Binary toxins

Binary toxins are unique concerning their structure because they are comprised of two individual, nonlinked proteins represented by an enzyme component and a binding/translocation component. The two components are secreted by the bacterium and assemble upon the surface of targeted eukaryotic cells to form an active toxin complex. For this to occur, both protein components of binary toxins act in a precisely concerted manner. The binding component first engages the cell-surface receptor and then mediates translocation of enzyme compo-nent(s) from the outside of a cell, through acidified endosomes, and into the host cell cytosol where it modifies the substrate (for review see Barth ). 

The following sections of this review will now transition into a unique group of protein toxins, the SEs and TSST-1. These proteins secreted by S. aureus vcovk in a different fashion versus the aforementioned binary toxins. In fact, these staphylococcal toxins do not enter a cell and do not directly injure the targeted cell surface. Toxin damage is insidiously indirect and caused by an over zealous response executed by the host s immune system. 

In a very different fashion from the clostridial/bacillus binary toxins, S. superantigens such as the SEs... 

Clostridium difficile is a commensal Gram-positive anaerobic bacterium of the human intestine, found in about 2-5% of the population. C. difficile is the most serious cause of antibiotic-associated diarrhoea and can lead to pseudomembranous colitis, a severe infection of the colon, often resulting from eradication of the normal gut flora by antibiotics. Discontinuation of causative antibiotic treatment is often curative. In more serious cases, oral administration of metronidazole or vancomycin is the treatment of choice. The bacterium produces several known toxins, including enterotoxin (toxin A) and cytotoxin (toxin B), both of which are responsible for the diarrhoea and inflammation seen in infected patients another toxin, binary toxin, has also been described. 

Considine RV, Simpson LL (1991) Cellular and molecular actions of binary toxins possessing ADP-ribosyltronsferase activity. In Toxicon 29 913-36... 

Pollard TD, Almo S, Quirk S etal. (1994) Structure of actin binding proteins Insights about function at atomic resolution. In Annu. Rev. Cell Biol. 10 207-49 Popoff MR, Rubin EJ, Gill DM et al. (1988) Actin-specific ADP-ribosyltransferase produced by a Clostridium difficile strain. In Infect. Immun. 56 2299-306 Popoff MR, Boquet P (1988) Clostridium spiroforme toxin is a binary toxin which ADP-ribosylates cellular actin. In Biochem. Biophys. Res. Commun. 152 1361—8 Reuner KH, Presek P, Boschek CB et al. (1987) Botulinum C2 toxin ADP-ribosylates actin and disorganizes the microfilament network in intact cells. In Eur. J. Cell Biol. 43 134-40... 

Simpson LL, Stiles BG, Zapeda HH et al. (1987) Molecular basis for the pathological actions of Clostridium perfringens lota toxin. In Infect. Immun. 55 118-22 Simpson LL (1989) The binary toxin produced by Clostridium botulinum enters cells by receptor-mediated endocytosis to exert its pharmacologic effects. In J. Pharmacol. Exp. Ther. 251 1223-8... 

The existence of binary toxins raises the possibility that unique chimeric substances can be formed that have implicit within them "receptors-on-demand". The underlying concepts to support this premise are as follows. First, the light chain component of C2 toxin could be modified so that it no longer expressed enzymatic activity. When so modified, it could be envisioned as a tissue-targeting domain, as explained below. Next, a novel pharmacologically active domain could be attached to the light chain. 

The first report in this area compared the actions of botulinum neurotoxin and botulinum binary toxin on transmission in the phrenic nerve-hemidiaphragm preparation (Simpson, 1982). There was the expected finding that neurotoxin blocked transmission by blocking acetylcholine release from nerve terminals, but the binary toxin had no effect. Apart from showing that C2 toxin did not block exocytosis, this study showed that the toxin did not act on the diaphragm to block muscle twitch. This observation is in keeping with the fact that the predominant form of actin in striated muscle is not that which is ADP-ribosylated by C2 toxin. 

Simpson LL (1989b) The binary toxin produced by Clostridium botulinum enters cells by receptor-mediated endocytosis to exert its pharmacologic effects. In J Pharmacol Exp Then 251 1223-8... 

Because C botulinum C2 toxin is a real binary toxin, studies of its effects on intact cells depend on the presence of both the binding (C2II) and enzyme component (C2I) (Reuner etai, 1987 Wiegers et al., 1991 Ohishi et al., 1984 Ohishi and Yanagimoto, 1992 Li et al., 1994 PrefDens etal., 1996). 

Popoff MR, Milward FW, Bancillon B, Boquet P (1989) Purification of the Clostridium spiroforme binary toxin and activity of the toxin on HEp-2 cells. Infect Immun 57 2462-2469. 

Anthrax toxin is composed of three proteins protective antigen (PA 83kDa), lethal factor (LF 90kDa), and edema factor (EF 89kDa). Individually, none of the three proteins are toxic but interact synergistically with at least one of the others. PA and LF (called LeTx) can cause lethal shock in experimental animals, and a mixture of PA and EF (edema toxin, EdTx) induces edema at the site of injection. Since two discrete units of the toxin are required for its action, the term binary toxin has been used to this and other bacterial toxins. Anthrax is unique from other binary toxins in that the binary moieties (EF and LF) interact only after being secreted from the bacteria. Further, EF and LF enter the cell via a single PA protein. Assembly of the three toxin proteins is initiated when PA binds to a proteinaceous cellular receptor and is activated by a member of the furin family of cellular proteases. The exact mechanisms of internalization of the toxin moieties are subject of scientific enquiry. Inside the cellular cytoplasm, EF (a calcium and calmodulin-dependent adenylate cyclase) causes a dramatic increase in intracellular cAMP concentrations and LF acts proteolytically to cleave certain MAPK kinases. 

Silva-Filha, M. H. Nielsen-Leroux, C. Charles, J. F. Binding kinetics of Bacillus sphaericus binary toxin to midgut brush-border membranes of Anopheles and Culex sp. mosquito larvae. Eur. J. Biochem. 1997 247, 754-761. 

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