Evidence that the transport of ricin to the cytoplasm is independent of both Rab6A and COPI

Intracellular Transport and Cytotoxicity of the Protein Toxin Ricin

Ricin can be isolated from the seeds of the castor bean plant (Ricinus communis). It belongs to the ribosome-inactivating protein (RIP) family of toxins classified as a bio-threat agent due to its high toxicity, stability and availability. Ricin is a typical A-B toxin consisting of a single enzymatic A subunit (RTA) and a binding B subunit (RTB) joined by a single disulfide bond. RTA possesses an RNA N-glycosidase activity; it cleaves ribosomal RNA leading to the inhibition of protein synthesis. However, the mechanism of ricin-mediated cell death is quite complex, as a growing number of studies demonstrate that the inhibition of protein synthesis is not always correlated with long term ricin toxicity. To exert its cytotoxic effect, ricin A-chain has to be transported to the cytosol of the host cell. This translocation is preceded by endocytic uptake of the toxin and retrograde traffic through the trans-Golgi network (TGN) and the endoplasmic reticulum (ER). In this article, we describe intracellular trafficking of ricin with particular emphasis on host cell factors that facilitate this transport and contribute to ricin cytotoxicity in mammalian and yeast cells. The current understanding of the mechanisms of ricin-mediated cell death is discussed as well. We also comment on recent reports presenting medical applications for ricin and progress associated with the development of vaccines against this toxin.

Subversion of Retrograde Trafficking by Translocated Pathogen Effectors

Intracellular bacterial pathogens subvert the endocytic bactericidal pathway to form specific replication-permissive compartments termed pathogen vacuoles or inclusions. To this end, the pathogens employ type III or type IV secretion systems, which translocate dozens, if not hundreds, of different effector proteins into their host cells, where they manipulate vesicle trafficking and signaling pathways in favor of the intruders. While the distinct cocktail of effectors defines the specific processes by which a pathogen vacuole is formed, the different pathogens commonly target certain vesicle trafficking routes, including the endocytic or secretory pathway. Recently, the retrograde transport pathway from endosomal compartments to the trans-Golgi network emerged as an important route affecting pathogen vacuole formation. Here, we review current insight into the host cell's retrograde trafficking pathway and how vacuolar pathogens of the genera Legionella, Coxiella, Salmonella, Chlamydia, and Simkania employ mechanistically distinct strategies to subvert this pathway, thus promoting intracellular survival and replication.

Uptake and processing of the cytolethal distending toxin by mammalian cells

The cytolethal distending toxin (Cdt) is a heterotrimeric holotoxin produced by a diverse group of Gram-negative pathogenic bacteria. The Cdts expressed by the members of this group comprise a subclass of the AB toxin superfamily. Some AB toxins have hijacked the retrograde transport pathway, carried out by the Golgi apparatus and endoplasmic reticulum (ER), to translocate to cytosolic targets. Those toxins have been used as tools to decipher the roles of the Golgi and ER in intracellular transport and to develop medically useful delivery reagents. In comparison to the other AB toxins, the Cdt exhibits unique properties, such as translocation to the nucleus, that present specific challenges in understanding the precise molecular details of the trafficking pathway in mammalian cells. The purpose of this review is to present current information about the mechanisms of uptake and translocation of the Cdt in relation to standard concepts of endocytosis and retrograde transport. Studies of the Cdt intoxication process to date have led to the discovery of new translocation pathways and components and most likely will continue to reveal unknown features about the mechanisms by which bacterial proteins target the mammalian cell nucleus. Insight gained from these studies has the potential to contribute to the development of novel therapeutic strategies.

Retrograde traffic from the Golgi to the endoplasmic reticulum

Proteins to be secreted are transported from the endoplasmic reticulum (ER) to the Golgi apparatus. The transport of these proteins requires the localization and activity of proteins that create ER exit sites, coat proteins to collect cargo and to reshape the membrane into a transport container, and address labels--SNARE proteins--to target the vesicles specifically to the Golgi apparatus. In addition some proteins may need export chaperones or export receptors to enable their exit into transport vesicles. ER export factors, SNAREs, and misfolded Golgi-resident proteins must all be retrieved from the Golgi to the ER again. This retrieval is also part of the organellar homeostasis pathway essential to maintaining the identity of the ER and of the Golgi apparatus. In this review, I will discuss the different processes in retrograde transport from the Golgi to the ER and highlight the mechanistic insights we have obtained in the last couple of years.

Eeyarestatin 1 interferes with both retrograde and anterograde intracellular trafficking pathways

BACKGROUND

The small molecule Eeyarestatin I (ESI) inhibits the endoplasmic reticulum (ER)-cytosol dislocation and subsequent degradation of ERAD (ER associated protein degradation) substrates. Toxins such as ricin and Shiga/Shiga-like toxins (SLTx) are endocytosed and trafficked to the ER. Their catalytic subunits are thought to utilise ERAD-like mechanisms to dislocate from the ER into the cytosol, where a proportion uncouples from the ERAD process, recovers a catalytic conformation and destroys their cellular targets. We therefore investigated ESI as a potential inhibitor of toxin dislocation.

METHODOLOGY AND PRINCIPAL FINDINGS

Using cytotoxicity measurements, we found no role for ES(I) as an inhibitor of toxin dislocation from the ER, but instead found that for SLTx, ESI treatment of cells was protective by reducing the rate of toxin delivery to the ER. Microscopy of the trafficking of labelled SLTx and its B chain (lacking the toxic A chain) showed a delay in its accumulation at a peri-nuclear location, confirmed to be the Golgi by examination of SLTx B chain metabolically labelled in the trans-Golgi cisternae. The drug also reduced the rate of endosomal trafficking of diphtheria toxin, which enters the cytosol from acidified endosomes, and delayed the Golgi-specific glycan modifications and eventual plasma membrane appearance of tsO45 VSV-G protein, a classical marker for anterograde trafficking.

CONCLUSIONS AND SIGNIFICANCE

ESI acts on one or more components that function during vesicular transport, whilst at least one retrograde trafficking pathway, that of ricin, remains unperturbed.

RNAi screening of Drosophila (Sophophora) melanogaster S2 cells for ricin sensitivity and resistance

The ribosome-inhibiting toxin ricin binds exposed β1→4 linked galactosyls on multiple glycolipids and glycoproteins on the cell surface of most eukaryotic cells. After endocytosis, internal cell trafficking is promiscuous, with only a small proportion of ricin proceeding down a productive (cytotoxic) trafficking route to the endoplasmic reticulum (ER). Here, the catalytic ricin A chain traverses the membrane to inactivate the cytosolic ribosomes, which can be monitored by measuring reduction in protein biosynthetic capacity or cell viability. Although some markers have been discovered for the productive pathway, many molecular details are lacking. To identify a more comprehensive set of requirements for ricin intoxication, the authors have developed an RNAi screen in Drosophila S2 cells, screening in parallel the effects of individual RNAi treatments alone and when combined with a ricin challenge. Initial screening of 806 gene knockdowns has revealed a number of candidates for both productive and nonproductive ricin trafficking, including proteins required for transport to the Golgi, plus potential toxin interactors within the ER and cytosol.

Biogenesis of the plant Golgi apparatus

It has long been assumed that the individual cisternal stacks that comprise the plant Golgi apparatus multiply by some kind of fission process. However, more recently, it has been demonstrated that the Golgi apparatus can be experimentally disassembled and the reformation process from the ER (endoplasmic reticulum) monitored sequentially using confocal fluorescence and electron microscopy. Some other evidence suggests that Golgi stacks may arise de novo in cells. In the present paper, we review some of the more recent findings on plant Golgi stack biogenesis and propose a new model for their growth de novo from ER exit sites.

Intracellular phospholipase A1gamma (iPLA1gamma) is a novel factor involved in coat protein complex I- and Rab6-independent retrograde transport between the endoplasmic reticulum and the Golgi complex

The mammalian intracellular phospholipase A(1) (iPLA(1)) family consists of three members, iPLA(1)alpha/PA-PLA(1), iPLA(1)beta/p125, and iPLA(1)gamma/KIAA0725p. Although iPLA(1)beta has been implicated in organization of the ER-Golgi compartments, little is known about the physiological role of its closest paralog, iPLA(1)gamma. Here we show that iPLA(1)gamma mediates a specific retrograde membrane transport pathway between the endoplasmic reticulum (ER) and the Golgi complex. iPLA(1)gamma appeared to be localized to the cytosol, the cis-Golgi, and the ER-Golgi intermediate compartment (ERGIC). Time-lapse microscopy revealed that a population of GFP-iPLA(1)gamma was associated with transport carriers moving out from the Golgi complex. Knockdown of iPLA(1)gamma expression by RNAi did not affect the anterograde transport of VSVGts045 but dramatically delayed two types of Golgi-to-ER retrograde membrane transport; that is, transfer of the Golgi membrane into the ER in the presence of brefeldin A and delivery of cholera toxin B subunit from the Golgi complex to the ER. Notably, knockdown of iPLA(1)gamma did not impair COPI- and Rab6-dependent retrograde transports represented by ERGIC-53 recycling and ER delivery of Shiga toxin, respectively. Thus, iPLA(1)gamma is a novel membrane transport factor that contributes to a specific Golgi-to-ER retrograde pathway distinct from presently characterized COPI- and Rab6-dependent pathways.

Intoxication of epithelial cells by plasmid-encoded toxin requires clathrin-mediated endocytosis

It has been shown that the autotransporter plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli (EAEC) produces cytotoxic and enterotoxic effects. Both effects can be explained by the proteolytic activity of Pet on its intracellular target alpha-fodrin (alphaII spectrin). In addition, Pet cytotoxicity and enterotoxicity depend on Pet serine protease activity, and on its internalization into epithelial cells. However, the mechanisms of Pet uptake by epithelial cells are unknown. Here, we show that Pet interacts with the plasma membrane of epithelial cells, and afterwards is detected inside the cells. Furthermore, Pet was internalized via clathrin-mediated endocytosis, since its internalization was inhibited by monodansylcadaverine and sucrose, but not by filipin or methyl-beta-cyclodextrin, which are drugs that interfere with protein entry via a clathrin-independent pathway. Additionally, Pet was immunoprecipitated by anti-clathrin antibodies, but not by anti-caveolin antibodies. Moreover, small interfering RNA (siRNA), designed to knock out clathrin gene expression in HEp-2 cells, prevented Pet internalization, and thereby the Pet-induced cytotoxic effect. However, the use of siRNA to knock out caveolin expression had no effect on Pet internalization, and the cytotoxic effect was clearly observed. Together, these data indicate that Pet secreted by EAEC binds to the cell surface via an unknown receptor, to be taken up by clathrin-mediated endocytosis, and exert its toxic effect in the cytoplasm.

"Alternative" endocytic mechanisms exploited by pathogens: new avenues for therapeutic delivery?

Some pathogens utilize unique routes to enter cells that may evade the intracellular barriers encountered by the typical clathrin-mediated endocytic pathway. Retrograde transport and caveolar uptake are among the better characterized pathways, as alternatives to clathrin-mediated endocytosis, that are known to facilitate entry of pathogens and potential delivery agents. Recent characterization of the trafficking mechanisms of prion proteins and certain bacteria may present new paradigms for strategizing improvements in therapeutic spread and retention of therapy. This review will provide an overview of such endocytic pathways, and discuss current and future possibilities in using these routes as a means to improve therapeutic delivery.

Identification and characterization of small molecules that inhibit intracellular toxin transport

Shiga toxin (Stx), cholera toxin (Ctx), and the plant toxin ricin are among several toxins that reach their intracellular destinations via a complex route. Following endocytosis, these toxins travel in a retrograde direction through the endosomal system to the trans-Golgi network, Golgi apparatus, and endoplasmic reticulum (ER). There the toxins are transported across the ER membrane to the cytosol, where they carry out their toxic effects. Transport via the ER from the cell surface to the cytosol is apparently unique to pathogenic toxins, raising the possibility that various stages in the transport pathway can be therapeutically targeted. We have applied a luciferase-based high-throughput screen to a chemical library of small-molecule compounds in order to identify inhibitors of Stx. We report two novel compounds that protect against Stx and ricin inhibition of protein synthesis, and we demonstrate that these compounds reversibly inhibit bacterial transport at various stages in the endocytic pathway. One compound (compound 75) inhibited transport at an early stage of Stx and Ctx transport and also provided protection against diphtheria toxin, which enters the cytosol from early endosomes. In contrast, compound 134 inhibited transport from recycling endosomes through the Golgi apparatus and protected only against toxins that access the ER. Small-molecule compounds such as these will provide insight into the mechanism of toxin transport and lead to the identification of compounds with therapeutic potential against toxins routed through the ER.

Pet, a non-AB toxin, is transported and translocated into epithelial cells by a retrograde trafficking pathway

The plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli is a 104-kDa autotransporter protein that exhibits proteolytic activity against the actin-binding protein alpha-fodrin. Intracellular cleavage of epithelial fodrin by Pet disrupts the actin cytoskeleton, causing both cytotoxic and enterotoxic effects. Intoxication requires the serine protease activity of Pet and toxin endocytosis from clathrin-coated pits. The additional events in the intracellular trafficking of Pet are largely uncharacterized. Here, we determined by confocal microscopy that internalized Pet is transferred from the early endosomes to the Golgi apparatus and then travels to the endoplasmic reticulum (ER). Pet associates with the Sec61p translocon before it moves into the cytosol as an intact, 104-kDa protein. This translocation process contrasts with the export of other ER-translocating toxins, in which only the catalytic A subunit of the AB toxin enters the cytosol. However, like intoxication with these AB toxins, Pet intoxication was inhibited in a subset of mutant CHO cell lines with aberrant activity in the ER-associated degradation pathway of ER-to-cytosol translocation. This is the first report which documents the cell surface-to-ER and ER-to-cytosol trafficking of a bacterial non-AB toxin.

Entry of protein toxins into mammalian cells by crossing the endoplasmic reticulum membrane: co-opting basic mechanisms of endoplasmic reticulum-associated degradation

The catalytic polypeptides of certain bacterial and plant protein toxins reach their substrates in the cytosol of mammalian cells by retro-translocation from the endoplasmic reticulum (ER). Emerging evidence indicates that these proteins subvert the ER-associated protein degradation (ERAD) pathway that normally removes misfolded or unassembled proteins from the ER, to achieve retrotranslocation. Upon entering the ER lumen, the toxins are unfolded to be perceived as ERAD substrates. Toxins that retro-translocate from the ER have an unusually low lysine content to avoid ubiquitin-mediated proteasomal degradation. This allows the exported toxins to refold into the proteasome-resistant, biologically active conformation, and leads to cellular intoxication.

Transport of Ricin from Endosomes to the Golgi Apparatus is Regulated by Rab6A and Rab6A′

Ricin is transported from early endosomes and/or the recycling compartment to the trans-Golgi network (TGN) and subsequently to the endoplasmic recticulum (ER) before it enters the cytosol and intoxicates cells. We have investigated the role of the Rab6 isoforms in retrograde transport of ricin using both oligo- and vector-based RNAi assays. Ricin transport to the TGN was inhibited by the depletion of Rab6A when the Rab6A messenger RNA (mRNA) levels were reduced by more than 40% and less than 75%. However, when Rab6A mRNA was reduced by more than 75% and Rab6A' mRNA was simultaneously up-regulated, the inhibition of ricin sulfation was abolished, indicating that the up-regulation of Rab6A' may compensate for the loss of Rab6A function. In addition, we found that a near complete depletion of Rab6A' gave approximately 40% reduction in ricin sulfation. The up-regulation of Rab6A mRNA levels did not seem to compensate for the loss of Rab6A' function. The depletion of both Rab6A and Rab6A' gave a stronger inhibition of ricin sulfation than what was observed knocking down the two isoforms separately. In conclusion, both Rab6A and Rab6A' seem to be involved in the transport of ricin from endosomes to the Golgi apparatus.

Retrograde transport pathways utilised by viruses and protein toxins

A model has been presented for retrograde transport of certain toxins and viruses from the cell surface to the ER that suggests an obligatory interaction with a glycolipid receptor at the cell surface. Here we review studies on the ER trafficking cholera toxin, Shiga and Shiga-like toxins, Pseudomonas exotoxin A and ricin, and compare the retrograde routes followed by these protein toxins to those of the ER trafficking SV40 and polyoma viruses. We conclude that there is in fact no obligatory requirement for a glycolipid receptor, nor even with a protein receptor in a lipid-rich environment. Emerging data suggests instead that there is no common pathway utilised for retrograde transport by all of these pathogens, the choice of route being determined by the particular receptor utilised.

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.

Protein toxins: intracellular trafficking for targeted therapy

The immunotoxin approach is based on the use of tumor-targeting ligands or antibodies that are linked to the catalytic (toxic) moieties of bacterial or plant protein toxins. In this review, we first discuss the current state of clinical development of immunotoxin approaches describing the results obtained with the two toxins most frequently used: diphtheria and Pseudomonas toxin-derived proteins. In the second part of the review, a novel concept will be presented in which the roles are inverted: nontoxic receptor-binding toxin moieties are used for the targeting of therapeutic and diagnostic compounds to cancer or immune cells. The cell biological basis of these novel types of toxin-based therapeutics will be discussed, and we will summarize ongoing preclinical and clinical testing.

Delivery into cells: lessons learned from plant and bacterial toxins

A number of protein toxins of bacterial and plant origin have cytosolic targets, and knowledge about these toxins have provided us with essential information about mechanisms that can be used to gain access to the cytosol as well as detailed knowledge about endocytosis and intracellular sorting. Such toxins include those that have two moieties, one (the B-moiety) that binds to cell surface receptors and another (the A-moiety) with enzymatic activity that enters the cytosol, as well as molecules that only have the enzymatically active moiety and therefore are inefficient in cell entry. The toxins discussed in the present article include bacterial toxins such as Shiga toxin and diphtheria toxin, as well as plant toxins such as ricin and ribosome-inactivating proteins without a binding moiety, such as gelonin. Toxins with a binding moiety can be used as vectors to translocate epitopes, intact proteins, and even nucleotides into the cytosol. The toxins fall into two main groups when it comes to cytosolic entry. Some toxins enter from endosomes in response to low endosomal pH, whereas others, including Shiga toxin and ricin, are transported all the way to the Golgi apparatus and the ER before they are translocated to the cytosol. Plant proteins such as gelonin that are without a binding moiety are taken up only by fluid-phase endocytosis, and normally they have a low toxicity. However, they can be used to test for disruption of endosomal membranes leading to cytosolic access of internalized molecules. Similarly to toxins with a binding moiety they are highly toxic when reaching the cytosol, thereby providing the investigator with an efficient tool to study endosomal disruption and induced transport to the cytosol. In conclusion, the protein toxins are useful tools to study transport and cytosolic translocation, and they can be used as vectors for transport to the interior of the cell.

Crossing the divide--transport between the endoplasmic reticulum and Golgi apparatus in plants

The transport of proteins between the endoplasmic reticulum (ER) and the Golgi apparatus in plants is an exciting and constantly expanding topic, which has attracted much attention in recent years. The study of protein transport within the secretory pathway is a relatively new field, dating back to the 1970s for mammalian cells and considerably later for plants. This may explain why COPI- and COPII-mediated transport between the ER and the Golgi in plants is only now becoming clear, while the existence of these pathways in other organisms is relatively well documented. We summarize current knowledge of these protein transport routes, as well as highlighting key differences between those of plant systems and those of mammals and yeast. These differences have necessitated the study of plant-specific aspects of protein transport in the early secretory pathway, and this review discusses recent developments in this area. Advances in live-cell-imaging technology have allowed the observation of protein movement in vivo, giving a new insight into many of the processes involved in vesicle formation and protein trafficking. The use of these new technologies has been combined with more traditional methods, such as protein biochemistry and electron microscopy, to increase our understanding of the transport routes in the cell.

The Steady-State Distribution of Glycosyltransferases Between the Golgi Apparatus and the Endoplasmic Reticulum is Approximately 90:10

Several lines of evidence support a novel model for Golgi protein residency in which these proteins cycle between the Golgi apparatus and the endoplasmic reticulum (ER). However, to preserve the functional distinction between the two organelles, this pool of ER-resident Golgi enzymes must be small. We quantified the distribution for two Golgi glycosyltransferases in HeLa cells to test this prediction. We reasoned that best-practice, quantitative solutions would come from treating images as data arrays rather than pictures. Using deconvolution and computer calculated organellar boundaries, the Golgi fraction for both endogenous beta1,4-galactosyltransferase and UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase 2 fused with green fluorescent protein (GFP) was 91% by fluorescence microscopy. Immunogold labeling followed by electron microscopy and model analysis yielded a similar value. Values reflect steady-state conditions, as inclusion of a protein synthesis inhibitor had no effect. These data strongly suggest that the fluorescence of a GFP chimera with an organellar protein can be a valid indicator of protein distribution and more generally that fluorescent microscopy can provide a valid, rapid approach for protein quantification. In conclusion, we find the ER pool of cycling Golgi glycosyltransferases is small and approximately 1/100 the concentration found in the Golgi apparatus.

Cisternal rab proteins regulate Golgi apparatus redistribution in response to hypotonic stress

We show that a physiological role of the extensively studied cisternal Golgi rab protein, rab6, is modulation of Golgi apparatus response to stress. Taking exposure of cells to hypotonic media as the best-known example of mammalian Golgi stress response, we found that hypotonic-induced tubule extension from the Golgi apparatus was sensitive to GDP-rab6a expression. Similarly, we found that Golgi tubulation induced by brefeldin A, a known microtubule-dependent process, was inhibited by GDP-restricted rab6a, rab6a', and rab33b, the most commonly studied cisternal rab proteins. These GDP-rab levels were sufficient to inhibit rab-induced redistribution of Golgi glycosyltransferases into the endoplasmic reticulum (ER), also a microtubule-dependent process, and to depress Golgi membrane association of the GTP-conformer of rab6. Nocodazole-induced Golgi scattering, a microtubule-independent process, also was inhibited by GDP-rab6a expression. In comparison, we found similar GDP-rab expression levels had little inhibitory effect on another microtubule-independent process, constitutive recycling of Golgi resident proteins to the ER. We conclude that Golgi cisternal rabs, and in particular rab6a, are regulators of the Golgi response to stress and presumably the molecular targets of stress-activated signaling pathway(s). Moreover, we conclude that rab6a can regulate select microtubule-independent processes as well as microtubule-dependent processes.

p97 Is in a complex with cholera toxin and influences the transport of cholera toxin and related toxins to the cytoplasm

Certain protein toxins, including cholera toxin, ricin, and Pseudomonas aeruginosa exotoxin A, are transported to the lumen of the endoplasmic reticulum where they retro-translocate across the endoplasmic reticulum membrane to enter the cytoplasm. The mechanism of retrotranslocation is poorly understood but may involve the endoplasmic reticulum-associated degradation pathway. The AAA ATPase p97 (also called valosin-containing protein) participates in the retro-translocation of cellular endoplasmic reticulum-associated degradation substrates and is therefore a candidate to participate in the retrotranslocation of protein toxins. To investigate whether p97 functions in toxin delivery to the cytoplasm, we measured the sensitivity to toxins of cells expressing either wild-type p97 or a dominant ATPase-defective p97 mutant under control of a tetracycline-inducible promoter. The rate at which cholera toxin and related toxins entered the cytoplasm was reduced in cells expressing the ATPase-defective p97, suggesting that the toxins might interact with p97. To detect interaction, the cholera toxin A chain was immunoprecipitated from cholera toxin-treated Vero cells, and co-immunoprecipitation of p97 was assessed by immunoblotting. The immunoprecipitates contained both cholera toxin A chain and p97, evidence that the two proteins are in a complex. Altogether, these results provide functional and structural evidence that p97 participates in the transport of cholera toxin to the cytoplasm.

Ricin: the endoplasmic reticulum connection

Ricin is a potent, plant-derived, ribosome inactivating protein. To target ribosomes in the mammalian cytosol, ricin must firstly negotiate the endomembrane system of the cell to reach the endoplasmic reticulum. Here, the toxin is reduced and the catalytic A chain is recognised by ER components that facilitate its membrane translocation to the cytosol. To be toxic, ricin A chain must then avoid degradation, a conundrum made more tricky in that ubiquitination and proteasomal degradation are normally tightly coupled to the translocation process. This mini-review summarises current understanding of these events.

Retrograde transport of cholera toxin from the plasma membrane to the endoplasmic reticulum requires the trans-Golgi network but not the Golgi apparatus in Exo2-treated cells

Cholera toxin (CT) follows a glycolipid-dependent entry pathway from the plasma membrane through the trans-Golgi network (TGN) to the endoplasmic reticulum (ER) where it is retro-translocated into the cytosol to induce toxicity. Whether access to the Golgi apparatus is necessary for transport to the ER is not known. Exo2 is a small chemical that rapidly blocks anterograde traffic from the ER to the Golgi and selectively disrupts the Golgi apparatus but not the TGN. Here we use Exo2 to determine the role of the Golgi apparatus in CT trafficking. We find that under the condition of complete Golgi ablation by Exo2, CT reaches the TGN and moves efficiently into the ER without loss in toxicity. We propose that even in the absence of Exo2 the glycolipid pathway that carries the toxin from plasma membrane into the ER bypasses the Golgi apparatus entirely.