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Fungi in the basidiomycetous genus Amanita owe their high mammalian toxicity to the bicyclic octapeptide amatoxins such as α-amanitin. Amatoxins and the related phallotoxins (such as the heptapeptide phalloidin) are encoded by members of the "MSDIN" gene family and are synthesized on ribosomes as short (34- to 35-amino-acid) proproteins. Antiamanitin antibodies and confocal microscopy were used to determine the cellular and subcellular localizations of amanitin accumulation in basidiocarps (mushrooms) of the Eastern North American destroying angel (Amanita bisporigera). Consistent with previous studies, amanitin is present throughout the basidiocarp (stipe, pileus, lamellae, trama, and universal veil), but it is present in only a subset of cells within these tissues. Restriction of amanitin to certain cells is especially marked in the hymenium. Several lines of evidence implicate a specific prolyl oligopeptidase, A. bisporigera POPB (AbPOPB), in the initial processing of the amanitin and phallotoxin proproteins. The gene for AbPOPB is restricted taxonomically to the amatoxin-producing species of Amanita and is clustered in the genome with at least one expressed member of the MSDIN gene family. Immunologically, amanitin and AbPOPB show a high degree of colocalization, indicating that toxin biosynthesis and accumulation occur in the same cells and possibly in the same subcellular compartments.

Amatoxins, the lethal constituents of poisonous mushrooms in the genus Amanita, are bicyclic octapeptides. Two genes in A. bisporigera, AMA1 and PHA1, directly encode alpha-amanitin, an amatoxin, and the related bicyclic heptapeptide phallacidin, a phallotoxin, indicating that these compounds are synthesized on ribosomes and not by nonribosomal peptide synthetases. alpha-Amanitin and phallacidin are synthesized as proproteins of 35 and 34 amino acids, respectively, from which they are predicted to be cleaved by a prolyl oligopeptidase. AMA1 and PHA1 are present in other toxic species of Amanita section Phalloidae but are absent from nontoxic species in other sections. The genomes of A. bisporigera and A. phalloides contain multiple sequences related to AMA1 and PHA1. The predicted protein products of this family of genes are characterized by a hypervariable "toxin" region capable of encoding a wide variety of peptides of 7-10 amino acids flanked by conserved sequences. Our results suggest that these fungi have a broad capacity to synthesize cyclic peptides on ribosomes.

Abstract: Amatoxins, including α-amanitin, are bicyclic octapeptides found in mushrooms (Agaricomycetes, Agaricales) of certain species in the genera Amanita, Galerina, Lepiota, and Conocybe. Amatoxins and the chemically similar phallotoxins are synthesized on ribosomes in Amanita bisporigera, Amanita phalloides, and Amanita ocreata. In order to determine if amatoxins are synthesized by a similar mechanism in another, distantly related mushroom, we obtained genome survey sequence data from a monokaryotic isolate of Galerina marginata, which produces α-amanitin. The genome of G. marginata contains two copies of the α-amanitin gene (GmAMA1-1 and GmAMA1-2). The α-amanitin proprotein sequences of G. marginata (35 amino acids) are highly divergent from AMA1 of A. bisporigera except for the toxin region itself (IWGIGCNP in single-letter amino acid code) and the amino acids immediately upstream (N[A/S]TRLP). G. marginata does not contain any related toxin-encoding sequences besides GmAMA1-1 and GmAMA1-2. DNA from two other α-amanitin-producing isolates of Galerina (G. badipes and G. venenata) hybridized to GmAMA1, whereas DNA from the toxin non-producing species Galerina hybrida did not. Expression of the GmAMA1 genes was induced by growth on low carbon. RNASeq evidence indicates that both copies of GmAMA1 are expressed approximately equally. A prolyl oligopeptidase (POP) is strongly implicated in processing of the cyclic peptide toxins of A. bisporigera and Conocybe apala. G. marginata has two predicted POP genes; one, like AbPOPB of A. bisporigera, is present only in the toxin-producing isolates of Galerina and the other, like AbPOPA of A. bisporigera, is present in all species. Our results indicate that G. marginata biosynthesizes amatoxins on ribosomes by a pathway similar to Amanita species, involving a genetically encoded proprotein of 35 amino acids that is post-translationally processed by a POP. However, due to the high degree of divergence, the evolutionary relationship between AMA1 in the genera Amanita and Galerina is unclear. [Copyright &y& Elsevier]