A hydrophobin (ABH3) specifically secreted by vegetatively growing hyphae of Agaricus bisporus (common white button mushroom)

Aerial mycelium and hyphal strands of Agaricus bisporus, strain U1, exhibited a rodlet pattern at their surfaces characteristic for assembled class I hydrophobins. An SDS-insoluble/trifluoroacetic-acid-soluble fraction from strands was found to contain one abundant protein with an apparent molecular mass on gel of 19 kDa. Two sequences for this protein (ABH3), typical of class I hydrophobins, could be deduced by sequencing cDNA clones obtained by RT-PCR. The two forms of the protein could be assigned to different alleles present in the two homokaryons that constitute the heterokaryotic U1 strain. ABH3 displays all the in vitro properties of a typical class I hydrophobin such as SC3 from Schizophyllum commune but is not glycosylated or otherwise post-translationally modified because the molecular mass values deduced from the amino acid sequence (9228 and 9271 Da) and derived from mass spectrometry were in good agreement. The ABH3 transcript was found to be present in the vegetative mycelium of both primary and secondary mycelium but not in the fruiting bodies, whereas the reverse was found for the ABH1 hydrophobin. Using an S. commune mutant with a disrupted SC3 gene it was found that ABH3 can substitute for SC3 in inducing formation of aerial hyphae, suggesting a role of ABH3 in the emergence of aerial hyphae and strands in A. bisporus.