Cerato-platanin family proteins: one function for multiple biological roles?

Cerato-platanin family proteins (CPPs) have been found in more than 50 fungal genomes and seem to exist only in fungi. CPPs can activate defense responses in plants and have been reported to act as pathogen/microbe-associated molecular patterns (PAMPs/MAMPs). However, to date it is not easy to answer the question of why fungi produce these proteins. CPPs are abundantly secreted outside the hyphae but can also be found localized in the fungal cell walls. CPPs do not show any enzymatic activity but, for some pathogenic fungi, they are important for virulence on their plant hosts. It is generally believed that CPPs have at least two roles in fungi: one in growth and development, which should justify their presence in the cell wall, and one, more elusive, that should explain their secretion and the interaction with plants. But what is their function? This Opinion Article will attempt to answer these questions: can we hypothesize a basal function that characterizes CPPs and allows them to have multiple biological roles depending on the context (fungal cell wall or extracellular environment)? Can this function imply a role in plant colonization? This article will begin by considering the points of similarity between CPPs and expansins, which are proteins without enzymatic activity that, in plants, mediate the turgor-driven extension of the cell walls. Subsequently, on the basis of recent results obtained on cellulosic materials, a basal function for CPPs and its biological implications will be suggested: CPPs might act in an expansin-like manner on non-covalent interactions and cause the loosening of fungal and plant cell walls. With this function, CPPs could act in all those processes requiring remodeling and enlargement of the fungal cell wall; when secreted, they could facilitate the mechanical penetration of the plant cell wall and thus colonization. This assumption could explain why knockout mutants have never revealed differences in morphological traits compared to wild types (because fungi also have expansin-like genes in their genomes), and why some mutants instead revealed differences when tested on host plants (because CPPs would be very effective during plant colonization, and this may actually be their main role). The article will contain a summary table of the mutants obtained up to date for CPP genes, with their phenotypic characterization and the presence of expansin-like genes in each fungal genome.