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==Binding of pertussis toxin to its cellular targets== | ==Binding of pertussis toxin to its cellular targets== | ||
After being secreted, pertussis toxin (PTX) can interact with almost all mammalian cells, which explains the variety of biological activities of the toxin. <br /> | After being secreted, pertussis toxin (PTX) can interact with almost all mammalian cells, which explains the variety of biological activities of the toxin. <br /> | ||
No specific receptors for PTX have been identified but many cell surface '''sialoglycoproteins''' are involved in the binding of PTX | No specific receptors for PTX have been identified but many cell surface '''sialoglycoproteins''' are involved in the binding of PTX | ||
<ref name="Peppler (1988)"> | |||
Armstrong,G. D.,Howard,L. A. & M S Peppler (1988) Use of glycosyltransferases to restore pertussis toxin receptor activity to asialoagalactofetuin. J. Biol. Chem., 263: 8677-8684. | |||
</ref> | |||
, together with '''glycoproteins''': sugar moieties allow the recognition of the toxin and the carbohydrate sequence '''NeuAcα(2,6)-Galβ4GlcNAc''' is particularly important but sugar sequence alone is not sufficient | |||
<ref name="Peppler (1988)"> | |||
Brennan, M. J., David, J. L., Kenimer, J. G., & Manclark, C. R. (1988). Lectin-like binding of pertussis toxin to a 165-kilodalton Chinese hamster ovary cell glycoprotein. Journal of Biological Chemistry, 263(10), 4895-4899. | |||
</ref> | |||
<br /> | |||
PTX binds its target cells through the ''B oligomer'' : ''S2'' and ''S3'' subunits contains at least two '''carbohydrate-binding sites''' | PTX binds its target cells through the ''B oligomer'' : ''S2'' and ''S3'' subunits contains at least two '''carbohydrate-binding sites''' | ||
<ref name="Read (1994)"> | |||
Stein, P. E., Boodhoo, A., Armstrong, G. D., Heerze, L. D., Cockle, S. A., Klein, M. H., & Read, R. J. (1994). Structure of a pertussis toxin–sugar complex as a model for receptor binding. Nature Structural & Molecular Biology, 1(9), 591-596. | |||
</ref> | |||
. The N-terminal regions of these subunits are involved in receptor binding and the C-terminal domains of S2 and S3 adopt a fold found in other carbohydrate-binding proteins [73]. <br /> | |||
The B oligomer of PTX is involved in some biological activities of the toxin, independently of the enzyme activity. Thus '''Asn105''' in ''S2'' and '''Lys103''' in ''S3'' are important for the mitogenic activity of pertussis toxin on murine T lymphocytes | The B oligomer of PTX is involved in some biological activities of the toxin, independently of the enzyme activity. Thus '''Asn105''' in ''S2'' and '''Lys103''' in ''S3'' are important for the mitogenic activity of pertussis toxin on murine T lymphocytes | ||
<ref name="Locht93"> | |||
Lobet, Y., Feron, C., Dequesne, G., Simoen, E., Hauser, P., & Locht, C. (1993). Site-specific alterations in the B oligomer that affect receptor-binding activities and mitogenicity of pertussis toxin. The Journal of experimental medicine, 177(1), 79-87. | |||
</ref> | |||
, but not on human T cells | |||
<ref name="Klein93"> | |||
Loosmore, S., Zealey, G., Cockle, S., Boux, H., Chong, P. E. L. E., Yacoob, R., & Klein, M. (1993). Characterization of pertussis toxin analogs containing mutations in B-oligomer subunits. Infection and immunity, 61(6), 2316-2324. | |||
</ref>. | |||
==Toxin entry and trafficking in target cells== | ==Toxin entry and trafficking in target cells== | ||
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<references/> | <references/> | ||
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Pertussis Toxin (PTX) is a toxin produced and secreted by the bacteria Bordetella pertussis, also known as the whooping cough agent. It is a complex soluble bacterial Holotoxin, composed of 5 subuntits (named S1 to S5 according to their decreasing molecular weights), arranged in an A-B structure. The A part contains the enzymatically active S1 subunit, which catalyzes ADP-ribosylation of α subunit of trimeric G proteins, thereby disturbing major metabolic functions of the target cells, leading to a variety of biological activities. The B oligomer is composed by 1S2:1S3:2S4:1S5 and is responsible for binding of the toxin to target cell receptors and for intracellular traficking. This toxin is a major virulence factor of B. pertussis and is a component of vaccines against whooping cough.
Binding of pertussis toxin to its cellular targetsBinding of pertussis toxin to its cellular targets
After being secreted, pertussis toxin (PTX) can interact with almost all mammalian cells, which explains the variety of biological activities of the toxin.
No specific receptors for PTX have been identified but many cell surface sialoglycoproteins are involved in the binding of PTX
[1]
, together with glycoproteins: sugar moieties allow the recognition of the toxin and the carbohydrate sequence NeuAcα(2,6)-Galβ4GlcNAc is particularly important but sugar sequence alone is not sufficient
[1]
PTX binds its target cells through the B oligomer : S2 and S3 subunits contains at least two carbohydrate-binding sites
[2]
. The N-terminal regions of these subunits are involved in receptor binding and the C-terminal domains of S2 and S3 adopt a fold found in other carbohydrate-binding proteins [73].
The B oligomer of PTX is involved in some biological activities of the toxin, independently of the enzyme activity. Thus Asn105 in S2 and Lys103 in S3 are important for the mitogenic activity of pertussis toxin on murine T lymphocytes [3] , but not on human T cells [4].
Toxin entry and trafficking in target cellsToxin entry and trafficking in target cells
After binding to the target cell receptors, PTX enters the cells via receptor mediated endocytosis, and then follows the retrograde transport system, involving both the Golgi apparatus and the endoplasmic reticulum [5].[75,76].
Electron microscopy studies have shown that PTX enters the cells via coated pits [5]. But for now, PTX does not contain a clearly identified translocation domain in the B moiety.
S1 is able to bind to phospholipids bilayers [78], suggesting that it may directly interact with the target cell membranes and mediate its translocation, and also that the B oligomer is not essential for this step. Results obtained from cell transfection experiments support this hypothesis [80,81].
Binding of ATP to PTX [82] destabilizes the S1–B oligomer interactions and results in the release of S1 from the holotoxin [83]. This was proposed to occur in the endoplasmic reticulum, as it contain ATP and disulfide isomerases, that may reduce the intramolecular disulphide bonds of S1, therefore help to release the subunit from the holotoxin [85,86].
These observations imply that the holotoxin traffics via the endosomal pathway and Golgi apparatus to the endoplasmic reticulum, where it meet ATP and disulphide isomerase, leading to the release of the S1 subunit. S1 then translocate directly through the endoplasmic reticulum membrane into the cytosol (Fig ?)
Mechanism of pertussis toxinMechanism of pertussis toxin
Pertussis toxin (PTX) acts on target cells through its A protomer which contains the enzymatically active S1 subunit.
This subunit catalyzes ADP-ribosylation of the α-subunit of trimeric G proteins, which disturbs functions of the target cells and therefore lead to various biological effects.
In facts, substrates of PTX are regulators of the membrane-bound adenylate cyclase. These G proteins bind GTP in order to transduce signals in the cell. When ADP-ribosylation by PTX occurs, the downregulation of the adenylate cyclase activity is inhibited. This inhibition leads to increase cAMP levels in cells, which explains the amount of biological activities of the toxin.
-> picture of the active site of the S1 subunit
labeled in green: key residues surrounding the NAD-binding cavity (Arg9, Trp26, Cys41)
labeled in blue: catalytic residues (His35, Glu129)
The ADP-ribosylation of trimeric G proteins occurs on a cysteine residue in the C-terminal part of the α-subunit [88].
For that, the donor substrate used by PTX is NAD+, which binds the toxin through Trp26 [93,94], Arg9 [95] and Cys41 [92] located in the active site of S1.
Concerning the acceptor substrate, it binds to the toxin through residues 180-219 in the C-terminal region of S1 [91]. These residues show indeed a high affinity for the G protein and are involved in the catalysis of the ADP-ribosylation [97].
In the S1 subunit, the catalytic residues His35 [96,97] and Glu129 [98] have been identified: His35 is involved in the ionization of the nucleophilic thiol of the cysteine residue in the G protein via its ε-N [99] and the carboxylate group of the Glu129 side chain is in contact with the 2'-ribo-hydroxyl of the NAD+ [90].
Toxic effects of pertussis toxinToxic effects of pertussis toxin
Pertussis toxin (PTX) is especially toxic because of its ADP-ribosylation activity on trimeric G proteins but only the Gα subunits in the Gi/0 family are substrates for the toxin.
The ADP-ribosylation of this subunit leads to the loss of inhibition of adenylate cyclase activity, which drive to an increase of cAMP in target cells.
cAMP is primary in many biological processes that's why its accumulation leads to the disruption of cellular metabolism and pathological events, according to infected cells.
Thus biological activities of PTX are especially histamine sensitization, islet activation and lymphocytosis.
See AlsoSee Also
ReferenceReference
- ↑ 1.0 1.1
Armstrong,G. D.,Howard,L. A. & M S Peppler (1988) Use of glycosyltransferases to restore pertussis toxin receptor activity to asialoagalactofetuin. J. Biol. Chem., 263: 8677-8684.
Cite error: Invalid
<ref>tag; name "Peppler (1988)" defined multiple times with different content - ↑ Stein, P. E., Boodhoo, A., Armstrong, G. D., Heerze, L. D., Cockle, S. A., Klein, M. H., & Read, R. J. (1994). Structure of a pertussis toxin–sugar complex as a model for receptor binding. Nature Structural & Molecular Biology, 1(9), 591-596.
- ↑ Lobet, Y., Feron, C., Dequesne, G., Simoen, E., Hauser, P., & Locht, C. (1993). Site-specific alterations in the B oligomer that affect receptor-binding activities and mitogenicity of pertussis toxin. The Journal of experimental medicine, 177(1), 79-87.
- ↑ Loosmore, S., Zealey, G., Cockle, S., Boux, H., Chong, P. E. L. E., Yacoob, R., & Klein, M. (1993). Characterization of pertussis toxin analogs containing mutations in B-oligomer subunits. Infection and immunity, 61(6), 2316-2324.
- ↑ 5.0 5.1 El Baya, A., Linnemann, R., von Olleschik-Elbheim, L., Robenek, H., & Schmidt, M. A. (1997). Endocytosis and retrograde transport of pertussis toxin to the Golgi complex as a prerequisite for cellular intoxication. European journal of cell biology, 73(1), 40.