The Biological Roles of Class II Hydrophobins at the Cell Wall of Filamentous Ascomycetes

One of the most elusive aspects of fungal cell wall biology has been dilucidating the specific functions of most cell wall proteins. Class II hydrophobins are small amphipathic proteins produced exclusively by filamentous ascomycetes. A comparative synthesis of their biological functions across species has been lacking for this specific group of proteins. This review examines the available genetic, cell biological, and biochemical evidence on Class II hydrophobins in filamentous ascomycetes, with emphasis on Sordariomycetes, organized by both gene copy number and functional category. Gene deletion studies across species consistently reveal phenotypic effects on conidiation, desiccation resistance, host adhesion, and hyphal architecture, with severity scaling inversely with gene copy number. The evidence points to Class II hydrophobins acting as structural components integrated into or closely associated with the cell wall matrix, modulating its hydrophobic properties and mechanical characteristics. Abolishing these cell wall modulating properties can result in phenotypes related to fungal development and fitness. Understanding the contributions of Class II hydrophobins and other similar proteins to cell wall biology is directly relevant to fungal pathogenicity, host recognition, and antifungal strategies targeting the wall surface.