Cholera toxin receptors

P-Casein, synthesis, 151,153/ Caseinomacropeptide, laige-scale preparation and application, 211-219 amino acid composition, 219t biological functions, 211 inhibition of cholera toxin-receptor binding, 211-214 isolation procedure, 215,217/218 molecular weights, 215,216/ potential applications, 219 preparation, 215-219 recovery, 218/,219r... 

Phospholipids adsorb onto positively charged amidine PS latex initially as a monolayer but subsequently as lipid bilayers [49]. By incorporating the cholera toxin receptor monosialoganglioside (GMl) into these bilayers, specific binding of cholera toxin has been achieved [50]. 

Craig, S. W., and Cuatrecasas, P., 1975, Mobility of cholera toxin receptors on rat lymphocyte membranes, Proc. Natl. Acad. Sci. USA 72 3844. 

Protein toxins acting intracellularly are often composed of two subunits (A/B model). One subunit is catalytic (A-subunit) and the other is responsible for binding and cell entry (B-subunit). Following binding to an extracellular membrane receptor, the toxins are endocytosed. From the endosomes, the A-subunit is directly (pH dqDendent) transferred into the cytosol (e.g., diphtheria toxin and anthrax toxin) or the toxin is transported in a retrograde manner via the golgi to the ER (e.g., cholera toxin), where translocation into the cytosol occurs [1]. 

Figure 14-13. G i ganglioside, a monosialoganglio-side, the receptor in human intestine for cholera toxin.

Glycosphingolipids are constituents of the outer leaflet of plasma membranes and are important in cell adhesion and cell recognition. Some are antigens, eg, ABO blood group substances. Certain gangliosides function as receptors for bacterial toxins (eg, for cholera toxin, which subsequently activates adenylyl cyclase). 

Fig. 12. Tentative model of the signal transduction chain that links the perception of pectic fragments to defense responses in carrot cells. Abbreviations apy, heterotrimeric G protein CaM, calmodulin 4CL, 4-coumarate-CoA ligase CTX, cholera toxin FC, fusicoccine GDP-P-S and GTP-y-S, guanosine 5 -0-(2-thiodiphosphate) and guanosine 5 -0-(3-thiotriphosphate) IP3, 1,4,5-inositol trisphosphate PAL, phenylalanine ammonia-lyase PLC, phospholipase C PR, pathogenesis related PTX, pertussis toxin Rc, receptor SP, staurosporine. Activation and inhibition are symbolized by + and -respectively.

Stable analogs of GTP and GDP can be used to study the role of the G-protein, as indicated above. Thus, stable GTP analogs enhance agonist-induced receptor-mediated effects and slow their reversal, as shown in Figure 7.6. Pertussis and cholera toxins can also be used to inhibit or activate certain G-proteins, as indicated. 

A. Bemardi, L. Carrettoni, A. Grosso Ciponte, D. Montib, and S. Sonnino, Second generation mimics of ganglioside GM1 as artificial receptors for Cholera Toxin Replacement of the sialic acid moiety, Bioorg. Med. Chem. Lett., 10 (2000) 2197-2200. 

J. C. Pickens, E. A. Merritt, M. Ahn, C. L. M. J. Verlinde, W. G. J. Hoi, and E. Fan, Anchor-based design of improved cholera toxin and E. coli heat-labile enterotoxin receptor binding antagonists that display multiple binding modes, Chem. Biol., 9 (2002) 215-224. 

V. cholerae B subunits of the cholera toxin (CTB) Potato Receptor-binding activity. Immunogenic in mice when delivered orally. Mice challenged intraileally with CT showed up to 60% reduction in diarrheal fluid accumulation in small intestines. 117, 118... 

Figure 6.3. Mechanism of action of heterotrimeric G-proteins. Upon receptor occupancy, the Ga-subunit binds GTP in exchange for GDP, and then moves in the membrane until it encounters its target enzyme, shown here as adenylate cyclase (alternatively, a phospholipase). The activated target enzyme then becomes functional. Inherent GTPase activity within the a-subunit then hydrolyses bound GTP to GDP, and the a-subunit dissociates from its target enzyme (which becomes inactive) and rebinds the / - and ysubunits. Upon continued receptor occupancy, further catalytic cycles of GTP exchange and target enzyme activation may occur. The scheme shown is for a stimulatory G-protein (Got,), but similar sequences of events occur with inhibitory G-proteins (Gcx,) except that the interaction of the a-subunit with adenylate cyclase will result in its inhibition. The sites of action of pertussis and cholera toxins are shown.

Classical bacterial exotoxins, such as diphtheria toxin, cholera toxin, clostridial neurotoxins, and the anthrax toxins are enzymes that modify their substrates within the cytosol of mammalian cells. To reach the cytosol, these toxins must first bind to different cell-surface receptors and become subsequently internalized by the cells. To this end, many bacterial exotoxins contain two functionally different domains. The binding (B-) domain binds to a cellular receptor and mediates uptake of the enzymatically active (A-) domain into the cytosol, where the A-domain modifies its specific substrate (see Figure 1). Thus, three important properties characterize the mode of action for any AB-type toxin selectivity, specificity, and potency. Because of their selectivity toward certain cell types and their specificity for cellular substrate molecules, most of the individual exotoxins are associated with a distinct disease. Because of their enzymatic nature, placement of very few A-domain molecules in the cytosol will normally cause a cytopathic effect. Therefore, bacterial AB-type exotoxins which include the potent neurotoxins from Clostridium tetani and C. botulinum are the most toxic substances known today. However, the individual AB-type toxins can greatly vary in terms of subunit composition and enzyme activity (see Table 2). 

C. botulinum toxins belong to the AB group of toxins, which also includes diphtheria toxin, pseudomonas exotoxin A, anthrax toxin, Shiga(like) toxin, cholera toxin, pertussis toxin, and plant toxins, e.g., ricin. Moiety A has an enzymatic activity and usually modified cellular-target entering cytosol. Moiety B consists of one or more components and binds the toxin to surface receptors, and is responsible for translocation of the A component into cells. AB toxins are produced in a non-active form and are activated by a split between two cysteine residues within a region (Falnes and Sandvig, 2000). 

Cyclic AMP (adenosine 3, 5 -cyclic monophosphate) is anotter secondary messenger that acts as an intracellular mediator for many different hormones, communicating the signal through the cyclic AMP-dependent protein kinase. This, in turn, phosphorylates other proteins at ine and threonine residues. Certain cell-surfece receptors act by increasing the concentration of intracellular cyclic AMP. A long-duration sudden increase of intracellular cyclic AMP takes place with cholera toxins in intestinal epithelial cells. Other cell-surfece receptors play the opposite role of decreasing the concentration of cyclic AMP. 

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