activity and cytotoxicity as an A-B dimer. In conclusion, altering the contact residue of the high-affinity galactose-binding site of ricin B chain from Asn-255 to Ala-255 abrogates more than 99% of its lectin activity and the cytotoxicity of the A-B heterodimer to ricin-sensitive cells.     

Identification of three oligosaccharide binding sites in ricin.

The galactoside-binding sites of ricin B chain can be blocked by affinity-directed chemical modification using a reactive ligand derived from asialoglycopeptides containing triantennary N-linked oligosaccharides. The terminal galactosyl residue of one branch of the triantennary oligosaccharide is modified to contain a reactive dichlorotriazine moiety. Two separate galactoside-binding sites have been clearly established in the ricin B chain by X-ray crystallography [Rutenber, E., and Robertus, J. D. (1991) Proteins 10, 260-269], and it is necessary to covalently attach two such reactive ligands to the B chain to block its binding to galactoside affinity matrixes. A method was developed using thiol-specific labeling of the ligand combined with subsequent immunoaffinity chromatography which allowed the isolation of ricin B chain peptides covalently linked to the ligand from proteolytic digests of purified blocked ricin. The sites of covalent attachment of the two ligands in blocked ricin were inferred from sequence analysis to be Lys 62 in domain 1 of the B chain and Tyr 148 in domain 2. A minor species of blocked ricin contains a third covalently attached ligand. From the analysis of peptides derived from blocked ricin enriched in this species, it is inferred that Tyr 67 in domain 1 is the specific site on the ricin B chain where a third reactive ligand becomes covalently linked to the protein. These results are interpreted as providing support for the notion that the ricin B chain has three oligosaccharide binding sites.     

Evidence for a nucleation step in a lipid-protein interaction-kinetics of the incorporation of polymyxin into phosphatidic acid bilayer vesicles

Incubation of 8-anilino-1-naphthalene sulfonic acid with ricin and its isolated A and B polypeptide chains showed an increase in fluorescence at 470 nm. The A chain induced more fluorescence enhancement than either ricin or ricin B chain. The addition of B chain to A chain resulted in decreased fluorescence enhancement which was pH dependent. Sephadex gel filtration showed that A and B chain efficiently reassociated and the reassociation was not dependent on formation of the interchain disulfide bond and could not be prevented by high salt concentration.     

Homology between ricin and Ricinus communie agglutinin: Amino terminal sequence analysis and protein synthesis inhibition studies

The existence of three forms of ricin and two forms of the Ricinus communis agglutinin (RCA) was established using cation exchange chromatography, isoelectric focusing, and polyacrylamide gel electrophoresis. The preparation of the RCA we obtained was 60–75 times more potent than ricin in the agglutination of erythrocytes, but was about 4% as effective as an inhibitor of cell-free protein synthesis. When reduced with 2-mercaptoethanol, the RCA was activated 3000-fold as an inhibitor of cell-free protein synthesis, whereas ricin was activated about 600-fold by the same treatment. A mixture of the RCA A chains was about one-fifth as effective as the ricin A chain in the inhibition of cell-free protein synthesis. The purified polypeptide subunits of the castor bean lectins were subjected to automated Edman degradation. The sequence for 17 of the first 19 residues of the agglutinin A chain was determined. The first seven residues of the ricin A chain were determined and they are identical with those of the RCA A chain. Nineteen turns of Edman degradation on the RCA B chain resulted in the identification of 18 amino acids. The sequence determined for the first 17 residues of the ricin B chain was identical with that of the RCA B chain. It is likely that the identity of the ricin/RCA A and B chain sequences extends further along the polypeptide chains than the sequences we have determined. The similar structural and catalytic potentials of the RCA and ricin suggest that they bear a precursor-product relationship.     

Blocked and non-blocked ricin immunotoxins against the CD4 antigen exhibit higher cytotoxic potency than a ricin A chain immunotoxin potentiated with ricin B chain or with a ricin B chain immunotoxin

Summary An immunotoxin consisting of ricin A chain linked to the monoclonal antibody M-T151, recognising the CD4 antigen, was weakly toxic to the human T-lymphoblastoid cell line CEM in tissue culture. The incorporation of [³H]leucine by CEM cells was inhibited by 50% at an M-T151-ricin-A-chain concentration (IC₅₀) of 4.6 nM compared with an IC₅₀ of 1.0 pM for ricin. In contrast, immunotoxins made by linking intact ricin to M-T151 in such a way that the galactose-binding sites of the B chain subunit were either blocked sterically by the antibody component or were left unblocked, were both powerfully cytotoxic with IC₅₀ values of 20–30 pM. The addition of ricin B chain to CEM cells treated with M-T151—ricin-A-chain enhanced cytotoxicity by only eight-fold indicating that isolated B chain potentiated the action of the A chain less effectively than it did as an integral component of an intact ricin immunotoxin. Ricin B chain linked to goat anti-(mouse immunoglobulin) also potentiated weakly.Lactose completely inhibited the ability of isolated ricin B chain to potentiate the cytotoxicity of M-T151—ricin-A-chain and partially (3- to 4-fold) inhibited the cytotoxicity of the blocked and non-blocked ricin immunotoxins. Thus, in this system, the galactose-binding sites of the B chain contributed to cell killing regardless of whether isolated B chain was associated with the A chain immunotoxin or was present in blocked or non-blocked form as part of an intact ricin immunotoxin. The findings suggest that the blocked ricin immunotoxin may become unblocked after binding to the target antigen to re-expose the cryptic galactose-binding sites. However, the unblocking cannot be complete because the maximal inhibition of [³H]leucine incorporation by the blocked immunotoxin was only 80% compared with greater than 99% inhibition by the non-blocked immunotoxin.     

Modification of ricin A chain, by addition of endoplasmic reticulum (KDEL) or Golgi (YQRL) retention sequences, enhances its cytotoxicity and translocation

 A pKK expression system in Escherichia coli was used to produce recombinant ricin A chain (rRTA) and rRTA modified by addition of organelle-specific amino acid retention sequences, including KDEL (an endoplasmic reticulum, ER, lumen retention signal), KKMP (an ER membrane retention signal), YQRL (a trans-Golgi network retention signal) and KFERQ (a lysosome-targeting signal) to the C terminus of rRTA. The toxicities of these RTA mutants were assessed in Jurkat cells following fluid-phase endocytosis. rRTA-KDEL and rRTA-YQRL were significantly more cytotoxic for Jurkat cells than rRTA, rRTA-KKMP or rRTA-KFERQ. This difference did not result from signal(KDEL or YQRL)-mediated binding of these RTA mutants to the cell surface. Reconstituted ER and Golgi vesicles have been employed to assess translocation of rRTA and mutant rRTA. RTA-KDEL and RTA-YQRL respectively exhibited 6.7-fold and 6.1-fold more protection against papain digestion in reconstituted ER vesicles and 2.2-fold and 1.8-fold more protection in reconstituted Golgi vesicles, than unmodified rRTA. These mutants were reassociated with ricin B chain to form holotoxins. The mutant RTA-KDEL and RTA-YQRL holotoxins were 3.8-fold and 1.5-fold more cytotoxic for target cells, respectively, than ricin produced using unmodified rRTA. Our results suggest that both ER and the trans-Golgi network may play important roles in the intracellular trafficking and translocation of ricin A chain. Received: 14 August 1997 / Accepted: 14 October 1997     

Novel sugar-binding specificity of the type XIII xylan-binding domain of a family F/10 xylanase from Streptomyces olivaceoviridis E-86

The type XIII xylan-binding domain (XBD) of a family F/10 xylanase (FXYN) from Streptomyces olivaceoviridis E-86 was found to be structurally similar to the ricin B chain which recognizes the non-reducing end of galactose and specifically binds to galactose containing sugars. The crystal structure of XBD [Fujimoto, Z. et al. (2000) J. Mol. Biol. 300, 575-585] indicated that the whole structure of XBD is very similar to the ricin B chain and the amino acids which form the galactose-binding sites are highly conserved between the XBD and the ricin B chain. However, our investigation of the binding abilities of wt FXYN and its truncated mutants towards xylan demonstrated that the XBD bound xylose-based polysaccharides. Moreover, it was found that the sugar-binding unit of the XBD was a trimer, which was demonstrated in a releasing assay using sugar ranging in size from xylose to xyloheptaose. These results indicated that the binding specificity of the XBD was different from those of the same family lectins such as the ricin B chain. Somewhat surprisingly, it was found that lactose could release the XBD from insoluble xylan to a level half of that observed for xylobiose, indicating that the XBD also possessed the same galactose recognition site as the ricin B chain. It appears that the sugar-binding pocket of the XBD has evolved from the ancient ricin super family lectins to bind additional sugar targets, resulting in the differences observed in the sugar-binding specificities between the lectin group (containing the ricin B chain) and the enzyme group.     

The specific binding of ricin and its polypeptide chains to rat liver ribosomes and ribosomal subunits

Ricin from Ricinus communis was isolated and the binding of ³H-reductively alkylated or ¹²⁵I-iodinated ricin was studied by incubating the toxic protein with ribosomes and isolating the ricin-ribosome complex by centrifugation. Neither of the labeled ricin derivatives nor ³H-labeled A chain bound Escherichia coli ribosomes, but both bound rat liver ribosomes in a reproducible manner. ³H-labeled ricin bound in a ratio of 1 mol/mol of ribosomes with a dissociation constant of 3 μm as calculated from a Scatchard plot. Similarly, ³H-labeled B chain isolated from ricin also bound in a one-to-one complex with a dissociation constant of 1 μm. The binding of ricin and ricin B chain was sensitive to lactose, while the binding of reduced ricin or ricin A chain was not prevented by lactose. Reduced ¹²⁵I-labeled ricin in the presence of lactose and ³H-labeled A chain bound with a ratio of 2 mol/mol of ribosomes. It was further demonstrated that ³H-labeled ricin A chain bound only to the 60S ribosomal subunit and not to the 40S ribosomal subunit. The dissociation constant for the binding was 2 μm both in the presence and absence of lactose and 2 mol of A chain were bound per mole of 60S ribosomal subunit.     

Endoplasmic reticulum-associated degradation of ricin A chain has unique and plant-specific features

Proteins that fail to fold in the endoplasmic reticulum (ER) or cannot find a pattern for assembly are often disposed of by a process named ER-associated degradation (ERAD), which involves transport of the substrate protein across the ER membrane (dislocation) followed by rapid proteasome-mediated proteolysis. Different ERAD substrates have been shown to be ubiquitinated during or soon after dislocation, and an active ubiquitination machinery has been found to be required for the dislocation of certain defective proteins. We have previously shown that, when expressed in tobacco (Nicotiana tabacum) protoplasts, the A chain of the heterodimeric toxin ricin is degraded by a pathway that closely resembles ERAD but is characterized by an unusual uncoupling between the dislocation and the degradation steps. Since lysine (Lys) residues are a major target for ubiquitination, we have investigated the effects of changing the Lys content on the retrotranslocation and degradation of ricin A chain in tobacco protoplasts. Here we show that modulating the number of Lys residues does not affect recognition events within the ER lumen nor the transport of the protein from this compartment to the cytosol. Rather, the introduced modifications have a clear impact on the degradation of the dislocated protein. While the substitution of the two Lys residues present in ricin A chain with arginine slowed down degradation, the introduction of four extra lysyl residues had an opposite effect and converted the ricin A chain to a standard ERAD substrate that is disposed via a process in which dislocation and degradation steps are tightly coupled.     

Expression of ricin B chain in Escherichia coli

DNA encoding ricin B chain was fused to that encoding the E. coli OmpA signal peptide using the expression secretion vector pIN-111-ompA. When induced, E. coli cells transformed with the recombinant plasmid express ricin B chain. The recombinant product accumulates in the periplasmic space in a soluble, biologically active form.     

Ricinus communis agglutinin B chain contains a fucosylated oligosaccharide side chain not present on ricin B chain

Ricinus communis agglutinin (RCA) B chain, in contrast to ricin B chain, contains fucose. Since both RCA and ricin B chain lose two oligosaccharide side chains when treated with β-endo N-acetylglucosaminidase H, it is proposed that fucose is present on a third oligosaccharide. This third oligosaccharide is not present on the ricin B chain and accounts for the larger relative molecular mass of the RCA B chain.Ricinus communis agglutininRicinB chainFucose     

The tRNA N2,N2-dimethylguanosine-26 methyltransferase encoded by gene trm1 increases efficiency of suppression of an ochre codon in Schizosaccharomyces pombe

The plant toxin ricin has proven valuable as a membrane marker in studies of endocytosis as well as studies of different intracellular transport steps. The toxin, which consists of two polypeptide chains, binds by one chain (the B-chain) to both glycolipids and glycoproteins with terminal galactose at the cell surface. The other chain (the A-chain) enters the cytosol and inhibits protein synthesis enzymatically. After binding the toxin is endocytosed by different mechanisms, and it is transported via endosomes to the Golgi apparatus and the endoplasmic reticulum before translocation of the A-chain to the cytosol. The different transport steps have been analyzed by studying trafficking of ricin as well as modified ricin molecules.     

Ricin A chain reaches the endoplasmic reticulum after endocytosis

Ricin is a potent ribosome inactivating protein and now has been widely used for synthesis of immunotoxins. To target ribosome in the mammalian cytosol, ricin must firstly retrograde transport from the endomembrane system to reach the endoplasmic reticulum (ER) where the ricin A chain (RTA) is recognized by ER components that facilitate its membrane translocation to the cytosol. In the study, the fusion gene of enhanced green fluorescent protein (EGFP)-RTA was expressed with the pET-28a (+) system in Escherichia coli under the control of a T7 promoter. The fusion protein showed a green fluorescence. The recombinant protein can be purified by metal chelated affinity chromatography on a column of NTA. The rabbit anti-GFP antibody can recognize the fusion protein of EGFP-RTA just like the EGFP protein. The cytotoxicity of EGFP-RTA and RTA was evaluated by the MTT assay in HeLa and HEP-G2 cells following fluid-phase endocytosis. The fusion protein had a similar cytotoxicity of RTA. After endocytosis, the subcellular location of the fusion protein can be observed with the laser scanning confocal microscopy and the immuno-gold labeling Electro Microscopy. This study provided important evidence by a visualized way to prove that RTA does reach the endoplasmic reticulum.