Characterization of ordered aggregates of cerato-platanin and their involvement in fungus-host interactions

Background: The "cerato-platanin family" consists of fungal-secreted proteins that are involved in various stages of the host-fungus interaction and act as phytotoxins, elicitors of defense responses and allergens. Cerato-platanin (CP) is a moderately hydrophobic protein secreted and localized in the cell wall of Ceratocystis platani, the causal agent of a severe disease of Platanus. These properties make CP like the hydrophobins: these are self-assembling proteins that form a surface coating which is involved in the formation of aerial hyphae and in adherence to surfaces. Methods: CP aggregation was monitored by ThT, circular dichroism, and AFM. The eliciting activity of CP aggregates was assayed on leaves and cells. Results: The CP self-assembles forming amyloid-like aggregates via a nucleated growth mechanism which is joined up with a cleavage of the N-terminus. The ovoidal shape and the lack of a clear transition toward an all-β structure distinguish these aggregates from typical amyloid fibrils. Moreover, CP aggregates interact with hydrophobic surfaces and enhance the hypersensitive response of Platanus. Conclusion and general significance: CP forms "ordered aggregates" for which the soluble prefibrillar structures are the end point of the aggregation process, and do not evolve to insoluble fibrils. An involvement in host-microbe interaction is also suggested.