Actin toxin

Fig. 3. Arp2/3 binds to liposomes in an actin-dependent manner. Shown is a Coomassie-blue stained gel of sedimented liposomes following incubation with cytosol. Activation of GTPases with GTP7S causes the appearance of additional protein bands on the gel. Protein sequencing and mass spectrometry were used to identify several of these proteins including actin and the Arp2/3 subunit, ARPC2. The actin toxins cytochalasin D (CytD) and latrunculin A (Lat A) reduce the levels of both actin and the ARPC2. The experiment indicates that actin plays an important role in the binding of Arp2/3 complex to the liposomal membranes.

Zigmond, 1988). The ATP-hydrolysis that accompanies actin polymerization, ATP —> ADP + Pj, and the subsequent release of the cleaved phosphate (Pj) are believed to act as a clock (Pollard et ah, 1992 Allen et ah, 1996), altering in a time-dependent manner the mechanical properties of the filament and its propensity to depolymerize. Molecular dynamics simulations suggested a so-called back door mechanism for the hydrolysis reaction ATP ADP - - Pj in which ATP enters the actin from one side, ADP leaves from the same side, but Pj leaves from the opposite side, the back door (Wriggers and Schulten, 1997b). This hypothesis can explain the effect of the toxin phalloidin which blocks the exit of the putative back door pathway and, thereby, delays Pi release as observed experimentally (Dancker and Hess, 1990). 

Another subfamily of ADP-iibosylating toxins modifies G-actin (at Argl77), thereby inhibiting actin polymerization. Members of this family are, for example, C. botulinum C2 toxin and Clostridium perfringens iota toxin. These toxins are binary in structure. They consist of an enzyme component and a separate binding component, which is structurally related to the binding component of anthrax toxin [3]. 

C. botulinum C2-toxin and related toxins Actin ADP-ribosylation Inhibition of actin polymerization... 

Barbieri JT, Riese MJ, Aktories K (2002) Bacterial toxins that modify the actin cytoskeleton. Annu Rev Cell Dev Biol 18 315-344... 

Several toxins produced by marine sponges cause the destabilization of F-actin. They contain a macrocyclic ring and an aliphatic chain, by which they bind to actin protomers. The toxins that include reidispongiolides,... 

Allingham JS, Zampella A, D Auria MV et al (2005) Structures of microfilament destabilizing toxins bound to actin provide insight into toxin design and activity. Proc Natl Acad Sci USA 102 14527-14532... 

The ETa receptor activates G proteins of the Gq/n and G12/i3 family. The ETB receptor stimulates G proteins of the G and Gq/11 family. In endothelial cells, activation of the ETB receptor stimulates the release of NO and prostacyclin (PGI2) via pertussis toxin-sensitive G proteins. In smooth muscle cells, the activation of ETA receptors leads to an increase of intracellular calcium via pertussis toxin-insensitive G proteins of the Gq/11 family and to an activation of Rho proteins most likely via G proteins of the Gi2/i3 family. Increase of intracellular calcium results in a calmodulin-dependent activation of the myosin light chain kinase (MLCK, Fig. 2). MLCK phosphorylates the 20 kDa myosin light chain (MLC-20), which then stimulates actin-myosin interaction of vascular smooth muscle cells resulting in vasoconstriction. Since activated Rho... 

The cellular/molecular mechanism of action for these cyclic peptide toxins is now an area of active research in several laboratories. These peptides cause striking ultrastructural changes in isolated hepatocytes (95) including a decrease in the polymerization of actin. This effect on the cells cytoskeletal system continues to be investigated and recent work indirectly supports the idea that these toxins interact with the cells cytoskeletal system (86,96). Why there is a specificity of these toxins for liver cells is not clear although it has been suggested that the bile uptake system may be at least partly responsible for penetration of the toxin into the cell (92). 

EAggEC children in the developing world 20-48 h persistent FliC - inflammation EAST-1 - guanylate cyclase -t secretion heat-labile toxin Ca2+-dependent actin phosphorylation cytoskeletal damage Pet - histopathologic effects on human intestinal mucosa... 

C. difficile history of antibiotic use, advanced age, underlying illness 5-10 days of antibacteria treatment (range 1st day to 10 weeks of antibiotics) mild to severe inflammatory diarrhea toxins A and B monoglucosylation of Rho protein - disruption of actin cytoskeleton —> mucosal disruption. - COX-2 - prostaglandin E2 —> synthesis of inflammatory cytokines... 

A third type of bacterial toxin, diphtheria toxin, catalyzes the ADP-ribosylation of eukaryotic elongation factor (EFTU), a type of small G protein involved in protein synthesis (Table 19-2). The functional activity of the elongation factor is inhibitedby this reaction. Finally, a botulinum toxin ADP-ribosylates and disrupts the function of the small G protein Rho, which appears to be involved in assembly and rearrangement of the actin cytoskeleton (Table 19-2). These toxins maybe involved in neuropathy (see Ch. 36) and membrane trafficking (see Ch. 9). 

Idzko M. Dichmann S, Ferrari D, Di Virgilio F. la Sala A, Girolomoni G. Panther E. Norgauer J Nucleotides induce chemotaxis and actin polymerization in immature but not mature human dendritic cells via activation of pertussis toxin-sensitive P2y receptors. Blood 2002 100 925-932. 

Because of the strict requirement for actin, the most commonly used inhibitors of macropinocytosis are the cytochalasins, especially cytochalasin D or toxin C. These substances also block phagocytosis and intracellular trafficking along actin filaments. Therefore, the results from these experiments are described in the trafficking section Actin Dependence on Liposome Uptake. ... 

Toxin B targets a Rho GTPase that is involved in the regulation of the actin cytoskeleton (79). 

The involvement of the actin cytoskeleton in liposome endocytosis is studied by using cytochalasins, latrunculin, or toxin C2 to polymerize actin filaments. For a review on actin assembly and endocytosis, see Ref. (135). 

The binary Clostridium botulinum toxin C2 blocks the function of actin filaments similarly to cytochalasin D (92). Treatment is for one to four hours in concentrations of 50ng/100ng or 100ng/200ng prior to adding liposomes. 

Barth H, Blocker D, Aktories K. The uptake machinery of clostridial actin ADP-ribosylating toxins—a cell delivery system for fusion proteins and polypeptide drugs. Naunyn Schmiedebergs Arch Pharmacol 2002 366(6) 501-512. 

Clostridium botulinum C2 toxin The prototype of a binary actin-ADP-ribosylating toxin 155... 

Potent Virulence Factors Directly Attack the Actin Cytoskeleton of Mammalian Cells Actin-ADP-Ribosylating Toxins... 

Figure 2 The actin-ADP-ribosylating toxins, (a) Molecular mode of action. The actin-ADP-ribosylating toxins covalently transfer an ADP-ribose moiety from NAD+ onto Arg177 of G-actin in the cytosol of targeted cells. Mono-ADP-ribosylated G-actin acts as a capping protein and inhibits the assembly of nonmodified actin into filaments. Thus, actin polymerization is blocked at the fast-growing ends of actin filaments (plus or barbed ends) but not at the slow growing ends (minus or pointed ends). This effect ultimately increases the critical concentration necessary for actin polymerization and tends to depolymerize F-actin. Finally, all actin within an intoxicated cell becomes trapped as ADP-ribosylated G-actin.

In 1980, C. botulinum C2 toxin was the first binary actin-ADP-ribosylating toxin to be described in the literature. The C2 toxin, produced by C. botulinum types C and D, consists of two nonlinked components... 

The binary nature of iota toxin from C. perfringens type E was first explored in 1986 by Stiles and Wilkins. The overall mode of action for iota toxin is widely comparable to C2 toxin. The binding/translocation component iota b (Ib) facilitates cellular uptake of the enzyme component iota a (la) in a like manner as previously described for C2 toxin. la, just as C2I, specifically mono-ADP-ribosylates G-actin at Argl77. ... 

Leira, F., et al., Development of a F actin-based live-cell fluorimetric microplate assay for diarrheic shellfish toxins. Anal. Biochem., 317, 2, 129, 2003. 

---