The Filament-specific Rep1-1 Repellent of the Phytopathogen Ustilago maydis Forms Functional Surface-active Amyloid-like Fibrils

Abstract

Repellents of the maize pathogen Ustilago maydis are involved in formation of hydrophobic aerial hyphae and in cellular attachment. These peptides, called Rep1-1 to Rep1-11, are encoded by the rep1 gene and result from cleavage of the precursor protein Rep1 during passage of the secretion pathway. Using green fluorescent protein as a reporter, we here show that rep1 is expressed in filaments and not in the yeast form of U. maydis. In situ hybridization localized rep1 mRNA in the apex of the filament, which correlates with the expected site of secretion of the repellents into the cell wall. We also produced a synthetic peptide, Rep1-1. This peptide reduced the water surface tension to as low as 36 mJ m ؊2 . In addition, it formed amyloid-like fibrils as was shown by negative staining, by thioflavin T fluorescence, and by x-ray diffraction. These fibrils were not soluble in SDS but could be dissociated with trifluoroacetic acid. The repellents in the hyphal cell wall had a similar solubility and also stained with thioflavin T, strongly indicating that they are present as amyloid fibrils. However, such fibrils could not be observed at the hyphal surface. This can be explained by the fact that the Rep1-1 filaments decrease in length at increasing concentrations. Taken together, we have identified the second class of fungal proteins that form functional amyloid-like filaments at the hyphal surface.Ustilago maydis is the causal agent of smut in Zea mays (maize) and Euchlaena mexicana (Mexican teosinte). A filamentous pathogenic dikaryon is formed upon fusion of compatible yeast-like sporidia. Differentiation in the plant leads to the formation of diploid teliospores, which undergo meiosis ultimately resulting in haploid sporidia (see Refs. 1-3).Fusion of haploid cells and development of an infectious dikaryon are controlled by the a and b mating type loci. The a locus regulates cell fusion through a pheromone-based recognition system (4), and the b locus controls post-fusion steps of pathogenic development, including hyphal growth. The latter locus encodes two unrelated homeodomain proteins, bE and bW, that form heterodimers when they are derived from different alleles (5, 6). This heterodimer regulates a number of genes, among which the rep1 gene (7-10). This gene is highly expressed resulting in 2.5% of the mRNA. It encodes a preproprotein that, after processing at KEX2 recognition sites, results in 11 secreted peptides with a high sequence similarity. These peptides are localized in the cell wall of filaments, in an SDSinsoluble, but trifluoroacetic acid-extractable form (10). They are involved in formation of hydrophobic aerial hyphae (10) and in hyphal attachment to hydrophobic surfaces (11); as such they have functionally replaced hydrophobins in U. maydis (11).Hydrophobins, which are not related to the repellents, fulfill a wide spectrum of functions in fungal development (12). They do so by forming an amphipathic protein film, which consists of amyloid-like fibrils (13-15). We here show that the repellent Rep1...

Yeast Cell Adhesion Molecules Have Functional Amyloid-Forming Sequences

“…These protein aggregates can also occur naturally in adhesive bacterial curli (3), melanosomes (14), condensed peptide hormone arrays (24), as regulatory prions in yeast (2, 5), and fungal hydrophobins, which are nonantigenic coats to some fungi (1,33,39). Nevertheless, such natural occurrences are relatively few, considering the negative free energy for amyloid formation (28).…”

Lectin mapping reveals stage-specific display of surface carbohydrates in in vitro and haemolymph-derived cells of the entomopathogenic fungus Beauveria bassiana

“…Although this is speculative, this structure may mask the hyphal bodies from the host immune system, allowing for the free circulation of these cells. Surface fibrillar structures, either proteinaceous or carbohydrate in nature, some of which are thought to contribute to virulence and immune evasion, have long been noted on yeasts and A. Wanchoo, M. W. Lewis and N. O. Keyhani other fungi (Hazen & Hazen, 1993;Latgé et al, 1988;Latgé, 2007;Osumi, 1998;Takeo et al, 1993;Teertstra et al, 2009). Further characterization of the composition and potential role of the B. bassiana material is warranted.…”

Assembly of the Fungal SC3 Hydrophobin into Functional Amyloid Fibrils Depends on Its Concentration and Is Promoted by Cell Wall Polysaccharides

“…It may well be that other proteins that form functional amyloids at a microbial surface such as repellents of Ustilago maydis (30), chaplins of Streptomyces coelicolor (31), and curli of Escherichia coli (32) are also stimulated by cell wall polysaccharides to adopt this conformation. This is strengthened by the fact that disease-associated amyloid formation is promoted by glycosaminoglycans (polysaccharides containing repeating units of disaccharides) and proteoglycans (glycosaminoglycans bound to a core protein) (33,34).…”