Identification of an a-factor-like peptide mating pheromone secreted by the heterothallic ascomycete Aspergillus fumigatus

Fungal sexuality accompanied by the formation of fruiting bodies that contain fertile meiospores relies on a complex sequence of events. In heterothallic ascomycetes, mating-type systems serve as regulatory means to secure that compatible isolates of opposite gender fuse to enter the sexual phase in their life cycle. This intricate process requires reciprocal secretion and recognition of pheromones, small peptides that are processed from precursors to become secreted into the cellular vicinity. Identification of mating pheromones of fungal origin with their cognate receptors is generally achieved by genome mining and homology searches, based on considerable conservation on the protein sequence level. In the taxonomic class of the Eurotiomycetes this approach had failed for peptides that would resemble a-factor-like pheromones due to their small size and low sequence conservation. Accordingly, the existence and nature of an a-factor-like peptide secreted by the heterothallic mould Aspergillus fumigatus had not been revealed to date. Sexuality of this opportunistic human pathogen is genetically determined by a bipolar mating-type system encoding master regulators in an exclusive manner. By making use of consistent transcriptional profiling data, we could identify an unannotated candidate gene ppgB (pheromone precursor gene B) encoding the presumed but so far elusive a-factor pheromone of A. fumigatus. The deduced peptide is 24 amino acids in length and comprises a canonical CaaX box motif at its C-terminus. Transcription patterns of ppgB and functional analyses of its hydrophobic product by employing a suitable test system that is based on pheromone-sensitive yeast cells strongly support the hypothesis that PpgB serves as prototype for the long-sought a-factor like pheromone of the aspergilli. The identification of A. fumigatus PpgB closes a substantial knowledge gap with respect to cellular recognition and sexual propagation of Eurotiomycete fungi.