CELL-WALL GROWTH AND PROTEIN SECRETION IN FUNGI

Secretion of proteins is a vital process in fungi. Because hyphal walls form a diffusion barrier for proteins, a mechanism different from diffusion probably exist to transport proteins across the wall. In Schizophyllum commune, evidence has been obtained for synthesis at the hyphal apex of wall components, 1,3-beta-glucan and chitin, as separate components. These become subapically cross-linked by formation of covalent and noncovalent bonds, producing a rigid wall (steady-state walt growth). Because the wall at the apex apparently grows by apposition of plastic wall material, proteins excreted at the apex may pass the wall by being carried with the flow of wall material (bulk flow), making pores in the wall less important than previously thought. A large portion of excreted proteins leaves hyphae at the growing apices, another portion is retained by the wall and slowly released from the mature wall into the environment. Among proteins that can be permanently retained by the wall are the hydrophobins that self-assemble at the outer wall surface when confronted with a hydrophilic-hydrophobic interface. They were shown to mediate both the emergence of aerial hyphae and the attachment of hyphae to hydrophobic substrates.