Translocation mediated by domain II of Pseudomonas exotoxin A: transport of barnase into the cytosol.

Pseudomonas exotoxin A (PE) is a protein toxin composed of three structural domains. Functional analysis of PE has revealed that domain I is the cell-binding domain and that domain III functions in ADP ribosylation. Domain II was originally designated as the translocation domain, mediating the transfer of domain III to the cytosol, because mutations in this domain result in toxin molecules with normal cell-binding and ADP-ribosylation activities but which are not cytotoxic. However, the results do not rule out the possibility that regions of PE outside of domain II also participate in the translocation process. To investigate this problem, we have now constructed a toxin in which domain III of PE is replaced with barnase, the extracellular ribonuclease of Bacillus amyloliquefaciens. This chimeric toxin, termed PE1-412-Bar, is cytotoxic to a murine fibroblast cell line and to a murine hybridoma resistant to the ADP-ribosylation activity of PE. A mutant form of PE1-412-Bar with an inactivating mutation in domain II at position 276 was significantly less toxic. Because the cytotoxic effect of PE1-412-Bar was due to the ribonuclease-activity of barnase molecules which had been translocated to the cytosol, we conclude that domain II of PE is not only essential but also probably sufficient to carry out the translocation process.     

Analysis of sequences in domain II of Pseudomonas exotoxin A which mediate translocation

Pseudomonas exotoxin (PE) contains 613 amino acids that are arranged into 3 structural domains. PE exerts its cell-killing effects in a series of steps initiated by binding to the cell surface and internalization into endocytic vesicles. The toxin is then cleaved within domain II near arginine-279, generating a C-terminal 37-kDa fragment that is translocated into the cytosol where it ADP-ribosylates elongation factor 2 and arrests protein synthesis. In this study, we have focused on the functions of PE which are encoded by domain II. We have used the chimeric toxin TGF alpha-PE40 to deliver the toxin's ADP-ribosylating activity to the cell cytosol. Deletion analysis revealed that sequences from 253 to 345 were essential for toxicity but sequences from 346 to 364 were dispensable. Additional point mutants were constructed which identified amino acids 339 and 343 as important residues while amino acids 344 and 345 could be altered without loss of cytotoxic activity. Our data support the idea that domain II functions by first allowing PE to be processed to a 37-kDa fragment and then key sequences such as those identified in this study mediate the translocation of ADP-ribosylation activity to the cytosol.     

Alanine scanning mutagenesis identifies surface amino acids on domain II of Pseudomonas exotoxin required for cytotoxicity, proper folding, and secretion into periplasm.

Pseudomonas exotoxin A (PE) is a single polypeptide chain that contains 613 amino acids and is arranged into three major structural domains. Domain Ia is responsible for cell recognition, domain II for translocation of PE across the membrane, and domain III for ADP-ribosylation of elongation factor 2. Recombinant PE can be produced in Escherichia coli and is efficiently secreted into the periplasm when an OmpA signal sequence is present. To investigate the role of the amino acids located on the surface of domain II in the action of the toxin against mammalian cells, we substituted alanine for each of the 27 surface amino acids present in domain II. Surprisingly, all 27 mutant proteins had some alteration in cytotoxicity when tested on human A431 or MCF7 cells or mouse L929 cells. Native PE has a compact structure and therefore is relatively protease resistant and very little ADP-ribosylation activity is detected in the absence of the denaturing agents like urea and dithiothreitol. Several of the mutations resulted in altered protease sensitivity of the toxin. Seven of the mutant molecules exhibited ADP-ribosylation activity without urea and dithiothreitol, indicating they are partially unfolded. Out of these seven mutants, six had increased cytotoxic activity on at least one of the target cell lines and the other retained its native cytotoxic potency.     

Cell-mediated cleavage of Pseudomonas exotoxin between Arg279 and Gly280 generates the enzymatically active fragment which translocates to the cytosol.

Pseudomonas exotoxin (PE) is a three-domain toxin which is cleaved by a cellular protease within cells and then reduced to generate two prominent fragments (Ogata, M., Chaudhary, V. K., Pastan, I., and FitzGerald, D. J. (1990) J. Biol. Chem. 265, 20678-20685). The N-terminal fragment is 28 kDa in size and contains the binding domain. The 37-kDa C-terminal fragment, which translocates to the cytosol, contains the translocation domain and the ADP-ribosylation domain. Cleavage followed by reduction is essential for toxicity since mutant forms of the toxin that cannot be cleaved by cells are nontoxic. Previous results with these mutants suggest that cleavage occurred in an arginine-rich (arginine residues are at positions 274, 276, and 279) disulfide loop near the beginning of the translocation domain, but the exact site of cleavage was not determined. Since very few molecules of the 37-kDa fragment are generated within cells it was not possible to determine the site of cleavage by performing a conventional N-terminal sequence analysis of the 37-kDa fragment. Two experimental approaches were used to overcome this limitation. First, existing amino acids near the cleavage sites were replaced with methionine residues; this was followed by the addition of [35S]methionine-labeled versions of these toxins to cells. The pattern of radioactive toxin fragments recovered from the cells indicated that the toxin was cleaved either just before or just after Arg279. Second, [3H]leucine-labeled toxin was produced and added to the cells. Sequential Edman degradations were performed on the small amount of radioactive 37-kDa fragment that could be recovered from toxin-treated cells. A peak of radioactivity in the fifth fraction indicated that leucine was the 5th amino acid on the C-terminal side of the cleavage site. This result confirmed that cleavage was between Arg279 and Gly280.     

A deletion within the translocation domain of Pseudomonas exotoxin A enhances translocation efficiency and cytotoxicity concomitantly

Pseudomonas exotoxin A (PE) is a cytotoxin composed of three structural domains. Domain I is responsible for cell binding, domain II for membrane translocation enabling access to the cytosol, and domain III for the catalytic inactivation of protein synthesis, which results in cell death. To investigate the role of the six alpha-helices (A-F) that form the translocation domain, we deleted them successively one at a time. All mutants showed native cell-binding and catalytic activities, indicating that deletions specifically affected translocation activity. This step of the intoxication procedure was examined directly using a cell-free translocation assay, and indirectly by monitoring cytotoxicity. Translocation activity and log(cytotoxicity) were highly correlated, directly indicating that translocation is rate limiting for PE intoxication. Deletion of B, C and D helices resulted in non-toxic and non-translocating molecules, whereas mutants lacking the A or E helix displayed significant cytotoxicity albeit 500-fold lower than native PE. We concluded that B, C and D helices, which make up the core of domain II, are essential, whereas the more peripheral A and E helices are comparatively dispensable. The last helix (F) is inhibitory for translocation because its deletion produced a mutant displaying a translocation activity 60% higher than PE, along with a three- to sixfold increase in cytotoxicity in all tested cell lines. This toxin is the most in vitro active PE mutant obtained until now. Finally, partial duplication of domain II did not give rise to a more actively translocated PE, but rather to a threefold less active molecule.     

Agonist-induced internalization of the platelet-activating factor receptor is dependent on arrestins but independent of G-protein activation. Role of the C terminus and the (D/N)PXXY motif.

As with most G-protein-coupled receptors, repeated agonist stimulation of the platelet-activating factor receptor (PAFR) results in its desensitization, sequestration, and internalization. In this report, we show that agonist-induced PAFR internalization is independent of G-protein activation but is dependent on arrestins and involves the interaction of arrestins with a limited region of the PAFR C terminus. In cotransfected COS-7 cells, both arrestin-2 and arrestin-3 could be coimmunoprecipitated with PAFR, and agonist stimulation of PAFR induced the translocation of both arrestin-2 and arrestin-3. Furthermore, coexpression of arrestin-2 with PAFR potentiated receptor internalization, whereas agonist-induced PAFR internalization was inhibited by a dominant negative mutant of arrestin-2. The coexpression of a minigene encoding the C-terminal segment of the receptor abolished PAF-induced arrestin translocation and inhibited PAFR internalization. Using C terminus deletion mutants, we determined that the association of arrestin-2 with the receptor was dependent on the region between threonine 305 and valine 330 because arrestin-2 could be immunoprecipitated with the mutant PAFRstop330 but not PAFRstop305. Consistently, stop330 could mediate agonist-induced arrestin-2 translocation, whereas stop305 could not. Two other deletion mutants with slightly longer regions of the C terminus, PAFRstop311 and PAFRstop317, also failed to induce arrestin-2 translocation. Finally, the PAFR mutant Y293A, containing a single substitution in the putative internalization motif DPXXY in the seventh transmembrane domain (which we had shown to be able to internalize but not to couple to G-proteins) could efficiently induce arrestin translocation. Taken together, our results indicate that ligand-induced PAFR internalization is dependent on arrestins, that PAFR can associate with both arrestin-2 and -3, and that their translocation involves interaction with the region of residues 318-330 in the PAFR C terminus but is independent of G-protein activation.     

A recombinant form of Pseudomonas exotoxin directed at the epidermal growth factor receptor that is cytotoxic without requiring proteolytic processing.

Pseudomonas exotoxin A is composed of three structural domains that mediate cell recognition (I), membrane translocation (II), and ADP-ribosylation (III). Within the cell, the toxin is cleaved within domain II to produce a 37-kDa carboxyl-terminal fragment, containing amino acids 280-613, which is translocated to the cytosol and causes cell death. In this study, we constructed a mutant protein (PE37), composed of amino acids 280-613 of Pseudomonas exotoxin A, which does not require proteolysis to translocate. PE37 was targeted specifically to cells with epidermal growth factor receptors by inserting transforming growth factor-alpha (TGF-alpha) after amino acid 607 near the carboxyl terminus of Pseudomonas exotoxin A. PE37/TGF-alpha was very cytotoxic to cells with epidermal growth factor receptors. It was severalfold more cytotoxic than a derivative of full-length Pseudomonas exotoxin A containing TGF-alpha in the same position, probably because the latter requires intracellular proteolytic processing to exhibit its cytotoxicity, and proteolytic processing is not 100% efficient. Deletion of 2, 4, or 7 amino acids from the amino terminus of PE37/TGF-alpha greatly diminished cytotoxic activity, indicating the need for a proper amino-terminal sequence. In addition, a mutant containing an internal deletion of amino acids 314-380 was minimally active, indicating that other regions of domain II are also required for the cytotoxic activity of Pseudomonas exotoxin A.     

Identification of the carboxyl-terminal amino acids important for the ADP-ribosylation activity of Pseudomonas exotoxin A

The ADP-ribosylation domain of Pseudomonas exotoxin A (PE) has been identified to reside in structural domain III (residues 405-613) and a portion of domain Ib (residues 385-404) of the molecule (Hwang, J., FitzGerald, D. J., Adhya, S., and Pastan, I. (1987) Cell 48, 129-136). To further determine the carboxyl end region essential for ADP-ribosylation activity, we constructed sequential deletions at the carboxyl-terminal of PE. Our results show that a clone with a deletion of the carboxyl-terminal amino acid residues from Arg-609 to Lys-613 and replaced with Arg-Asn retained wild-type PE ADP-ribosylation activity. Deletion of the terminal amino acid residues from Ala-596 to Lys-613 and replaced with Val-Ile-Asn reduced ADP-ribosylation activity by 75%, while deletions of 36 or more amino acids from the carboxyl terminus completely lose their ADP-ribosylation activity. These modified PEs were also examined for their ability to block PE cytotoxicity. Our results shown that modified PEs which lost their ADP-ribosylation activity correspondingly lost their cytotoxicity. Furthermore, extracts containing PE fragments without ADP-ribosylation activity were able to block the cytotoxic activity of intact PE. Our results thus indicate that carboxyl-terminal amino acids in the Ser-595 region are crucial for ADP-ribosylation activity and, consequently, cytotoxicity of PE. The modified PEs which have lost their ADP-ribosylation activity may also be a route to new PE vaccines.     

Acid-triggered membrane insertion of Pseudomonas exotoxin A involves an original mechanism based on pH-regulated tryptophan exposure

Exposure to low endosomal pH during internalization of Pseudomonas exotoxin A (PE) triggers membrane insertion of its translocation domain. This process is a prerequisite for PE translocation to the cytosol where it inactivates protein synthesis. Although hydrophobic helices enable membrane insertion of related bacterial toxins such as diphtheria toxin, the PE translocation domain is devoid of hydrophobic stretches and the structural features triggering acid-induced membrane insertion of PE are not known. Here we have identified a molecular device that enables PE membrane insertion. This process is promoted by exposure of a key tryptophan residue. At neutral pH, this Trp is buried in a hydrophobic pocket closed by the smallest alpha-helix of the translocation domain. Upon acidification, protonation of the Asp that is the N-cap residue of the helix leads to its destabilization, enabling Trp side chain insertion into the endosome membrane. This tryptophan-based membrane insertion system is surprisingly similar to the membrane-anchoring mechanism of human annexin-V and could be used by other proteins as well.     

Transcytosis of iota-toxin across polarized CaCo-2 cells

Iota-toxin from Clostridium perfringens type E is a binary toxin consisting of two independent proteins, an enzymatic Ia and binding Ib component. Ia catalyses ADP-ribosylation of actin monomers, thus disrupting the actin cytoskeleton. In this report, we show that Ia plus Ib applied apically or basolaterally induce a rapid decrease in the transepithelial resistance (TER) of CaCo-2 cell monolayers and disorganization of actin filaments as well as the tight and adherens junctions. Ib alone, on the apical or basolateral side, slowly decreased the TER without affecting the actin cytoskeleton, possibly via pore formation. Interestingly, the two iota-toxin components inoculated separately on each cell surface induced cytopathic effects and a TER decrease. Anti-Ib sera, raised against the whole molecule or the Ia docking domain and applied to the opposite cell side versus Ib, neutralized the TER decrease. In addition, radioactive Ib incubated in the basolateral compartment was detected on the apical side by selective cell surface biotinylation. This argues for a transcytotic routing of Ib to mediate internalization of Ia from the opposite cell surface. Bafilomycin A1 also prevented the cytopathic effects of Ia and Ib applied separately to each cell side, possibly by blocking translocation of Ia into the cytosol and/or the intracellular transport of Ib. Ib is either routed into the cell independently of Ia, trans-cytosed and permanently exposed on the opposite cell surface or continuously recycled between an endosomal compartment and the cell surface.     

Arg9-peptide facilitates the internalization of an anti-CEA immunotoxin and potentiates its specific cytotoxicity to target cells

Arginines-containing membrane translocational signals (MTS), such as Tat(47-57) and VP22(267-300), and synthetic arginine-rich peptides have been reported to be efficient carriers for transporting various types of biomolecules into cytoplasmic and nuclear compartments of living cells. Among those arginines-containing MTS, a 9-mer arginine peptide (Arg9) was proved the most economical and efficient. We fused Arg9-peptide to an anti-CEA (Carcinoembryonic antigen) immunotoxin, PE35/CEA(Fv)/KDEL, at the position between the toxin moiety and the binding moiety. Strong binding and internalization of this fusion protein was observed in all detected cell lines, but little cytotoxicity to the cells that lack the CEA molecules on the cell surface was detected. However, the cytotoxicity besides the binding activity of the fusion protein to specific tumor cells expressing large amount of CEA molecules on the cell surface was improved markedly, indicating that the Arg9-peptide is capable of facilitating the receptor-mediated endocytosis of this immunotoxin, which leads to the increase of the specific cytotoxicity of this immunotoxin.     

Rapid agonist-induced desensitization and internalization of the A(2B) adenosine receptor is mediated by a serine residue close to the COOH terminus

The G(s)-coupled rat A(2B) adenosine receptor (A(2B)-AR) was epitope-tagged at the NH(2) terminus with hemagglutinin (HA) and subjected to progressive deletions or point mutations of the COOH terminus in order to determine regions of the receptor that contribute to agonist-induced desensitization and internalization. When expressed stably in Chinese hamster ovary cells, a mutant receptor in which the final 2 amino acids were deleted, the Leu(330)-stop mutant, underwent rapid agonist-induced desensitization and internalization as did the wild type (WT) receptor. However, the Phe(328) and the Gln(325)-stop mutants were resistant to rapid agonist-induced desensitization and internalization. Co-expression of arrestin-2-green fluorescent protein (arrestin-2-GFP) with WT receptor or Leu(330)-stop mutant resulted in rapid translocation of arrestin-2-GFP from cytosol to membrane upon agonist addition. On the other hand, agonist activation of the Phe(328)-stop or Gln(325)-stop mutant did not result in translocation of arrestin-2-GFP from cytosol. A COOH terminus point mutant, S329G, was also unable to undergo rapid agonist-induced desensitization and internalization, indicating that Ser(329) is a critical residue for these processes. A further deletion mutant (Ser(326)-stop) unexpectedly underwent rapid agonist-induced desensitization and internalization. However, activation of this mutant did not promote translocation of arrestin-2-GFP from cytosol to membrane. In addition, whereas WT receptor internalization was markedly inhibited by co-expression of dominant negative mutants of arrestin-2 (arrestin-2-(319-418)), dynamin (dynamin K44A), or Eps-15 (EDelta95-295), Ser(326)-stop receptor internalization was only inhibited by dominant negative mutant dynamin. Taken together these results indicate that Ser(329), close to the COOH terminus of the rat A(2B)-AR, is critical for the rapid agonist-induced desensitization and internalization of the receptor. However, deletion of the COOH terminus also uncovers a motif that is able to redirect internalization of the receptor to an arrestin- and clathrin-independent pathway.     

Cell surface tumor endothelium marker 8 cytoplasmic tail-independent anthrax toxin binding,proteolytic processing,oligomer formation,and internalization

The interaction of anthrax toxin protective antigen (PA) and target cells was assessed, and the importance of the cytosolic domain of tumor endothelium marker 8 (TEM8) in its function as a cellular receptor for PA was evaluated. PA binding and proteolytic processing on the Chinese hamster ovary cell surface occurred rapidly, with both processes nearly reaching steady state in 5 min. Remarkably, the resulting PA63 fragment was present on the cell surface only as an oligomer, and furthermore, the oligomer was the only PA species internalized, suggesting that oligomerization of PA63 triggers receptor-mediated endocytosis. Following internalization, the PA63 oligomer was rapidly and irreversibly transformed to an SDS/heat-resistant form, in a process requiring an acidic compartment. This conformational change was functionally correlated with membrane insertion, channel formation, and translocation of lethal factor into the cytosol. To explore the role of the TEM8 cytosolic tail, a series of truncated TEM8 mutants was transfected into a PA receptor-deficient Chinese hamster ovary cell line. Interestingly, all of the cytosolic tail truncated TEM8 mutants functioned as PA receptors, as determined by PA binding, processing, oligomer formation, and translocation of an lethal factor fusion toxin into the cytosol. Moreover, cells transfected with a TEM8 construct truncated before the predicted transmembrane domain failed to bind PA, demonstrating that residues 321-343 are needed for cell surface anchoring. Further evidence that the cytosolic domain plays no essential role in anthrax toxin action was obtained by showing that TEM8 anchored by a glycosylphosphatidylinositol tail also functioned as a PA receptor.     

Structure and function relationship of Pseudomonas exotoxin A. An immunochemical study

We have raised antisera against Pseudomonas exotoxin A (PE) and domains Ia and III to study the structure-function relationships of PE. Anti-PE antibody (AbPE) was shown to abolish the ADP-ribosylation activity of PE. However, neither antidomain Ia antibody nor antidomain III antibody inhibited the ADP-ribosylation activity of PE. This suggests that the inhibition of ADP-ribosylation by AbPE results from the binding of AbPE to the region between domains Ia and III. Since the binding of AbPE to PE did not inhibit NAD hydrolysis in the absence of elongation factor 2, the inhibitory effect of AbPE on ADP-ribosylation may be due to steric hindrance rather than a direct action on the catalytic function. Thus, the interface between domain Ia and III may be the site of entry of elongation factor 2 during ADP-ribosylation. The antibodies were also used to study both the inhibitory effects of PE on protein synthesis and its cytotoxic activity. Either AbPE or antidomain Ia antibody, but not antidomain III antibody, was able to reverse the inhibition of protein synthesis by PE and to block its cytotoxicity. In addition, rabbits immunized with domain Ia acquired tolerance against 100 micrograms of PE injected subcutaneously. These results suggest that domain Ia is the cell-binding domain of PE and may be used for vaccination against PE-mediated diseases.     

Protective antigen-binding domain of anthrax lethal factor mediates translocation of a heterologous protein fused to its amino- or carboxy-terminus

The edema factor (EF) and lethal factor (LF) components of anthrax toxin require anthrax protective antigen (PA) for binding and entry into mammalian cells. After internalization by receptor-mediated endocytosis, PA facilitates the translocation of EF and LF across the membrane of an acidic intracellular compartment. To characterize the translocation process, we generated chimeric proteins composed of the PA recognition domain of LF (LF_N; residues 1–255) fused to either the amino-terminus or the carboxy-terminus of the catalytic chain of diphtheria toxin (DTA). The purified fusion proteins retained ADP-ribosyltransferase activity and reacted with anti-sera against LF and diphtheria toxin. Both fusion proteins strongly inhibited protein synthesis in CHO-K1 cells in the presence of PA, but not in its absence, and they showed similar levels of activity. This activity could be inhibited by adding LF or the LF_N fragment (which blocked the interaction of the fusion proteins with PA), by adding inhibitors of endo-some acidification known to block entry of EF and LF into cells, or by introducing mutations that attenuated the ADP-ribosylation activity of the DTA moiety. The results demonstrate that LF_N fused to either the amino-terminus or the carboxy-terminus of a heterologous protein retains its ability to complement PA in mediating translocation of the protein to the cytoplasm. Besides its importance in understanding translocation, this finding provides the basis for constructing a translocation vector that mediates entry of a variety of heterologous proteins, which may require a free amino- or carboxy-terminus for biological activity, into the cytoplasm of mammalian cells.     

Redirecting Pseudomonas exotoxin

Pseudomonas exotoxin (PE) is a three-domain bacterial toxin that kills mammalian cells by gaining entry to the cytosol and inactivating protein synthesis. The pathway of toxin entry includes binding to a surface receptor, internalization via coated pits and endosomes, proteolytic processing, reduction of disulfide bonds and finally the translocation of an enzymatically active C-terminal fragment to the cytosol. Once in the cytosol this fragment inhibits protein synthesis by ADP ribosylating elongation factor 2. Because of its potency PE and its derivatives have been directed to kill various target cells. It is hoped this strategy will lead to the development of a novel kind of therapeutic agent for the treatment of various human diseases including cancer, AIDS and various immunological disorders.     

Phosphatidylethanolamine critically supports internalization of cell-penetrating protein C inhibitor

Although their contribution remains unclear, lipids may facilitate noncanonical routes of protein internalization into cells such as those used by cell-penetrating proteins. We show that protein C inhibitor (PCI), a serine protease inhibitor (serpin), rapidly transverses the plasma membrane, which persists at low temperatures and enables its nuclear targeting in vitro and in vivo. Cell membrane translocation of PCI necessarily requires phosphatidylethanolamine (PE). In parallel, PCI acts as a lipid transferase for PE. The internalized serpin promotes phagocytosis of bacteria, thus suggesting a function in host defense. Membrane insertion of PCI depends on the conical shape of PE and is associated with the formation of restricted aqueous compartments within the membrane. Gain- and loss-of-function mutations indicate that the transmembrane passage of PCI requires a branched cavity between its helices H and D, which, according to docking studies, precisely accommodates PE. Our findings show that its specific shape enables cell surface PE to drive plasma membrane translocation of cell-penetrating PCI.     

A proper amino terminus of diphtheria toxin is important for cytotoxicity

A series of deletions and substitutions were made at the 5' end of the gene fusion between the first 388 codons of diphtheria toxin (DT) and a cDNA encoding human IL2. The chimeric protein (DT388-IL2) was expressed and purified from E. coli and found to be very cytotoxic to a human T cell line, HUT 102, that expresses a large number of IL2 receptors. Deletion of the first five amino acids of DT resulted in a non-cytotoxic chimeric protein that had both ADP-ribosylation activity and IL2 receptor binding activity. Deletion of the first two amino acids of DT had little effect on cytotoxicity, while deletion of the first four amino acids or of two acidic residues at positions 3 and 4 greatly reduced cytotoxicity. Unexpectedly, a mutant containing a single leucine in place of the first two amino acids (gly, ala) was 2-3 fold more active. The amino terminus of DT may participate in the translocation of the A chain to the cytosol in a manner similar to Pseudomonas exotoxin (PE) in which a specific C-terminal sequence has been proposed to be involved in its cytotoxicity.     

Functional analysis of domains II, Ib, and III of Pseudomonas exotoxin

Pseudomonas exotoxin is composed of three structural domains that are responsible for cell recognition, membrane translocation, and ADP-ribosylation. The substitution of the cell recognition domain (domain Ia) with a growth factor such as transforming growth factor alpha (TGF alpha), creates a cell-specific cytotoxic agent, TGF alpha-PE40, which kills cells bearing epidermal growth factor (EGF) receptors. We have used TGF alpha-PE40 to define the role of sequences in domains II, Ib, and III. Various mutations were made in these domains and mutant forms of TGF alpha-PE40 expressed in Escherichia coli. Mutant proteins were then tested for their ADP-ribosylation, EGF receptor-binding, and cell-killing activities. Additionally, the amino boundary of domain III, which contains the ADP-ribosylation activity, was determined by deletion analysis. Data indicate that (i) the functional amino terminus of domain III is near amino acid 400; (ii) deletion of various regions in domain II or conversion of cysteines 265 and 268 to serines results in a loss of cytotoxicity which ranged from 10-fold to more than 150-fold, indicating that domain II is essential for full expression of cytotoxicity; (iii) deletion of the amino terminus of domain Ib results in a molecule with somewhat increased cytotoxic activity, indicating that domain Ib is not essential for the cytotoxic effect of TGF alpha-PE40; and (iv) TGF alpha-PE40, produced by denaturing and refolding of insoluble material from inclusion bodies, binds better to EGF receptors and is about 10-fold more cytotoxic to cells bearing EGF receptors than is the secreted form of soluble TGF alpha-PE40.     

Fusions of anthrax toxin lethal factor to the ADP-ribosylation domain of Pseudomonas exotoxin A are potent cytotoxins which are translocated to the cytosol of mammalian cells.

The lethal factor (LF) and edema factor (EF) components of anthrax toxin are toxic to animal cells only if internalized by interaction with the protective antigen (PA) component. PA binds to a cell surface receptor and is proteolytically cleaved to expose a binding site for LF and EF. To study how LF and EF are internalized and trafficked within cells, LF was fused to the translocation and ADP-ribosylation domains (domains II and III, respectively) of Pseudomonas exotoxin A. LF fusion proteins containing Pseudomonas exotoxin A domains II and III were less toxic than those containing only domain III. Fusion proteins with a functional endoplasmic reticulum retention sequence, REDLK, at the carboxyl terminus of domain III were less toxic than those with a nonfunctional sequence, LDER. The most potent fusion protein, FP33, had an EC50 = 2 pM on Chinese hamster ovary cells, exceeding that of native Pseudomonas exotoxin A (EC50 = 420 pM). Toxicity of all the fusion proteins required the presence of PA and was blocked by monensin. These data suggest that LF and LF fusion proteins are efficiently translocated from acidified endosomes directly to the cytosol without trafficking through other organelles, as is required for Pseudomonas exotoxin A. This system provides a potential vehicle for importing diverse proteins into the cytosol of mammalian cells.