Amanitin toxic blocks both the initiation and elongation steps of transcription  and was nicked named the “death cap” mushroom.  The toxin binds to sites residing beneath the bridge helix towards the cleft of the largest Pol II subunit, the Rpb1 and Rpb2.  The bridge helix residue His-A816 is interacted indirectly with the backbone group 4,5-dihydroxyisoleucine 3 of alpha-amanitin due to hydrogen bonds.  There is also several hydrogen bonds present between alpha-amanitin and the region of Rpb1 adjacent to the bridge helix.  These bindings constrain the position of the bridge helix of the Rpb2 side of the cleft.  The rate of translocation of Pol II on DNA is reduced greatly to only a few nucleotides per minute.  A lack in hydoxyl group of hydroxyproline2 and hydroxyl group of 4,5-dihroxyisoleucine 3 is 20,000 less inhibitory than normal alpha amanitin.  This reduction is due to the alteration of hydroxyproline 2 because alterations of 4,5-dihydroxylisoleucine 3 only cause a 100 fold decrease in inhibition.  Other alterations can be done to reduce inhibition such as shortening of the side chain of isoleucine-6 in alpha-amanitin which reduces inhibition by 1,000 times.  In active Pol II is not due to a blockage of the cleft where nucleotides enter, but due to a blockage of the translocation bridge helix.  Inhibiting translocation inhibits RNA polymerization thus resulting in cellular death.  [16]

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