Alpha-amanitin is a produced by the poisonous mushroom: Amanita phalloides. This mushroom is also known as the death cup or the destroying angel. Alpha amanitin has a very strong affinity to RNA polymerase II, but not as strong of an affinity for either RNA RNA polymerase III, and has no affinity for polymerase I, or prokaryotic RNA polymerase. Upon ingesting the mushrooms, symptoms are reported between 10-24 hours. These initial symptoms include: diarrhea and cramps. These symptoms usually go away, but by the fourth or fifth day the alpha-amanitin toxin has severe effects on the liver and kidneys. This leads to a total system failure in both organs and death usually occurs about a week after ingestion of the mushrooms. Diagnosis is difficult and treatments available include antibiotics and pumping of the stomach right after ingestion.
The main inhibition of alpha-amanitin on transcription is the inability for RNA polymerase II to translocate once the DNA base is correctly paired with the RNA base in transcription. The binding of alpha-amanitin permits the entry of NTPs through the funnel and the pore and also allows for entry and exit of both the RNA and DNA strands. The binding of alpha-amanitin specifically effects the bridge helix chain of RNA polymerase II. The hydroxyproline 2 group forms a hydrogen bond with Glu822 on the bridge helix. Other hydrogen bonds that this toxin forms are with amino acids Arg726, Gln767, Gln768, and Ser769. The hydrogen bonding between the toxin and Gln768 also effects residue His816, which is located on the bridge helix (2). Normally, the bridge helix changes conformation to allow a translocation event to occur in the RNA polymerase II active site, but since the alpha-amanitin interacts with the bridge helix, this event does not occur. Even though, alpha-amanitin has these major effects on translocation, it is not completely abolished. Translocation occurs at about a rate of a few nucleotides per minute compared to the several thousand in an unaffected RNA polymerase II enzyme. Since the rate of transcription is significantly slowed to this extent, the cell cannot produce mRNA transcripts at an efficient rate, ultimately destroying the cell.
This menu shows different orientations of the alpha-amanitin binding to the RNA polymerase II.
The orange residue in the Amanitin Binding Pocket view is of the critical residue Glu822, which is located in the bridge helix, and the alpha-amanitin toxin is shown in blue.
PDB ID: 1K83.pdb