Unfolding of diphtheria toxin. Identification of hydrophobic sites exposed on lowering of pH by photolabeling

Clinical targeting recombinant immunotoxins for cancer therapy

Recombinant immunotoxins (RITs) are proteins that contain a toxin fused to an antibody or small molecules and are constructed by the genetic engineering technique. RITs can bind to and be internalized by cells and kill cancerous or non-cancerous cells by inhibiting protein synthesis. A wide variety of RITs have been tested against different cancers in cell culture, xenograft models, and human patients during the past several decades. RITs have shown activity in therapy of several kinds of cancers, but different levels of side effects, mainly related to vascular leak syndrome, were also observed in the treated patients. High immunogenicity of RITs limited their long-term or repeat applications in clinical cases. Recent advances in the design of immunotoxins, such as humanization of antibody fragment, PEGylation, and modification of human B- and T-cell epitopes, are overcoming the above mentioned problems, which predict the use of these immunotoxins as a potential therapeutic method to treat cancer patients.

Recombinant immunotoxins containing truncated bacterial toxins for the treatment of hematologic malignancies

Immunotoxins are molecules that contain a protein toxin and a ligand that is either an antibody or a growth factor. The ligand binds to a target cell antigen, and the target cell internalizes the immunotoxin, allowing the toxin to migrate to the cytoplasm where it can kill the cell. In the case of recombinant immunotoxins, the ligand and toxin are encoded in DNA that is then expressed in bacteria, and the purified immunotoxin contains the ligand and toxin fused together. Among the most active recombinant immunotoxins clinically tested are those that are targeted to hematologic malignancies. One agent, containing human interleukin-2 and truncated diphtheria toxin (denileukin diftitox), has been approved for use in cutaneous T-cell lymphoma, and has shown activity in other hematologic malignancies, including leukemias and lymphomas. Diphtheria toxin has also been targeted by other ligands, including granulocyte-macrophage colony-stimulating factor and interleukin-3, to target myelogenous leukemia cells. Single-chain antibodies containing variable heavy and light antibody domains have been fused to truncated Pseudomonas exotoxin to target lymphomas and lymphocytic leukemias. Recombinant immunotoxins anti-Tac(Fv)-PE38 (LMB-2), targeting CD25, and RFB4(dsFv)-PE38 (BL22, CAT-3888), targeting CD22, have each been tested in patients. Major responses have been observed after failure of standard chemotherapy. The most successful application of recombinant immunotoxins today is in hairy cell leukemia, where BL22 has induced complete remissions in most patients who were previously treated with optimal chemotherapy.

Behavior of the N-Terminal Helices of the Diphtheria Toxin T Domain during the Successive Steps of Membrane Interaction

During intoxication of a cell, the translocation (T) domain of the diphtheria toxin helps the passage of the catalytic domain across the membrane of the endosome into the cytoplasm. We have investigated the behavior of the N-terminal region of the T domain during the successive steps of its interaction with membranes at acidic pH using tryptophan fluorescence, its quenching by brominated lipids, and trypsin digestion. The change in the environment of this region was monitored using mutant W281F carrying a single native tryptophan at position 206 at the tip of helix TH1. The intrinsic propensity to interact with the membrane of each helix of the N-terminus of the T domain, TH1, TH2, TH3, and TH4, was also studied using synthetic peptides. We showed the N-terminal region of the T domain was not involved in the binding of the domain to the membrane, which occurred at pH 6 mainly through hydrophobic effects. At that stage of the interaction, the N-terminal region remained strongly solvated. Further acidification eliminated repulsive electrostatic interactions between this region and the membrane, allowing its penetration into the membrane by attractive electrostatic interactions and hydrophobic effects. The peptide study indicated the nature of forces contributing to membrane penetration. Overall, the data suggested that the acidic pH found in the endosome not only triggers the formation of the molten globule state of the T domain required for membrane interaction but also governs a progressive penetration of the N-terminal part of the T domain in the membrane. We propose that these physicochemical properties are necessary for the translocation of the catalytic domain.

Immunotoxins for targeted cancer therapy

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncated Pseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.

Ion channel formation and membrane-linked pathologies of misfolded hydrophobic proteins: the role of dangerous unchaperoned molecules

1. Protein-membrane interaction includes the interaction of proteins with intrinsic receptors and ion transport pathways and with membrane lipids. Several hypothetical interaction models have been reported for peptide-induced membrane destabilization, including hydrophobic clustering, electrostatic interaction, electrostatic followed by hydrophobic interaction, wedge x type incorporation and hydrophobic mismatch. 2. The present review focuses on the hypothesis of protein interaction with lipid membranes of those unchaperoned positively charged and misfolded proteins that have hydrophobic regions. We advance the hypothesis that protein misfolding that leads to the exposure of hydrophobic regions of proteins renders them potentially cytotoxic. Such proteins include prion, amyloid beta protein (AbetaP), amylin, calcitonin, serum amyloid and C-type natriuretic peptides. These proteins have the ability to interact with lipid membranes, thereby inducing membrane damage and cell malfunction. 3. We propose that the most significant mechanism of membrane damage induced by hydrophobic misfolded proteins is mediated via the formation of ion channels. The hydrophobicity based toxicity of several proteins linked to neurodegenerative pathologies is similar to those observed for antibacterial toxins and viral proteins. 4. It is hypothesized that the membrane damage induced by amyloids, antibacterial toxins and viral proteins represents a common mechanism for cell malfunction, which underlies the associated pathologies and cytotoxicity of such proteins.

Does fusion of domains from unrelated proteins affect their folding pathways and the structural changes involved in their function? A case study with the diphtheria toxin T domain

We investigated whether the structural and functional behaviors of two unrelated protein domains were modified when fused. The IgG-binding protein ZZ derived from staphylococcal protein A was fused to the N- and/or C-terminus of the diphtheria toxin transmembrane domain (T). T undergoes a conformational change from a soluble native state at neutral pH to a molten globule-like state at acidic pH, leading to its interaction with membranes. We found that this molten globule state was not connected to the GdnHCl-induced unfolding pathway of T. The pH-induced transition of T, and also the unfolding of T and ZZ at neutral and acidic pH, were unchanged whether the domains were isolated or fused. The position of ZZ, however, influenced the solubility of T near its pK(i). SPR measurements revealed that T has a high affinity for membranes, isolated or within the fusion proteins (K(D)< 10(-11) M). This work shows that in the case of T and ZZ, the fusion of protein domains with different stabilities does not alter the structural changes involved in folding and function. This supports the use of T as a soluble membrane anchor.

Exploitation of intracellular pH gradients in the cellular delivery of macromolecules

Most cellular components such as the cytoplasm, endosomes, lysosomes, endoplasmic reticulum, Golgi bodies, mitochondria, and nuclei are known to maintain their own characteristic pH values. These pH values range from as low as 4.5 in the lysosome to about 8.0 in the mitochondria. Given these proton gradients around a neutral pH, weak acids, and bases with a pKa between 5.0 and 8.0 can exhibit dramatic changes in physicochemical properties. These compounds can be conjugated as such to macromolecules or incorporated into polymeric or liposomal formulations to promote the efficient cellular delivery of macromolecules. Mechanistically, the carrier molecules can facilitate favorable membrane partition, membrane fusion, transient pore formation, or membrane disruption. Drug carriers equipped with such pH-sensitive triggers and switches are able to significantly enhance the cellular delivery of macromolecules in vitro. However, the successful application of these molecules for efficient delivery in vivo requires the design of noncytotoxic, nonimmunogenic, serum compatible and biochemically labile carriers, systematic analysis of their mechanisms of action, and extensive animal studies.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Organization of diphtheria toxin in membranes. A hydrophobic photolabeling study

Diphtheria toxin (DT) is a disulfide linked AB-toxin consisting of a catalytic domain (C), a membrane-inserting domain (T), and a receptor-binding domain (R). It gains entry into cells by receptor-mediated endocytosis. The low pH ( approximately 5.5) inside the endosomes induces a conformational change in the toxin leading to insertion of the toxin in the membrane and subsequent translocation of the C domain into the cell, where it inactivates protein synthesis ultimately leading to cell death. We have used a highly reactive hydrophobic photoactivable reagent, DAF, to identify the segments of DT that interact with the membrane at pH 5.2. This reagent readily partitions into membranes and, on photolysis, indiscriminately inserts into lipids and membrane-inserted domains of proteins. Subsequent chemical and/or enzymatic fragmentation followed by peptide sequencing allows for identification of the modified residues. Using this approach it was observed that T domain helices, TH1, TH8, and TH9 insert into the membrane. Furthermore, the disulfide link was found on the trans side leaving part of the C domain on the trans side. This domain then comes out to the cis side via a highly hydrophobic patch corresponding to residues 134-141, originally corresponding to a beta-strand in the solution structure of DT. It appears that the three helices of the T domain could participate in the formation of a channel from a DT-oligomer, thus providing the transport route to the C domain after the disulfide reductase separates the two chains.