Atomic Structures of Anthrax Prechannel Bound with Full-Length Lethal and Edema Factors

Cancer Drug Delivery Systems Using Bacterial Toxin Translocation Mechanisms

Recent advances in targeted cancer therapy hold great promise for both research and clinical applications and push the boundaries in finding new treatments for various currently incurable cancers. However, these therapies require specific cell-targeting mechanisms for the efficient delivery of drug cargo across the cell membrane to reach intracellular targets and avoid diffusion to unwanted tissues. Traditional drug delivery systems suffer from a limited ability to travel across the barriers posed by cell membranes and, therefore, there is a need for high doses, which are associated with adverse reactions and safety concerns. Bacterial toxins have evolved naturally to specifically target cell subtypes via their receptor binding module, penetrating the cell membrane efficiently through the membrane translocation process and then successfully delivering the toxic cargo into the host cytosol. They have, thus, been harnessed for the delivery of various drugs. In this review, we focus on bacterial toxin translocation mechanisms and recent progress in the targeted delivery systems of cancer therapy drugs that have been inspired by the receptor binding and membrane translocation processes of the anthrax toxin protective antigen, diphtheria toxin, and Pseudomonas exotoxin A. We also discuss the challenges and limitations of these studies that should be addressed before bacterial toxin-based drug delivery systems can become a viable new generation of drug delivery approaches in clinical translation.

Chloroquine-analogues block anthrax protective antigen channels in steady-state and kinetic studies

The tripartite anthrax toxin from Bacillus anthracis represents the prototype of A-B type of toxins, where the effector A (an enzymatic subunit) is transported with the help of a binding component B into a target cell. Anthrax toxin consists of three different molecules, two effectors, lethal factor (LF) and edema factor (EF) and the binding component also known as protective antigen (PA). PA forms heptamers or octamers following binding to host cell's receptors and mediates the translocation of the effectors into the cytosol via the endosomal pathway. The cation-selective PA₆₃-channel is able to reconstitute in lipid membranes and can be blocked by chloroquine and other heterocyclic compounds. This suggests that the PA₆₃-channel contains a binding site for quinolines. In this study, we investigated the structure-function relationship of different quinolines for the block of the PA₆₃-channel. The affinity of the different chloroquine analogues to the PA₆₃-channel as provided by the equilibrium dissociation constant was measured using titrations. Some quinolines had a much higher affinity to the PA₆₃-channel than chloroquine itself. We also performed ligand-induced current noise measurements using fast Fourier transformation to get insight in the kinetics of the binding of some quinolines to the PA₆₃-channel. The on-rate constants of ligand binding were around 10⁸M^-1·s^-1 at 150mM KCl and were only little dependent on the individual quinoline. The off-rates varied between 4s^-1 and 160s^-1 and depended much more on the structure of the molecules than the on-rate constants. The possible use of the 4-aminoquinolines as a therapy is discussed.

Neutralization of the anthrax toxin by antibody-mediated stapling of its membrane-penetrating loop

Anthrax infection is associated with severe illness and high mortality. Protective antigen (PA) is the central component of the anthrax toxin, which is one of two major virulence factors of Bacillus anthracis, the causative agent of anthrax disease. Upon endocytosis, PA opens a pore in the membranes of endosomes, through which the cytotoxic enzymes of the toxin are extruded. The PA pore is formed by a cooperative conformational change in which the membrane-penetrating loops of PA associate, forming a hydrophobic rim that pierces the membrane. Due to its crucial role in anthrax progression, PA is an important target for monoclonal antibody-based therapy. cAb29 is a highly effective neutralizing antibody against PA. Here, the cryo-EM structure of PA in complex with the Fab portion of cAb29 was determined. It was found that cAb29 neutralizes the toxin by clamping the membrane-penetrating loop of PA to the static surface-exposed loop of the D3 domain of the same subunit, thereby preventing pore formation. These results provide the structural basis for the antibody-based neutralization of PA and bring into focus the membrane-penetrating loop of PA as a target for the development of better anti-anthrax vaccines.

Anthrax toxin channel: What we know based on over 30 years of research

Protective antigen channel is the central component of the deadly anthrax exotoxin responsible for binding and delivery of the toxin's enzymatic lethal and edema factor components into the cytosol. The channel, which is more than three times longer than the lipid bilayer membrane thickness and has a 6-Å limiting diameter, is believed to provide a sophisticated unfoldase and translocase machinery for the foreign protein transport into the host cell cytosol. The tripartite toxin can be reengineered, one component at a time or collectively, to adapt it for the targeted cancer therapeutic treatments. In this review, we focus on the biophysical studies of the protective antigen channel-forming activity, small ion transport properties, enzymatic factor translocation, and blockage comparing it with the related clostridial binary toxin channels. We address issues linked to the anthrax toxin channel structural dynamics and lipid dependence, which are yet to become generally recognized as parts of the toxin translocation machinery.

Site-Specific Labeling and 19F NMR Provide Direct Evidence for Dynamic Behavior of the Anthrax Toxin Pore ϕ-Clamp Structure

The anthrax toxin protective antigen (PA), the membrane binding and pore-forming component of the anthrax toxin, was studied using ¹⁹F NMR. We site-specifically labeled PA with p-fluorophenylalanine (pF-Phe) at Phe427, a critically important residue that comprises the ϕ-clamp that is required for translocation of edema factor (EF) and lethal factor (LF) into the host cell cytosol. We utilized ¹⁹F NMR to follow low-pH-induced structural changes in the prepore, alone and bound to the N-terminal PA binding domain of LF, LF_N. Our studies indicate that pF-Phe427 is dynamic in the prepore state and then becomes more dynamic in the transition to the pore. An increase in dynamic behavior at the ϕ-clamp may provide the necessary room for movement needed in translocating EF and LF into the cell cytosol.

Assembly and Function of the Anthrax Toxin Protein Translocation Complex

Anthrax toxin is a major virulence factor of Bacillus anthracis, a Gram-positive bacterium which can form highly stable spores that are the causative agents of the disease, anthrax. While chiefly a disease of livestock, spores can be "weaponized" as a bio-terrorist agent, and can be deadly if not recognized and treated early with antibiotics. The intracellular pathways affected by the enzymes are broadly understood and are not discussed here. This chapter focuses on what is known about the assembly of secreted toxins on the host cell surface and how the toxin is delivered into the cytosol. The central component is the "Protective Antigen", which self-oligomerizes and forms complexes with its pay-load, either Lethal Factor or Edema Factor. It binds a host receptor, CMG2, or a close relative, triggering receptor-mediated endocytosis, and forms a remarkably elegant yet powerful machine that delivers toxic enzymes into the cytosol, powered only by the pH gradient across the membrane. We now have atomic structures of most of the starting, intermediate and final assemblies in the infectious process. Together with a major body of biophysical, mutational and biochemical work, these studies reveal a remarkable story of both how toxin assembly is choreographed in time and space.

Cryo-EM Uncovers Atomic Details for Loading Cell-Killing Enzymes to the Anthrax Toxin Pretranslocation Complex

In this issue of Structure, Zhou et al. report the structures of full-length lethal and edema factors, the cytotoxic components of the deadly anthrax toxin, in complex with the toxin's cell binding and delivery module, the protective antigen prechannel, providing an atomic description for the toxin recruitment prior to translocation.