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, ex hibited a rodlet pattern at their surfaces characteristic for assemble d class I hydrophobins. An SDS-insoluble/trifluoroacetic-acid-soluble fraction from strands was found to contain one abundant protein with a n apparent molecular mass on gel of 19 kDa. Two sequences for this pro tein (ABH3), typical of class I hydrophobins, could be deduced by sequ encing cDNA clones obtained by RT-PCR. The two forms of the protein co uld be assigned to different alleles present in the two homokaryons th at constitute the heterokaryotic U1 strain. ABH3 displays all the in v itro properties of a typical class I hydrophobin such as SC3 from Schi zophyllum commune but is not glycosylated or otherwise post-translatio nally modified because the molecular mass values deduced from the amin o 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 no t in the fruiting bodies, whereas the reverse was found for the ABH1 h ydrophobin. Using an S. commune mutant with a disrupted SC3 gene it wa s found that ABH3 can substitute for SC3 in inducing formation of aeri al hyphae, suggesting a role of ABH3 in the emergence of aerial hyphae and strands in A. bisporus.