                                       diphthoria toxin: protein domains

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   Diphtheria  toxin (DT) was once a major killer of children before immunizations against the virus began in
   the  late  1920s.   Since  then  researchers have discovered it structure (after they isolated it from the
   bacterium  that  produces it) and have learned a lot about the workings of the toxin.  The toxin has three
   domains- regions that have specific functions.  They are shown in the figure below.

                                                [diptheria.gif]

   The  receptor-binding  domain  attaches  itself to the receptor of the cell (a docking site outside of the
   cell  [e.g. a cell located on the lining of your throat]).  Then the transmembrane fragment penetrates the
   cell  wall  and drags the catalytic domain inside the cell.  Once inside, the toxin starts its dirty work.
   [This  information  was  gleaned  from the paper "The crystal structure of diphtheria toxin" by Eisenberg,
   Choe,  Bennett,  Fujii, Curmi, Kantardjieff, and Collier.  This paper was published in Nature, Volume 357,
   May 21, 1992.]
