A GLYCOSYLATION PATTERN THAT IS UNIQUE TO FIMBRIAE FROM THE TAXON MICROBOTRYALES

Eukaryotic fimbriae are extracellular structures that have been shown to play a key role in mating and pathogenesis. Previous work has established that the major component of the fimbriae of the anther smut fungus Miclobotiyum violaceum (Pers.) G. Deml & Oberw. (Ustilago violacea (Pers.) Rouss.) is a 74-kDa glycoprotein subunit. This study analyzes the carbohydrate moiety in the fimbrial subunit and reveals that it is composed of mannose and possibly some acetylated sugars that are anchored to an asparagine residue of the protein via two N-acetylglucosamine residues. The carbohydrate component comprises 35% of the fimbrial subunit and the aglycone has an apparent molecular mass of 47 kDa. Using polyclonal antibodies, we have examined the conservation of the two components of the fimbrial subunits (protein and carbohydrate) in four divisions of fungi. Antibodies raised against native intact fimbriae of M. violaceum detected only the unique glycosylation pattern of these fimbriae, whilst antibodies raised against denatured fimbrial subunits were able to detect protein epitopes. Our results indicate that the fimbrial subunits of M. violaceum are composed of a protein moiety that is highly conserved amongst fungal species and a carbohydrate moiety that has a pattern that is unique to members of the dicot-infecting smut fungi. The possible implications of a conserved glycosylation pattern and the limited host range of this phylogenetic line of smut fungi are discussed in relation to current attempts to reclassify these species into the taxon Microbotryales.