Interactions by the Fungal Flo11 Adhesin Depend on a Fibronectin Type III-like Adhesin Domain Girdled by Aromatic Bands

• Flo11 harbors a fibronectin type III-like domain that confers adhesion
• Flo11A domain has an inside-out topology reminiscent of hydrophobins
• pH-dependent homo- and heterophilic interactions via surface-exposed aromatic bands
• Flo11 clusters on cell surfaces to form spacer-like cell-cell contacts

SummarySaccharomyces cerevisiae harbors a family of GPI-anchored cell wall proteins for interaction with its environment. The flocculin Flo11, a major representative of these fungal adhesins, confers formation of different types of multicellular structures such as biofilms, flors, or filaments. To understand these environment-dependent growth phenotypes on a molecular level, we solved the crystal structure of the N-terminal Flo11A domain at 0.89-Å resolution. Besides a hydrophobic apical region, the Flo11A domain consists of a β sandwich of the fibronectin type III domain (FN3). We further show that homophilic Flo11-Flo11 interactions and heterophilic Flo11-plastic interactions solely depend on the Flo11A domain and are strongly pH dependent. These functions of Flo11A involve an apical region with its surface-exposed aromatic band, which is accompanied by acidic stretches. Together with electron microscopic reconstructions of yeast cell-cell contact sites, our data suggest that Flo11 acts as a spacer-like, pH-sensitive adhesin that resembles a membrane-tethered hydrophobin.

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