TOXINS
CTx (cholera toxin)
Ricin
BASIC STRUCTURE
●
AB type protein toxin with subunits
A1 (catalytic) &A2 (linker) & cell
binding homopentamer.(1)
●
It is a ribosome inactivating
protein. Belongs to AB family
group of toxins.(7)
●
AB toxin having a single
535 residue
polypeptide(10)
●
AB​5​ subunit toxin.(4)
●
Have 2 subunits. A chain with r
RNA N-glycosidase. B chain
with lectin(7)
●
ADP-ribosyl transferase
that inhibits host protein
synthesis.(10)
FEATURES
●
CT A1 subunit targets the cytosol of
the host cell and attacks it.(1)
●
It is a water insoluble
glycoprotein.(6)
●
The toxin possess a crystal
structure.(11)
DOMAINS OF
TOXIN
●
Contains A & B subunits. A contains
A1(enzymatically active)& A2
domain (​α​- helix tail), whereas the
B has a pentameric ring(2)
●
Ricin B, lectin domain.(7)
●
Amine terminal C or
catalytic domain,
Intermediate T or
transmembrane domain &
carboxyl terminal R or
receptor binding
domain.(10)
CELL SURFACE
RECEPTORS
●
GM1 gangliosides that are present on
cell surface or plasma membrane.(1)
●
●
●
Targets G-protein receptors.(2)(3)
B chain binds to the galactose
possessing surface with
glycoproteins & glycolipids.(7)
●
Caveola present on the cell surface
binds
●
Binds to the misfolded cell
surface proteins.(9)
It binds to the hbEGFheparin binding epidermal
growth factor precursor on
the cell surfaces.(11)
●
Enters the cell by triggering the
endocytosis by a lipid raft
mechanism.(1)
●
●
●
Caveola endocytosis.(1)
Can be through clathrin
dependent & independent
pathways or through
endocytosis.(8)
The toxin after binding to
the cell receptors have
clathrin pits and
internalisation is in the
calathrin coated early
endosomal vesicles,
internalisation ca be by
endocytosis(12)
●
After entering the cell travels from
the endosome to the ER through the
Golgi, by the retrograde vesicular
transport.(1)
●
From early endosomes to the
Trans Golgi then to the ER.(8)
●
●
In the ER the subunits then
slips and A subunits retro
translocate from the ER to the
cytoplasm.(8)
After acidification of the
endosomal lumen the
domain rearranges and
binds to the membrane and
forms a hole.(11)
●
After that the disulphide
bonds of the fragments A
and B is reduced and are
released into the cytosol
and catalysis the
elongation factor 2 and
stops protein
synthesis.(11)
MECHANISM OF
INTERNALISATION
INTERCELLULAR
PATHWAY AND
INTERACTIONS
●
The A& B subunits reduces its
sulphide bond in the ER and helps in
the separation of the CTA1 from its
toxin by with the help of
chaperone.(1)
●
The interaction with certain factors
in the cytosol activates the CTA1
subunit.(1)
●
Interaction of GM1 with the
residues.(5)
DTx(Diphtheria toxin)
●
The RTA removes the
misfolding of the proteins that
are present in the ER.(8)
●
But the RTA present in the
cytosol folds to a catalytic
conformation that inactivates
the ribosomes.(8)
●
They interact with ER luminal
chaperone protein disulphide
isomerase
MECHANISM OF
TOXICITY
●
Toxicity begins by binding to the cell
surface receptors at high affinity.(5)
●
Later CTA1 is transported to the
cytosol to induce adenylate
cyclase.(5)
●
High level of c AMP activates CFTR
thus resulting in diarrhoea.(5)
●
It inactivates the ribosomes in
the mammals as it depends on
the pathway of exposure i.e.,
through inhalation is more
severe than ingestion.(6)
●
Inactivation of the ribosomes
results in the stopping of
protein synthesis and the cell
dies by apoptosis.(9)
●
The stopping of the
protein synthesis results in
the death of the cell by
apoptosis.(11)
REFERENCE
1. Cherubin P, et.al. Inhibition of Cholera Toxin and Other AB Toxins by Polyphenolic Compounds.
PLoS One​. 2016 Nov 9; 11(11):e0166477.
2. Bagley, K., et.al. The catalytic A1 domains of cholera toxin and heat-labile enterotoxin are potent
DNA adjuvants that evoke mixed Th1/Th17 cellular immune responses. ​Hum Vaccin Immunother​.
2015 Sep; 11(9): 2228–2240.
3. CA, D. & AK.K. Functions of cholera toxin B-subunit as a raft cross-linker. ​Essays Biochem.
2015; 57:135-45.
4. Wernick. N. L. B. Cholera Toxin: An Intracellular Journey into the Cytosol by Way of the
Endoplasmic Reticulum. ​Toxins (Basel).​ 2010 Mar; 2(3): 310–325.
5. J.S and J.H. Cholera toxin - a foe & a friend. ​Indian J Med Res.​ 2011 Feb; 133:153-63.
6. Moshiri. M,et.al. Ricin Toxicity: Clinical and Molecular Aspects. ​Rep Biochem Mol Biol.​ 2016
Apr; 4(2): 60–65.
7. Schieltz.D.M, et.al, Quantification of ricin, RCA and comparison of enzymatic activity in 18
Ricinus communis​ cultivars by isotope dilution mass spectrometry​. Toxicon​. 2015 Mar; 95: 72–83.
8. H.M. O’Hara et.al. Folding domains within the ricin toxin A subunit as targets of protective
antibodies. ​Vaccine​. 2010 Oct 8; 28(43):7035-46.
9. Spooner.R.A and Lord.J.M. Ricin Trafficking in Cells. ​Toxins (Basel).​ 2015 Jan; 7(1): 49–65.
10. Oh. K.J., et.al. Translocation of the catalytic domain of diphtheria toxin across planar
phospholipid bilayers by its own T domain​. Proc Natl Acad Sci U S A​. 1999 Jul 20; 96(15):
8467–8470.
11. Umata.T. et.al. The cytotoxic action of diphtheria toxin and its degradation in intact vero cells are
inhibited by bafilomycin A1 a specific inhibitor of vacuolar type H+ ATPase. ​The Journal of
biological chemistry.​ Vol. 265, No. 35, Issue of December 15, pp. 21940-21945, 199O.
UNIVERSITY COLLEGE DUBLIN
STUDENT NAME: GRACE MARIA BOBEN
STUDENT NUMBER: 17203587
MODULE NAME: ADVANCED CELL BIOLOGY
SUBMITTED ON: 9/04/2018
ASSIGNMENT TITLE: COMPARE AND CONTRAST BETWEEN CTx, DTx & Ricin