Major toxin belonging to the bicyclic octapeptides amatoxins that acts by binding non-competitively to RNA polymerase II and greatly slowing the elongation of transcripts from target promoters (PubMed:24613547). Processed by the macrocyclase-peptidase enzyme POPB to yield a toxic cyclic decapeptide (PubMed:24613547). POPB first removes 10 residues from the N-terminus (By similarity). Conformational trapping of the remaining peptide forces the enzyme to release this intermediate rather than proceed to macrocyclization (By similarity). The enzyme rebinds the remaining peptide in a different conformation and catalyzes macrocyclization of the N-terminal 8 residues (By similarity). The typical symptoms of amatoxin poisoning are gastro-intestinal distress beginning 6-12 hours after ingestion, a remission phase lasting 12-24 hours, and progressive loss of liver function culminating in death within 3-5 days (PubMed:12475187). One of the few effective treatments is liver transplantation (PubMed:12475187). Belongs to the MSDIN fungal toxin family.