7/21/2019 The Best Known and Studied Bacterial Toxin is the Diphtheria Toxin

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The best known and studied bacterial toxin is the diphtheriatoxin, produced byCorynebacterium diphtheriae. Diphtheria toxinis a bacterial exotoxin of the A/B prototype. It is produced assingle polypeptide chain with a molecular weight of !,!!!

daltons. The function of the protein is distinguishable into twoparts" subunit A, with a m.w. of #$,!!! daltons, contains theen%ymatic acti&ity for inhibition of elongation factor'# in&ol&ed inhost protein synthesis( subunit B, with a m.w. of )*,!!! daltons,is responsible for binding to the membrane of a susceptible hostcell. The B subunit possesses a region T +translocation domainwhich inserts into the endosome membrane thus securing therelease of the en%ymatic component into the cytoplasm.

7/21/2019 The Best Known and Studied Bacterial Toxin is the Diphtheria Toxin

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-igure $. Diphtheria Toxin +Dtx. A +red is the catalytic domain(B +yellow is the binding domain which displays the receptor forcell attachment( T +blue is the hydrophobic domain responsiblefor insertion into the endosome membrane to secure the release

of A. The protein is illustrated in its closed configuration.

In vitro, the nati&e toxin is produced in an inacti&e form whichcan be acti&ated by the proteolytic en%yme trypsin in thepresence of thiol +reducing agent. The en%ymatic acti&ity of-ragment A is masked in the intact toxin. -ragment B is reuiredto enable -ragment A to reach the cytoplasm of susceptible cells.The 0 terminal end of -ragment B is hydrophilic and containsdeterminants that interact with specific membrane receptors on

sensiti&e cell membranes and the 1'terminal end of -ragment B+called the T domain is strongly hydrophobic. The specificmembrane receptor for the B fragment has been shown to be atransmembranous heparin'binding protein on the susceptiblecell2s surface.

The diphtheria toxin enters its target cells by either direct entryor receptor mediated endocytosis. The first step is the irre&ersiblebinding of the 0'terminal hydrophilic portion of -ragment B +AA3)#'4)4 to the receptor. During 567, the whole toxin is thentaken up in an endocytic &esicle. In the &esicle, the p8 drops toabout 4 which allows unfolding of the A and B chains. Thisexposes hydrophobic regions of both the A and B chains that caninsert into the &esicle membrane. The result is exposure of the Achain to the cytoplasmic side of the membrane. There, reductionand proteolytic clea&age releases the A chain in the cytoplasm.The A fragment is released as an extended chain but regains itsacti&e +en%ymatic globular conformation in the cytoplasm. The A

chain cataly%es the AD9 ribosylation of elongation factor'# +7-'#as shown in -igure #.

7/21/2019 The Best Known and Studied Bacterial Toxin is the Diphtheria Toxin

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-igure #. 7ntry and acti&ity of diphtheria toxin +Dtx insusceptible cells. The B domain of the toxin binds to a cognatereceptor on a susceptible cell. The toxin is taken up in an

endosome by receptor mediated encocytosis. Acidification of theendocytic &esicle allows unfolding of the A and B chains exposingthe hydrophobic T domain of the toxin. The T domain inserts intothe endosome membrane translocating the A fragment into the

cytoplasm where it regains its en%ymatic configuration. Theen%ymatic A component utili%es 1AD as a substrate. It cataly%esthe attachment of the AD9'ribose portion of 1AD to elongationfactor +7-'# which inacti&ates its function in protein synthesis.