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14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology

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Abstract:

At least four properties of bacterial protein toxins make them suitable as cell biological and pharmacological tools. First, the toxins enter cells without damaging the cell integrity. Second, the toxins possess high specificity. A high cell specificity is most often based on a toxin-specific membrane-binding domain and on specific receptors present on the surface of eukaryotic target cells. Actin, another important eukaryotic substrate for ADP-ribosylation by bacterial toxins, is not a GTP-binding protein but an ATP-binding protein. Because all these nucleotide-binding proteins are functionally important cellular proteins, the toxins, which allow their selective covalent modification, are widely used as tools. The actin cytoskeleton is the target of various bacterial toxins that affect the microfilament protein either directly by ADP-ribosylation or indirectly by modifying the regulatory mechanisms involved in the organization of the actin cytoskeleton. Actin, which is one of the most abundant proteins in eukaryotic cells, is the major component of the microfilament system. The toxin effect should occur with some delay of at least 15 to 30 min. This time is necessary for the translocation of the toxin. Moreover, it should be tested whether actin is in fact ADP-ribosylated by the toxin. Hydrolysis of bound GTP terminates the active state of the GTPases. It has been shown that especially Rho subfamily GTPases are targets for bacterial protein toxins. Recently, the genes for toxins were introduced into some crop plants in an effort to protect them from insect attack.

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14

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Figures

Image of Figure 14.1
Figure 14.1

ADP-ribosylation of G proteins by CT and PT. CT ADP-ribosylates the α subunits of Gs proteins. ADP-ribosylation blocks the GTPase activity of Gsα and activates the G protein persistently. CT is used as a tool to radiolabel Gs proteins and to manipulate the signaling via Gs. PT ADP-ribosylates the α subunits of Gi,o proteins, thereby blocking the receptor-mediated activation of the G protein. Thus, inhibition of a specific signal transduction process by PT indicates the involvement of “PT-sensitive” G proteins in this signaling pathway.

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14
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Image of Figure 14.2
Figure 14.2

Model of the action of the actin-ADP-ribosylating C2 toxin. The trypsin-activated binding component (C2IIa) forms heptamers, binds to the cell surface receptor of the target cell, and is the binding site for the enzyme component C2I. The toxin-receptor complex is internalized. At the acidic pH of endosomes, the binding component inserts into the membrane and allows the translocation of C2I into the cytosol, where monomeric actin is ADP-ribosylated. ADP-ribosylation of actin blocks its polymerization (“trapping” of the monomeric form). Moreover, ADP-ribosylated actin monomers bind like capping proteins to the plus ends of actin filaments, thereby inhibiting polymerization of unmodified actin monomers (“capping”). Because the minus ends of filaments are free, actin can be released at this site. The released actin is immediately ADP-ribosylated and trapped. Thus, the major consequence of ADP-ribosylation is breakdown of the microfilaments.

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14
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Image of Figure 14.3
Figure 14.3

Influence of C2 toxin and toxin B on morphology of RBL cells. Stimulation of immunoglobulin E-primed RBL cells by antigen (2,4-dinitrophenyl-bovine serum albumin [DNP-BSA]) via the high-affinity antigen receptor (FcεRI) induces membrane ruffling. Pretreatment of cells with C2 toxin, which depolymerizes actin by ADP-ribosylation, causes dramatic morphological changes but increases regulated serotonin release. toxin B, which inactivates Rho family proteins by glucosylation, induces a similar morphology of RBL cells but completely blocks antigen-induced serotonin release. The experiment shows that inhibition of serotonin release by toxin B is not simply caused by an action on the actin cytoskeleton and indicates that Rho proteins are essentially involved in the signal transduction of the FcεRI receptor. Scanning electron micrographs courtesy of J. Wilting, Freiburg, Germany.

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14
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Image of Figure 14.4
Figure 14.4

Covalent modification of Rho proteins by bacterial protein toxins. C3-like transferases ADP-ribosylate RhoA, RhoB, and RhoC by using NAD as a cosubstrate. Rho family proteins are glucosylated by toxin A and B and by the hemorrhagic (HT) and lethal (LT) toxins of . The cosubstrate is UDP-glucose. While toxins A and B and HT glucosylate all Rho family members, LT modifies Rac and Cdc42 (depending on the producer strain) but not Rho. Additionally, Ras family proteins (e.g., Ras, Rap, and Ral) are substrates for glucosylation by LT. The α-toxin from catalyzes an -acetylglucosaminylation. Rho proteins (Rho, Rac, and Cdc42) are activated by cytotoxic necrotizing factors (CNF1 and CNF2) from and by the dermonecrotic toxin (DNT) from species. DNT is also able to activate Rho GTPases by transglutamination, e.g., the attachment of polyamines onto the GTPases.

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14
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Image of Figure 14.5
Figure 14.5

Regulatory GTPase cycle of Rho proteins. Rho GTPases are inactive in the GDP-bound form and active after GTP-GDP exchange. The nucleotide exchange is stimulated by guanine nucleotide exchange factors (GEF) and inhibited by guanine nucleotide dissociation inhibitors (GDI). The active state of the GTPase is terminated by hydrolysis of bound GTP. GTP hydrolysis is stimulated by GTPaseactivating proteins (GAP). Some of the various processes regulated by Rho GTPases are indicated. Effects of Rho-modifying toxins. C3 exoenzyme and C3-like transferases ADP-ribosylate Rho at Asn 41, inhibiting the activation of Rho by blocking GEF-induced activation and increasing the stability of the GDI/Rho complex. Glucosylation of Rho GTPases by large clostridial cytotoxins blocks the interaction of active Rho with effectors and causes inactivation of Rho GTPase-dependent processes. The cytotoxic necrotizing factor CNF and the dermonecrotic toxin DNT inhibit the switch-off mechanism of activated Rho GTPases by deamidation and transglutamination of a glutamine residue (Gln 63 of Rho), which is essential for GTP hydrolysis resulting in constitutively activated Rho. In addition, bacterial proteins, which are introduced into target cells by a type III system, are indicated. YopT possesses protease activity to cleave the isoprenylated C terminus of Rho GTPases, thereby inactivating the GTPases. SOP is a protein with GEF-like function to activate Rho. The YopE, the SptP, and the ExoS are bacterial GAPs, which inactivate Rho GTPases.

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14
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References

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Tables

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Table 14.1

Bacterial toxins frequently used as tools

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14
Generic image for table
Table 14.2

Enzyme activities and targets of the bacterial protein toxins most frequently used as tools

Citation: Aktories K. 2004. 14 Bacterial Protein Toxins as Tools in Cell Biology and Pharmacology, p 341-360. In Cossart P, Boquet P, Normark S, Rappuoli R (ed), Cellular Microbiology, Second Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817633.ch14

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