Addition of an endoplasmic reticulum retrieval sequence to ricin A chain significantly increases its cytotoxicity to mammalian cells

Targeting the Inside of Cells with Biologicals: Toxin Routes in a Therapeutic Context

Numerous toxins translocate to the cytosol in order to fulfil their function. This demonstrates the existence of routes for proteins from the extracellular space to the cytosol. Understanding these routes is relevant to multiple aspects related to therapeutic applications. These include the development of anti-toxin treatments, the potential use of toxins as shuttles for delivering macromolecular cargo to the cytosol or the use of drugs based on toxins. Compared with other strategies for delivery, such as chemicals as carriers for macromolecular delivery or physical methods like electroporation, toxin routes present paths into the cell that potentially cause less damage and can be specifically targeted. The efficiency of delivery via toxin routes is limited. However, low-delivery efficiencies can be entirely sufficient, if delivered cargoes possess an amplification effect or if very few molecules are sufficient for inducing the desired effects. This is known for example from RNA-based vaccines that have been developed during the coronavirus disease 2019 pandemic as well as for other approved RNA-based drugs, which elicited the desired effect despite their typically low delivery efficiencies. The different mechanisms by which toxins enter cells may have implications for their technological utility. We review the mechanistic principles of the translocation pathway of toxins from the extracellular space to the cytosol, the delivery efficiencies, and therapeutic strategies or applications that exploit toxin routes for intracellular delivery.

Construction and characterization of the recombinant immunotoxin RTA-4D5-KDEL targeting HER2/neu-positive cancer cells and locating the endoplasmic reticulum

The specific targeting of immunotoxins enables their wide application in cancer therapy. The A-chain of the ricin protein (RTA) is an N-glycosidase that catalyzes the removal of adenine from the 28S rRNA, preventing protein translation and leading to cell death. Ricin is highly toxic but can only exert its toxic effects from within the cytoplasm. In this study, we linked the anti-HER2 single-chain variable fragment 4D5 scFv and the endoplasmic reticulum-targeting peptide KDEL to the C-terminal of the RTA to construct immunotoxin RTA-4D5-KDEL. In vitro experiments showed that the anticancer effect of RTA-4D5-KDEL towards ovarian cancer cells SKOV-3 increased 440-fold and 28-fold relative to RTA and RTA-4D5, respectively. RTA-4D5-KDEL had a strong inhibitory effect on HER2-overexpressing SKOV-3 cells and caused little damage to normal HEK-293 cells and H460 lung cancer cells. Immunofluorescence experiments showed that the immunotoxin RTA-4D5 could specifically bind to SKOV-3 cells, but not to normal cells HEK-293. The immunotoxin RTA-4D5-KDEL could rapidly localize the recombinant protein to the endoplasmic reticulum. These results suggest that the recombinant immunotoxin RTA-4D5-KDEL has a strong inhibitory effect on ovarian cancer cells that overexpress HER2 but little harm to the normal cells.

Selective CXCR4+ Cancer Cell Targeting and Potent Antineoplastic Effect by a Nanostructured Version of Recombinant Ricin

Under the unmet need of efficient tumor-targeting drugs for oncology, a recombinant version of the plant toxin ricin (the modular protein T22-mRTA-H6) is engineered to self-assemble as protein-only, CXCR4-targeted nanoparticles. The soluble version of the construct self-organizes as regular 11 nm planar entities that are highly cytotoxic in cultured CXCR4⁺ cancer cells upon short time exposure, with a determined IC50 in the nanomolar order of magnitude. The chemical inhibition of CXCR4 binding sites in exposed cells results in a dramatic reduction of the cytotoxic potency, proving the receptor-dependent mechanism of cytotoxicity. The insoluble version of T22-mRTA-H6 is, contrarily, moderately active, indicating that free, nanostructured protein is the optimal drug form. In animal models of acute myeloid leukemia, T22-mRTA-H6 nanoparticles show an impressive and highly selective therapeutic effect, dramatically reducing the leukemia cells affectation of clinically relevant organs. Functionalized T22-mRTA-H6 nanoparticles are then promising prototypes of chemically homogeneous, highly potent antitumor nanostructured toxins for precise oncotherapies based on self-mediated intracellular drug delivery.

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

Plant ribosome-inactivating protein (RIP) toxins are EC3.2.2.22 N-glycosidases, found among most plant species encoded as small gene families, distributed in several tissues being endowed with defensive functions against fungal or viral infections. The two main plant RIP classes include type I (monomeric) and type II (dimeric) as the prototype ricin holotoxin from Ricinus communis that is composed of a catalytic active A chain linked via a disulphide bridge to a B-lectin domain that mediates efficient endocytosis in eukaryotic cells. Plant RIPs can recognize a universally conserved stem-loop, known as the α-sarcin/ ricin loop or SRL structure in 23S/25S/28S rRNA. By depurinating a single adenine (A4324 in 28S rat rRNA), they can irreversibly arrest protein translation and trigger cell death in the intoxicated mammalian cell. Besides their useful application as potential weapons against infected/tumor cells, ricin was also used in bio-terroristic attacks and, as such, constitutes a major concern. In this review, we aim to summarize past studies and more recent progresses made studying plant RIPs and discuss successful approaches that might help overcoming some of the bottlenecks encountered during the development of their biomedical applications.

Glycosylated Triterpenoids as Endosomal Escape Enhancers in Targeted Tumor Therapies

Protein-based targeted toxins play an increasingly important role in targeted tumor therapies. In spite of their high intrinsic toxicity, their efficacy in animal models is low. A major reason for this is the limited entry of the toxin into the cytosol of the target cell, which is required to mediate the fatal effect. Target receptor bound and internalized toxins are mostly either recycled back to the cell surface or lysosomally degraded. This might explain why no antibody-targeted protein toxin has been approved for tumor therapeutic applications by the authorities to date although more than 500 targeted toxins have been developed within the last decades. To overcome the problem of insufficient endosomal escape, a number of strategies that make use of diverse chemicals, cell-penetrating or fusogenic peptides, and light-induced techniques were designed to weaken the membrane integrity of endosomes. This review focuses on glycosylated triterpenoids as endosomal escape enhancers and throws light on their structure, the mechanism of action, and on their efficacy in cell culture and animal models. Obstacles, challenges, opportunities, and future prospects are discussed.

Yeast Reporter Assay to Identify Cellular Components of Ricin Toxin A Chain Trafficking

RTA, the catalytic A-subunit of the ribosome inactivating A/B toxin ricin, inhibits eukaryotic protein biosynthesis by depurination of 28S rRNA. Although cell surface binding of ricin holotoxin is mainly mediated through its B-subunit (RTB), sole application of RTA is also toxic, albeit to a significantly lower extent, suggesting alternative pathways for toxin uptake and transport. Since ricin toxin trafficking in mammalian cells is still not fully understood, we developed a GFP-based reporter assay in yeast that allows rapid identification of cellular components required for RTA uptake and subsequent transport through a target cell. We hereby show that Ypt6p, Sft2p and GARP-complex components play an important role in RTA transport, while neither the retromer complex nor COPIB vesicles are part of the transport machinery. Analyses of yeast knock-out mutants with chromosomal deletion in genes whose products regulate ADP-ribosylation factor GTPases (Arf-GTPases) and/or retrograde Golgi-to-ER (endoplasmic reticulum) transport identified Sso1p, Snc1p, Rer1p, Sec22p, Erv46p, Gea1p and Glo3p as novel components in RTA transport, suggesting the developed reporter assay as a powerful tool to dissect the multistep processes of host cell intoxication in yeast.

Augmenting the Efficacy of Immunotoxins and Other Targeted Protein Toxins by Endosomal Escape Enhancers

The toxic moiety of almost all protein-based targeted toxins must enter the cytosol of the target cell to mediate its fatal effect. Although more than 500 targeted toxins have been investigated in the past decades, no antibody-targeted protein toxin has been approved for tumor therapeutic applications by the authorities to date. Missing efficacy can be attributed in many cases to insufficient endosomal escape and therefore subsequent lysosomal degradation of the endocytosed toxins. To overcome this drawback, many strategies have been described to weaken the membrane integrity of endosomes. This comprises the use of lysosomotropic amines, carboxylic ionophores, calcium channel antagonists, various cell-penetrating peptides of viral, bacterial, plant, animal, human and synthetic origin, other organic molecules and light-induced techniques. Although the efficacy of the targeted toxins was typically augmented in cell culture hundred or thousand fold, in exceptional cases more than million fold, the combination of several substances harbors new problems including additional side effects, loss of target specificity, difficulties to determine the therapeutic window and cell type-dependent variations. This review critically scrutinizes the chances and challenges of endosomal escape enhancers and their potential role in future developments.

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

Cargo binding promotes KDEL receptor clustering at the mammalian cell surface

Transmembrane receptor clustering is a ubiquitous phenomenon in pro- and eukaryotic cells to physically sense receptor/ligand interactions and subsequently translate an exogenous signal into a cellular response. Despite that receptor cluster formation has been described for a wide variety of receptors, ranging from chemotactic receptors in bacteria to growth factor and neurotransmitter receptors in mammalian cells, a mechanistic understanding of the underlying molecular processes is still puzzling. In an attempt to fill this gap we followed a combined experimental and theoretical approach by dissecting and modulating cargo binding, internalization and cellular response mediated by KDEL receptors (KDELRs) at the mammalian cell surface after interaction with a model cargo/ligand. Using a fluorescent variant of ricin toxin A chain as KDELR-ligand (eGFP-RTA^H/KDEL), we demonstrate that cargo binding induces dose-dependent receptor cluster formation at and subsequent internalization from the membrane which is associated and counteracted by anterograde and microtubule-assisted receptor transport to preferred docking sites at the plasma membrane. By means of analytical arguments and extensive numerical simulations we show that cargo-synchronized receptor transport from and to the membrane is causative for KDELR/cargo cluster formation at the mammalian cell surface.

Rescue and Stabilization of Acetylcholinesterase in Skeletal Muscle by N-terminal Peptides Derived from the Noncatalytic Subunits

The vast majority of newly synthesized acetylcholinesterase (AChE) molecules do not assemble into catalytically active oligomeric forms and are rapidly degraded intracellularly by the endoplasmic reticulum-associated protein degradation pathway. We have previously shown that AChE in skeletal muscle is regulated in part post-translationally by the availability of the noncatalytic subunit collagen Q, and others have shown that expression of a 17-amino acid N-terminal proline-rich attachment domain of collagen Q is sufficient to promote AChE tetramerization in cells producing AChE. In this study we show that muscle cells, or cell lines expressing AChE catalytic subunits, incubated with synthetic proline-rich attachment domain peptides containing the endoplasmic reticulum retrieval sequence KDEL take up and retrogradely transport them to the endoplasmic reticulum network where they induce assembly of AChE tetramers. The peptides act to enhance AChE folding thereby rescuing them from reticulum degradation. This enhanced folding efficiency occurs in the presence of inhibitors of protein synthesis and in turn increases total cell-associated AChE activity and active tetramer secretion. Pulse-chase studies of isotopically labeled AChE molecules show that the enzyme is rescued from intracellular degradation. These studies provide a mechanistic explanation for the large scale intracellular degradation of AChE previously observed and indicate that simple peptides alone can increase the production and secretion of this critical synaptic enzyme in muscle tissue.

Immunotoxins: the role of the toxin

Immunotoxins are antibody-toxin bifunctional molecules that rely on intracellular toxin action to kill target cells. Target specificity is determined via the binding attributes of the chosen antibody. Mostly, but not exclusively, immunotoxins are purpose-built to kill cancer cells as part of novel treatment approaches. Other applications for immunotoxins include immune regulation and the treatment of viral or parasitic diseases. Here we discuss the utility of protein toxins, of both bacterial and plant origin, joined to antibodies for targeting cancer cells. Finally, while clinical goals are focused on the development of novel cancer treatments, much has been learned about toxin action and intracellular pathways. Thus toxins are considered both medicines for treating human disease and probes of cellular function.

Adapting yeast as model to study ricin toxin a uptake and trafficking

The plant A/B toxin ricin represents a heterodimeric glycoprotein belonging to the family of ribosome inactivating proteins, RIPs. Its toxicity towards eukaryotic cells results from the depurination of 28S rRNA due to the N-glycosidic activity of ricin toxin A chain, RTA. Since the extention of RTA by a mammalian-specific endoplasmic reticulum (ER) retention signal (KDEL) significantly increases RTA in vivo toxicity against mammalian cells, we here analyzed the phenotypic effect of RTA carrying the yeast-specific ER retention motif HDEL. Interestingly, such a toxin (RTA^HDEL) showed a similar cytotoxic effect on yeast as a corresponding RTA^KDEL variant on HeLa cells. Furthermore, we established a powerful yeast bioassay for RTA in vivo uptake and trafficking which is based on the measurement of dissolved oxygen in toxin-treated spheroplast cultures of S. cerevisiae. We show that yeast spheroplasts are highly sensitive against external applied RTA and further demonstrate that its toxicity is greatly enhanced by replacing the C-terminal KDEL motif by HDEL. Based on the RTA resistant phenotype seen in yeast knock-out mutants defective in early steps of endocytosis (∆end3) and/or in RTA depurination activity on 28S rRNA (∆rpl12B) we feel that the yeast-based bioassay described in this study is a powerful tool to dissect intracellular A/B toxin transport from the plasma membrane through the endosomal compartment to the ER.

Stability and sugar recognition ability of ricin-like carbohydrate binding domains

Lectins are a class of proteins known for their novel binding to saccharides. Understanding this sugar recognition process can be crucial in creating structure-based designs of proteins with various biological roles. We focus on the sugar binding of a particular lectin, ricin, which has two β-trefoil carbohydrate-binding domains (CRDs) found in several plant protein toxins. The binding ability of possible sites of ricin-like CRD has been puzzling. The apo and various (multiple) ligand-bound forms of the sugar-binding domains of ricin were studied by molecular dynamics simulations. By evaluating structural stability, hydrogen bond dynamics, flexibility, and binding energy, we obtained a detailed picture of the sugar recognition of the ricin-like CRD. Unlike what was previously believed, we found that the binding abilities of the two known sites are not independent of each other. The binding ability of one site is positively affected by the other site. While the mean positions of different binding scenarios are not altered significantly, the flexibility of the binding pockets visibly decreases upon multiple ligand binding. This change in flexibility seems to be the origin of the binding cooperativity. All the hydrogen bonds that are strong in the monoligand state are also strong in the double-ligand complex, although the stability is much higher in the latter form due to cooperativity. These strong hydrogen bonds in a monoligand state are deemed to be the essential hydrogen bonds. Furthermore, by examining the structural correlation matrix, the two domains are structurally one entity. Galactose hydroxyl groups, OH4 and OH3, are the most critical parts in both site 1α and site 2γ recognition.

Converting a marginally hydrophobic soluble protein into a membrane protein

δ-Helices are marginally hydrophobic α-helical segments in soluble proteins that exhibit certain sequence characteristics of transmembrane (TM) helices [Cunningham, F., Rath, A., Johnson, R. M. & Deber, C. M. (2009). Distinctions between hydrophobic helices in globular proteins and TM segments as factors in protein sorting. J. Biol. Chem., 284, 5395-402]. In order to better understand the difference between δ-helices and TM helices, we have studied the insertion of five TM-like δ-helices into dog pancreas microsomal membranes. Using model constructs in which an isolated δ-helix is engineered into a bona fide membrane protein, we find that, for two δ-helices originating from secreted proteins, at least three single-nucleotide mutations are necessary to obtain efficient membrane insertion, whereas one mutation is sufficient in a δ-helix from the cytosolic protein P450BM-3. We further find that only when the entire upstream region of the mutated δ-helix in the intact cytochrome P450BM-3 is deleted does a small fraction of the truncated protein insert into microsomes. Our results suggest that upstream portions of the polypeptide, as well as embedded charged residues, protect δ-helices in globular proteins from being recognized by the signal recognition particle-Sec61 endoplasmic-reticulum-targeting machinery and that δ-helices in secreted proteins are mutationally more distant from TM helices than δ-helices in cytosolic proteins.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.

Ricin A-chain requires c-Jun N-terminal kinase to induce apoptosis in nontransformed epithelial cells

Ricin is a toxin isolated from castor beans that has potential as a weapon of bioterrorism. This glycoprotein consists of an A-chain (RTA) that damages the ribosome and inhibits protein synthesis and a B-chain that plays a role in cellular uptake. Ricin activates the c-Jun N-terminal kinase (JNK) and p38 signaling pathways; however, a role for these pathways in ricin-induced cell death has not been investigated. Our goals were to determine if RTA alone could activate apoptosis and if the JNK and p38 pathways were required for this response. Comparable caspase activation was observed with both ricin and RTA treatment in the immortalized, nontransformed epithelial cell line, MAC-T. Ribosome depurination and inhibition of protein synthesis were induced in 2-4h with 1microg/ml RTA and within 4-6h with 0.1microg/ml RTA. Apoptosis was not observed until 4h of treatment with either RTA concentration. RTA activated JNK and p38 in a time- and concentration-dependent manner that preceded increases in apoptosis. Inhibition of the JNK pathway reduced RTA-induced caspase activation and poly(ADP-ribose) polymerase cleavage. In contrast, inhibition of the p38 pathway had little effect on RTA-induced caspase 3/7 activation. These studies are the first to demonstrate a role for the JNK signaling pathway in ricin-induced cell death. In addition, the MAC-T cell line is shown to be a sensitive in vitro model system for future studies using RTA mutants to determine relationships between RTA-induced depurination, ribotoxic stress, and apoptosis in normal epithelial cells.

Saporin suicide gene therapy

New genes useful in suicide gene therapy are those encoding toxins such as plant ribosome-inactivating proteins (RIPs), which can irreversibly block protein synthesis, triggering apoptotic cell death. Plasmids expressing a cytosolic saporin (SAP) gene from common soapwort (Saponaria officinalis) are generated by placing the region encoding the mature plant toxin under the control of strong viral promoters and may be placed under tumor-specific promoters. The ability of the resulting constructs to inhibit protein synthesis is tested in cultured tumor cells co-transfected with a luciferase reporter gene. SAP expression driven by the cytomegalovirus (CMV) promoter (pCI-SAP) demonstrates that only 10 ng ofplasmid DNA per 1.6 x 10(4) B16 melanoma cells drastically reduces luciferase reporter activity to 18% of that in control cells (1). Direct intratumoral injections are performed in an aggressive melanoma model. B16 melanoma-bearing mice injected with pCI-SAP complexed with lipofectamine or N-(2,3-dioleoyloxy-1-propyl) trimethylammonium methyl sulfate (DOTAP) show a noteworthy attenuation in tumor growth, and this effect is significantly augmented by repeated administrations of the DNA complexes. Here, we describe in detail this cost-effective and safe suicide gene approach.

Cytosolic chaperones influence the fate of a toxin dislocated from the endoplasmic reticulum

The plant cytotoxin ricin enters target mammalian cells by receptor-mediated endocytosis and undergoes retrograde transport to the endoplasmic reticulum (ER). Here, its catalytic A chain (RTA) is reductively separated from the cell-binding B chain, and free RTA enters the cytosol where it inactivates ribosomes. Cytosolic entry requires unfolding of RTA and dislocation across the ER membrane such that it arrives in the cytosol in a vulnerable, nonnative conformation. Clearly, for such a dislocated toxin to become active, it must avoid degradation and fold to a catalytic conformation. Here, we show that, in vitro, Hsc70 prevents aggregation of heat-treated RTA, and that RTA catalytic activity is recovered after chaperone treatment. A combination of pharmacological inhibition and cochaperone expression reveals that, in vivo, cytosolic RTA is scrutinized sequentially by the Hsc70 and Hsp90 cytosolic chaperone machineries, and that its eventual fate is determined by the balance of activities of cochaperones that regulate Hsc70 and Hsp90 functions. Cytotoxic activity follows Hsc70-mediated escape of RTA from an otherwise destructive pathway facilitated by Hsp90. We demonstrate a role for cytosolic chaperones, proteins typically associated with folding nascent proteins, assembling multimolecular protein complexes and degrading cytosolic and stalled, cotranslocational clients, in a toxin triage, in which both toxin folding and degradation are initiated from chaperone-bound states.

Effect of surface charge and density of distearylphosphatidylethanolamine-mPEG-2000 (DSPE-mPEG-2000) on the cytotoxicity of liposome-entrapped ricin: Effect of lysosomotropic agents

Ricin was encapsulated in various liposomes having neutral, negatively and positively charged and different density of DSPE-mPEG-2000 on the surface and cytotoxicity of ricin entrapped in these different charged liposomal formulations was studied in CHO pro(-) cells and compared with free ricin with a view to develop an optimum delivery system for ricin in vivo. It was observed that the cytotoxicity of ricin entrapped in various charged liposomes was significantly dependent on the charge on the surface of liposomes. The maximum cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes. Monensin enhances the cytotoxicity of ricin entrapped in various charged liposomes and the extent of enhancement of the cytotoxicity is significantly dependent on the charge on the surface of liposomes. Maximum potentiation (213.14-fold) of cytotoxicity of ricin was observed when it was delivered through positively charged liposomes followed by negatively charged (83.36-fold) and neutral (71.30-fold) liposomes, respectively. Studies on the kinetics of inhibition of protein synthesis by ricin entrapped in various charged liposomes revealed that lag period of inhibition of protein synthesis is significantly lengthened following delivery through various charged liposomes. However, in the presence of monensin, the lag period was reduced. There is a marginal variation in the cytotoxicity of ricin entrapped in various charged liposomes after incorporation of 5mol% of DSPE-mPEG-2000 on the surface. However, there is a significant variation in the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes in CHO pro(-) cells following incorporation of 5mol% DSPE-mPEG-2000 on the surface. Studies on the effect of variation of density of DSPE-mPEG-2000 on the surface of various charged liposomes on the enhancement of cytotoxicity of entrapped ricin by monensin in CHO pro(-) cells showed that the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes is significantly dependent on the density of DSPE-mPEG-2000 on their surface. It was also observed that the efficacies of monensin on the enhancement of cytotoxicity of ricin entrapped in various charged PEG-liposomes in CHO pro(-) cells was highly related to their amount of cell-association. The present study has clearly shown that by suitable alteration of liposomal lipid composition, charge and density of hydrophilicity it would be possible to direct liposomal ricin to specific cells for their selective elimination in combination with monensin.

Depletion of sphingolipids facilitates endosome to Golgi transport of ricin

It has been previously demonstrated that depletion of cholesterol inhibits endosome to Golgi transport. Whether this inhibition is due to disruption of sphingolipid- and cholesterol-containing lipid rafts that are selected for Golgi transport or whether there is a physical requirement of cholesterol for either membrane deformations, facilitating formation of transport vesicles, or for recruitment of cytosolic constituents is not obvious. To investigate this in more detail, we have studied endosome to Golgi transport of ricin in sphingolipid-deficient cells using either a mutant cell line that does not express serine palmitoyltransferase, the first enzyme in sphingolipid biosynthesis, or a specific inhibitor, myriocin, of the same enzyme. Depletion of sphingolipids gave an increased sensitivity to ricin, and this increased sensitivity was inhibited by addition of sphingolipids. Importantly, endosome to Golgi transport of ricin, measured as sulfation of a modified ricin molecule, was increased in sphingolipid-deficient cells. No effect was seen on other pathways taken by ricin. Interestingly, cholesterol depletion inhibited endosome to Golgi transport even in cells with reduced levels of sphingolipids, suggesting that cholesterol as such is required for formation of transport vesicles. Our results indicate that the presence of sphingolipids actually limits and may function to control endosome to Golgi transport of ricin.

Enhancement of the cytotoxicity of liposomal ricin by the carboxylic ionophore monensin and the lysosomotropic amine NH4Cl in Chinese hamster ovary cells

Ricin was encapsulated in negatively charged liposomes and its effect on the cytotoxicity was compared with native ricin in Chinese hamster ovarian (CHO) cells. The cytotoxicity of ricin, as measured by a marker protein synthesis (incorporation of 3H-leucine), was reduced markedly (300-fold) following encapsulation in liposomes. Lactose, a potent inhibitor of ricin cytotoxicity, had no effect on the binding, internalization, and cytotoxicity of liposomal ricin, indicating that liposomal ricin enter into mammalian cells by an alternative route, bypassing galactose-mediated endocytic pathway. Both monensin (a carboxylic ionophore) and NH4Cl (a lysosomotropic amine) markedly enhances the cytotoxicity of liposomal ricin, indicating endocytotic uptake of liposomal ricin. The degree of potentiation of the cytotoxicity of liposomal ricin by both monensin and NH4Cl was significantly higher (441- and 51-fold) as compared to native ricin (62.5- and 12.5-fold). The extent of exocytosis of free ricin was found to be much higher as compared to liposomal ricin; on the other hand, the extent of degradation of free and liposomal ricin was identical. Consequently, the intracellular level of liposomal ricin was increased to 3.5-fold. This higher level of intracellular liposomal ricin may allow more efficient ricin A-chain release into the cytosol under the influence of NH4Cl and monensin. Monensin-induced potentiation of liposomal ricin was prevented by brefeldin A, indicating that in the presence of monensin, the liposomal ricin was efficiently routed through the Golgi apparatus en route to the cytosol. Thus, liposomal ricin in combination with monensin may have potential application for selective elimination of malignant cells.

Saporin as a novel suicide gene in anticancer gene therapy

We used a non-viral gene delivery approach to explore the potential of the plant saporin (SAP) gene as an alternative to the currently employed suicide genes in cancer therapy. Plasmids expressing cytosolic SAP were generated by placing the region encoding the mature plant ribosome-inactivating protein under the control of cytomegalovirus (CMV) or simian virus 40 (SV40) promoters. Their ability to inhibit protein synthesis was first tested in cultured tumor cells co-transfected with a luciferase reporter gene. In particular, SAP expression driven by CMV promoter (pCI-SAP) demonstrated that only 10 ng of plasmid per 1.6 x 10(4) B16 cells drastically reduced luciferase activity to 18% of that in control cells. Direct intratumoral injection of pCI-SAP complexed with either lipofectamine or N-(2,3-dioleoyloxy-1-propyl) trimethylammonium methyl sulfate (DOTAP) in B16 melanoma-bearing mice resulted in a noteworthy attenuation of tumor growth. This antitumor effect was increased in mice that received repeated intratumoral injections. A SAP catalytic inactive mutant (SAP-KQ) failed to exert any antitumor effect demonstrating that this was specifically owing to the SAP N-glycosidase activity. Our overall data strongly suggest that the gene encoding SAP, owing to its rapid and effective action and its independence from the proliferative state of target cells might become a suitable candidate suicide gene for oncologic applications.

Binding diversity of the two binding sites of ricin B lectin

Ricin B is a galactose-binding protein, which contains two binding sites. We have compared the binding properties of the two binding sites of ricin B chain toward different mono- and disaccharide ligands. The free energies of binding are calculated using the free energy perturbation simulation (thermodynamic integration method) and linear interaction energy approach using CHARMM force field. The second binding site of the protein was found to be weaker compared to the first. The details of the hydrogen-bonding scheme suggested the origin of the epimeric specificity of the protein. The reason for the weaker binding capacity of the second binding site has been addressed.

Internalized Pseudomonas exotoxin A can exploit multiple pathways to reach the endoplasmic reticulum

Receptor-mediated internalization to the endoplasmic reticulum (ER) and subsequent retro-translocation to the cytosol are essential sequential processes required for the intoxication of mammalian cells by Pseudomonas exotoxin A (PEx). The toxin binds the alpha2-macroglobulin receptor/low-density lipoprotein receptor-related protein. Here, we show that in HeLa cells, PEx recruits a proportion of this receptor to detergent-resistant microdomains (DRMs). Uptake of receptor-bound PEx involves transport steps both directly from early endosomes to the trans-Golgi network (TGN) independently of Rab9 function and from late endosomes to the TGN in a Rab9-dependent manner. Furthermore, treatments that simultaneously perturb both Arf1-dependent and Rab6-dependent retrograde pathways show that PEx can use multiple routes to reach the ER. The Rab6-dependent route has only been described previously for cargo with lipid-sorting signals. These findings suggest that partial localization of PEx within DRM permits a choice of trafficking routes consistent with a model that DRM-associated toxins reach the ER on a lipid-dependent sorting pathway whilst non-DRM-associated PEx exploits the previously characterized KDEL receptor-mediated uptake pathway. Thus, unexpectedly, an ER-directed toxin with a proteinaceous receptor shows promiscuity in its intracellular trafficking pathways, exploiting routes controlled by both lipid- and protein-sorting signals.

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.

Saporin and ricin A chain follow different intracellular routes to enter the cytosol of intoxicated cells

Several protein toxins, such as the potent plant toxin ricin, enter mammalian cells by endocytosis and undergo retrograde transport via the Golgi complex to reach the endoplasmic reticulum (ER). In this compartment the catalytic moieties exploit the ER-associated degradation (ERAD) pathway to reach their cytosolic targets. Bacterial toxins such as cholera toxin or Pseudomonas exotoxin A carry KDEL or KDEL-like C-terminal tetrapeptides for efficient delivery to the ER. Chimeric toxins containing monomeric plant ribosome-inactivating proteins linked to various targeting moieties are highly cytotoxic, but it remains unclear how these molecules travel within the target cell to reach cytosolic ribosomes. We investigated the intracellular pathways of saporin, a monomeric plant ribosome-inactivating protein that can enter cells by receptor-mediated endocytosis. Saporin toxicity was not affected by treatment with Brefeldin A or chloroquine, indicating that this toxin follows a Golgi-independent pathway to the cytosol and does not require a low pH for membrane translocation. In intoxicated Vero or HeLa cells, ricin but not saporin could be clearly visualized in the Golgi complex using immunofluorescence. The saporin signal was not evident in the Golgi, but was found to partially overlap with that of a late endosome/lysosome marker. Consistently, the toxicities of saporin or saporin-based targeted chimeric polypeptides were not enhanced by the addition of ER retrieval sequences. Thus, the intracellular movement of saporin differs from that followed by ricin and other protein toxins that rely on Golgi-mediated retrograde transport to reach their retrotranslocation site.

The history of ricin, abrin and related toxins

Ricin, abrin and related plant toxins have played interesting and important roles in the history of clinical medicine and biomedical research. The use of these proteins in medical treatment since ancient times is reviewed. Later the proteins played important roles in the early days of immunological research and some of the fundamental principles of immunology were discovered with toxic proteins of this group. During the last three decades the mechanism of action of the toxins was elucidated. This led to a major effort to target the toxins to malignant cells. Ricin has been used in bioterrorism. Recently, the toxins have played important roles as experimental models to elucidate the intracellular trafficking of endocytosed proteins.

Cytotoxic ribosome-inactivating lectins from plants

A class of heterodimeric plant proteins consisting of a carbohydrate-binding B-chain and an enzymatic A-chain which act on ribosomes to inhibit protein synthesis are amongst the most toxic substances known. The best known example of such a toxic lectin is ricin, produced by the seeds of the castor oil plant, Ricinnus communis. For ricin to reach its substrate in the cytosol, it must be endocytosed, transported through the endomembrane system to reach the compartment from which it is translocated into the cytosol, and there avoid degradation making it possible for a few molecules to inactivate a large proportion of the ribosomes and hence kill the cell. Cell entry by ricin involves the following steps: (i) binding to cell-surface glycolipids and glycoproteins bearing beta-1,4-linked galactose residues through the lectin activity of the B-chain (RTB); (ii) uptake by endocytosis and entry into early endosomes; (iii) transfer by vesicular transport to the trans-Golgi network; (iv) retrograde vesicular transport through the Golgi complex and into the endoplasmic reticulum (ER); (v) reduction of the disulfide bond connecting the A- and B-chains; (vi) a partial unfolding of the A-chain (RTA) to enable it to translocate across the ER membrane via the Sec61p translocon using the pathway normally followed by misfolded ER proteins for targeting to the ER-associated degradation (ERAD) machinery; (vi) refolding in the cytosol into a protease-resistant, enzymatically active structure; (vii) interaction with the sarcin-ricin domain (SRD) of the large ribosome subunit RNA followed by cleavage of a single N-glycosidic bond in the RNA to generate a depurinated, inactive ribosome. In addition to the highly specific action on ribosomes, ricin and related ribosome-inactivating proteins (RIPs) have a less specific action in vitro on DNA and RNA substrates releasing multiple adenine, and in some instances, guanine residues. This polynucleotide:adenosine glycosidase activity has been implicated in the general antiviral, and specifically, the anti HIV-1 activity of several single-chain RIPs which are homologous to the A-chains of the heterodimeric lectins. However, in the absence of clear cause and effect evidence in vivo, such claims should be regarded with caution.

Expression of ricin A chain and ricin A chain-KDEL in Escherichia coli

Ricin and its A chains can be used to conjugate with monoclonal antibodies to prepare immunotoxins. Ricin A chain (RTA) and its modification RTA-KDEL (ER-retrieval signal) were expressed with the pKK223.3 system in Escherichia coli under control of a tac promoter. The recombinant proteins can be purified by one-step affinity chromatography on a column of Blue-Sepharose 6B. The toxicities of RTA and its mutant RTA-KDEL were evaluated by the MTT assay in HeLa, MCF, and ECV-304 cells following fluid-phase endocytosis. RTA-KDEL was somewhat more cytotoxic than RTA itself in the different cell lines. The results suggest that rRTA-KDEL may be useful for the synthesis of more potent immunotoxins.

A trans-Golgi network retention signal YQRL fused to ricin A chain significantly enhances its cytotoxicity

Ricin enters the cells by receptor-mediated endocytosis, followed by translocation across the membranes of intracellular organelles. A trans-Golgi retention peptide signal YQRL was fused to the C-terminus of ricin A chain (RTA) by polymerase chain reaction. The recombinant RTA and RTA-YQRL were expressed in Escherichia coli using plasmid pKK223.3 under the control of a tac promoter. The recombinant proteins were purified by affinity chromatography on a Blue-Sepharose 6B column. The cytotoxicities of RTA and the fusion toxin RTA-YQRL were measured by the MTT assay in HeLa, SKOV-3, and WISH cells following fluid-phase endocytosis. The rRTA-YQRL was 2-, 10-, and 40-fold more cytotoxic than rRTA itself in the three cell lines, respectively. The results indicate that addition of a TGN retention signal YQRL to the C-terminus of RTA can markedly increase its cytotoxicity, suggesting TGN may play an important role in the intracellular routing and translocation of RTA.

Cytotoxicity of ribosome-inactivating protein saporin is not mediated through alpha2-macroglobulin receptor

Saporin is a single chain ribosome-inactivating protein produced by the plant Saponaria officinalis. Several isoforms of saporin have been isolated from various parts of the plant. In the present study recombinant saporin isoforms 5 and 6 were produced in Escherichia coli. Saporin-6 was found to be more active than saporin-5 in its N-glycosidase, cytotoxic, and genomic DNA fragmentation activities. Earlier, saporin has been shown to bind low-density lipoprotein receptor-related protein (LRP), however, in this study the sensitivities of LRP-negative and LRP-positive cell lines were found to be similar towards saporin-6 toxicity suggesting the internalization of saporin not to be solely dependent on the expression of LRP on eukaryotic cells.

Secretory ribonucleases are internalized by a dynamin-independent endocytic pathway

Cytosolic internalization is a requirement for the toxicity of secretory ribonucleases. Here, we investigate the mechanism of internalization of Onconase (ONC), a toxic protein, and ribonuclease A (RNase A), a nontoxic homolog. Microscopy studies indicate that both ribonucleases readily bind to the cell surface and are internalized via acidic vesicles. Blocking dynamin-dependent endocytosis prevents transferrin internalization but does not hinder RNase A internalization. ONC and G88R RNase A, which is a toxic variant, demonstrate enhanced cytotoxicity in the absence of clathrin- and dynamin-mediated endocytosis. The cytosolic entry of ribonucleases does not require an acidic environment or transport to the ER and probably occurs from endosomes. Thus, common proteins - secretory ribonucleases - enter the cytosol by a pathway that is distinct from that of other known toxins.

Ricin

Ricin is a heterodimeric protein produced in the seeds of the castor oil plant (Ricinus communis). It is exquisitely potent to mammalian cells, being able to fatally disrupt protein synthesis by attacking the Achilles heel of the ribosome. For this enzyme to reach its substrate, it must not only negotiate the endomembrane system but it must also cross an internal membrane and avoid complete degradation without compromising its activity in any way. Cell entry by ricin involves a series of steps: (i) binding, via the ricin B chain (RTB), to a range of cell surface glycolipids or glycoproteins having beta-1,4-linked galactose residues; (ii) uptake into the cell by endocytosis; (iii) entry of the toxin into early endosomes; (iv) transfer, by vesicular transport, of ricin from early endosomes to the trans-Golgi network; (v) retrograde vesicular transport through the Golgi complex to reach the endoplasmic reticulum; (vi) reduction of the disulphide bond connecting the ricin A chain (RTA) and the RTB; (vii) partial unfolding of the RTA to render it translocationally-competent to cross the endoplasmic reticulum (ER) membrane via the Sec61p translocon in a manner similar to that followed by misfolded ER proteins that, once recognised, are targeted to the ER-associated protein degradation (ERAD) machinery; (viii) avoiding, at least in part, ubiquitination that would lead to rapid degradation by cytosolic proteasomes immediately after membrane translocation when it is still partially unfolded; (ix) refolding into its protease-resistant, biologically active conformation; and (x) interaction with the ribosome to catalyse the depurination reaction. It is clear that ricin can take advantage of many target cell molecules, pathways and processes. It has been reported that a single molecule of ricin reaching the cytosol can kill that cell as a consequence of protein synthesis inhibition. The ready availability of ricin, coupled to its extreme potency when administered intravenously or if inhaled, has identified this protein toxin as a potential biological warfare agent. Therapeutically, its cytotoxicity has encouraged the use of ricin in 'magic bullets' to specifically target and destroy cancer cells, and the unusual intracellular trafficking properties of ricin potentially permit its development as a vaccine vector. Combining our understanding of the ricin structure with ways to cripple its unwanted properties (its enzymatic activity and promotion of vascular leak whilst retaining protein stability and important immunodominant epitopes), will also be crucial in the development of a long awaited protective vaccine against this toxin.

Selective regulation of the Rab9-independent transport of ricin to the Golgi apparatus by calcium

Transport of ricin from endosomes to the Golgi apparatus occurs, in contrast to the transport of the mannose 6-phosphate receptor, by a Rab9-independent process. To characterize the pathway of ricin transport to the Golgi apparatus, we investigated whether it was regulated by calcium. As shown here, our data indicate that calcium is selectively involved in the regulation of ricin transport to the Golgi apparatus. Thapsigargin, which inhibits calcium transport into the ER, and the calcium ionophore A23187 both increased the transport of ricin to the Golgi apparatus by a factor of 20. By contrast, transport of the mannose 6-phosphate receptor to the Golgi apparatus was unaffected. Ricin and mannose 6-phosphate receptor transport were measured by quantifying the sulfation of modified forms of ricin and the mannose 6-phosphate receptor. The increased transport of ricin was reduced by wortmannin and LY294002, suggesting that phosphoinositide 3-kinase might be involved in transport of ricin to the Golgi apparatus. Together, these findings indicate that the different pathways to the Golgi apparatus utilized by ricin and the mannose 6-phosphate receptor are regulated by different mechanisms.

Ricin

The plant toxin ricin consists of two disulfide-linked polypeptides with different functions. The A-chain enters the cytosol and inactivates the ribosomes enzymatically, whereas the B-chain has lectin properties and binds to carbohydrates at the cell surface. This binding is a requirement for translocation of the A-chain to the cytosol. The bound toxin is endocytosed and transported retrograde through the Golgi apparatus to the endoplasmic reticulum where it appears to be translocated to the cytosol by the sec61p complex.

Ability of the Tat basic domain and VP22 to mediate cell binding, but not membrane translocation of the diphtheria toxin A-fragment

A number of proteins are able to enter cells from the extracellular environment, including protein toxins, growth factors, viral proteins, homeoproteins, and others. Many such translocating proteins, or parts of them, appear to be able to carry with them cargo into the cell, and a basic sequence from the HIV-1 Tat protein has been reported to provide intracellular delivery of several fused proteins. For evaluating the efficiency of translocation to the cytosol, this TAT-peptide was fused to the diphtheria toxin A-fragment (dtA), an extremely potent inhibitor of protein synthesis which normally is delivered efficiently to the cytosol by the toxin B-fragment. The fusion of the TAT-peptide to dtA converted the protein to a heparin-binding protein that bound avidly to the cell surface. However, no cytotoxicity of this protein was detected, indicating that the TAT-peptide is unable to efficiently deliver enzymatically active dtA to the cytosol. Interestingly, the fused TAT-peptide potentiated the binding and cytotoxic effect of the corresponding holotoxin. We made a fusion protein between VP22, another membrane-permeant protein, and dtA, and also in this case we detected association with cells in the absence of a cytotoxic effect. The data indicate that transport of dtA into the cell by the TAT-peptide and VP22 is inefficient.

Endosome to Golgi transport of ricin is regulated by cholesterol

We have here studied the role of cholesterol in transport of ricin from endosomes to the Golgi apparatus. Ricin is endocytosed even when cells are depleted for cholesterol by using methyl-beta-cyclodextrin (m beta CD). However, as here shown, the intracellular transport of ricin from endosomes to the Golgi apparatus, measured by quantifying sulfation of a modified ricin molecule, is strongly inhibited when the cholesterol content of the cell is reduced. On the other hand, increasing the level of cholesterol by treating cells with mbetaCD saturated with cholesterol (m beta CD/chol) reduced the intracellular transport of ricin to the Golgi apparatus even more strongly. The intracellular transport routes affected include both Rab9-independent and Rab9-dependent pathways to the Golgi apparatus, since both sulfation of ricin after induced expression of mutant Rab9 (mRab9) to inhibit late endosome to Golgi transport and sulfation of a modified mannose 6-phosphate receptor (M6PR) were inhibited after removal or addition of cholesterol. Furthermore, the structure of the Golgi apparatus was affected by increased levels of cholesterol, as visualized by pronounced vesiculation and formation of smaller stacks. Thus, our results indicate that transport of ricin from endosomes to the Golgi apparatus is influenced by the cholesterol content of the cell.

Cytotoxic activity of recombinant bFGF-rViscumin fusion proteins

A fusion protein (bFGF-rMLA), containing the mitogen basic fibroblast growth factor (bFGF) and the cytotoxic component of rViscumin (recombinant mistletoe lectin), the enzymatic A-chain (rMLA), was expressed in Escherichia coli, purified, and functionally characterized. bFGF-rMLA is cytotoxic for mouse B16 melanoma cells expressing the FGF receptor with an IC(50) value of approximately 1 nM. rMLA shows no significant effect on the viability of the B16 cells up to a concentration of 141 nM. Additionally, bFGF-rMLA was associated with the rViscumin B-chain (rMLB) in an in vitro folding procedure. The IC(50) value of bFGF-rMLA/rMLB to B16 cells in the presence of lactose-to block rMLB lectin activity-was 134 pM. Thus, it was possible to enhance the efficacy of a rViscumin A-chain mitotoxin through addition of rMLB. We conclude that rViscumin fusion proteins may be generally applicable for the receptor-specific inactivation of target cells and point out their potential in drug development.

Heat shock proteins and cancer immunotherapy

Vaccination with heat shock proteins from tumor have been shown to elicit an anti-tumor response. Current studies indicate that the immunogenicity of HSPs is derived from the antigenic peptides which they associate with. Mechanisms by which the HSP-peptide complexes induce an immune response and the possible role of HSPs in antigen presentation is discussed in this article. The use of HSP-peptide complexes can be used as tumor vaccines for cancer immunotherapy is reviewed.

Ribosome-mediated folding of partially unfolded ricin A-chain

After endocytic uptake by mammalian cells, the cytotoxic protein ricin is transported to the endoplasmic reticulum, whereupon the A-chain must cross the lumenal membrane to reach its ribosomal substrates. It is assumed that membrane traversal is preceded by unfolding of ricin A-chain, followed by refolding in the cytosol to generate the native, biologically active toxin. Here we describe biochemical and biophysical analyses of the unfolding of ricin A-chain and its refolding in vitro. We show that native ricin A-chain is surprisingly unstable at pH 7.0, unfolding non-cooperatively above 37 degrees C to generate a partially unfolded state. This species has conformational properties typical of a molten globule, and cannot be refolded to the native state by manipulation of the buffer conditions or by the addition of a stem-loop dodecaribonucleotide or deproteinized Escherichia coli ribosomal RNA, both of which are substrates for ricin A-chain. By contrast, in the presence of salt-washed ribosomes, partially unfolded ricin A-chain regains full catalytic activity. The data suggest that the conformational stability of ricin A-chain is ideally poised for translocation from the endoplasmic reticulum. Within the cytosol, ricin A-chain molecules may then refold in the presence of ribosomes, resulting in ribosome depurination and cell death.

Dependence of ricin toxicity on translocation of the toxin A-chain from the endoplasmic reticulum to the cytosol

Ricin acts by translocating to the cytosol the enzymatically active toxin A-chain, which inactivates ribosomes. Retrograde intracellular transport and translocation of ricin was studied under conditions that alter the sensitivity of cells to the toxin. For this purpose tyrosine sulfation of mutant A-chain in the Golgi apparatus, glycosylation in the endoplasmic reticulum (ER) and appearance of A-chain in the cytosolic fraction was monitored. Introduction of an ER retrieval signal, a C-terminal KDEL sequence, into the A-chain increased the toxicity and resulted in more efficient glycosylation, indicating enhanced transport from Golgi to ER. Calcium depletion inhibited neither sulfation nor glycosylation but inhibited translocation and toxicity, suggesting that the toxin is translocated to the cytosol by the pathway used by misfolded proteins that are targeted to the proteasomes for degradation. Slightly acidified medium had a similar effect. The proteasome inhibitor, lactacystin, sensitized cells to ricin and increased the amount of ricin A-chain in the cytosol. Anti-Sec61alpha precipitated sulfated and glycosylated ricin A-chain, suggesting that retrograde toxin translocation involves Sec61p. The data indicate that retrograde translocation across the ER membrane is required for intoxication.

Calnexin family members as modulators of genetic diseases

The endoplasmic reticulum (ER) is an intracellular compartment devoted to the synthesis, segregation and folding of soluble and membrane secretory proteins. Some mutations in these proteins lead to their incorrect or incomplete folding in the ER. The ER has a quality control system which detects misfolded proteins and then specifies their fate. Some mutated proteins are retained in the ER wherein they accumulate (Russell bodies for misfolded immunoglobulin heavy chains, the PiZZ for alpha 1-antitrypsin), others are retrotranslocated from the ER and degraded by the cytosolic proteasomal system, and yet other proteins are eventually secreted (in AZC-treated cells). In this review we summarize the role of ER resident proteins in quality control of mutated secretory proteins.