A Possible Role for Exocytosis in Aflatoxin Export in Aspergillus parasiticus

Abstract

Filamentous fungi synthesize bioactive secondary metabolites with major human health and economic impacts. Little is known about the mechanisms that mediate the export of these metabolites to the cell exterior. Aspergillus parasiticus synthesizes aflatoxin, a secondary metabolite that is one of the most potent naturally occurring carcinogens known. We previously demonstrated that aflatoxin is synthesized and compartmentalized in specialized vesicles called aflatoxisomes and that these subcellular organelles also play a role in the export process. In the current study, we tested the hypothesis that aflatoxisomes fuse with the cytoplasmic membrane to facilitate the release of aflatoxin into the growth environment. Microscopic analysis of A. parasiticus grown under aflatoxin-inducing and non-aflatoxin-inducing conditions generated several lines of experimental evidence that supported the hypothesis. On the basis of the evidence, we propose that export of the mycotoxin aflatoxin in Aspergillus parasiticus occurs by exocytosis, and we present a model to illustrate this export mechanism.Secondary metabolites are chemically diverse natural products synthesized by plants, fungi, bacteria, algae, and animals. Secondary metabolites have an enormous impact on humans. Antibiotics, for example, are essential elements of the multibillion-dollar pharmaceutical industry, whereas mycotoxins cause hundreds of millions of dollars in damage to agriculture annually (11,15). These chemicals help the producing organism to survive nutrient limitation (16). They also contribute to cellular defense mechanisms and development (11, 12), reduce cellular oxidative stress (10), and help maintain cellular homeostasis by regulating carbon flow in the cell (17).Many fungal secondary metabolites are exported outside the cell; examples include antibiotics and mycotoxins (3,14). We and others conducted extensive studies on the regulation of fungal secondary metabolism at the molecular (11, 15) and cellular (3, 7) levels. However, little is known about the mechanisms that mediate secondary metabolite export or why export occurs.The filamentous fungus Aspergillus parasiticus produces aflatoxin, a secondary metabolite and the most potent naturally occurring carcinogen known. More than 90% of aflatoxin is exported to the cell exterior (3), making A. parasiticus an excellent model for studying secondary metabolite export. We recently demonstrated that specialized trafficking vesicles called aflatoxisomes play a key role in aflatoxin synthesis and export (3). As synthesis initiates, vesicle-vacuole fusion is downregulated by the global regulator Velvet, resulting in the accumulation of aflatoxisomes which contain at least the last two functional enzymes in the aflatoxin pathway and sequester aflatoxin (3). Treatments that block vesicle-vacuole fusion increase the number of aflatoxisomes, increase the quantity of aflatoxin accumulated in aflatoxisomes, and increase aflatoxin export to the cell exterior (3). On the basis of these previous observations, we ...

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