A toxin that binds to rabbit muscle actin with a dissociation constant of 20 nM preventing actin depolymerization, causing cellular and tissue disruption, and death. The 2'-methylsulfonyltryptophan is thought to arise from cross-linking to Cys-6, as found in other Amanita peptide toxins, followed by sulfur methylation, beta-elimination, and oxidation. The cysteine, converted to 2,3-didehydroalanine, is hydrated to D-serine. LD(50) is 2.5 mg/kg by intraperitoneal injection into mice. This peptide is cyclic.