Promanullin is a cyclic nonribosomal peptide.

It is an amatoxin, all of which are found in the mushroom genus Amanita.

Like other amatoxins, proamanullin is an inhibitor of RNA polymerase II.

Promanullin has a specific attraction to the enzyme RNA polymerase II.

Upon ingestion, it binds to the RNA polymerase II enzyme, effectively causing cytolysis of hepatocytes (liver cells).