OSIP1 is a self-assembling DUF3129 protein required to protect fungal cells from toxins and stressors.

Secreted proteins are key players in fungal physiology and cell protection against external stressing agents and antifungals. Oak stress-induced protein 1 (OSIP1) is a fungal-specific protein with unknown function. By using Podospora anserina and Phanerochaete chrysosporium as models, we combined both in vivo functional approaches and biophysical characterization of OSIP1 recombinant protein. The P. anserina OSIP1Δ mutant showed an increased sensitivity to the antifungal caspofungin compared to the wild type. This correlated with the production of a weakened extracellular exopolysaccharide/protein matrix (ECM). Since the recombinant OSIP1 from P. chrysosporium self-assembled as fibers and was capable of gelation, it is likely that OSIP1 is linked to ECM formation that acts as a physical barrier preventing drug toxicity. Moreover, compared to the wild type, the OSIP1Δ mutant was more sensitive to oak extractives including chaotropic phenols and benzenes. It exhibited a strongly modified secretome pattern and an increased production of proteins associated to the cell-wall integrity signalling pathway, when grown on oak sawdust. This demonstrates that OSIP1 has also an important role in fungal resistance to extractive-induced stress.

Oak extractive-induced stress reveals the involvement of new enzymes in the early detoxification response of Phanerochaete chrysosporium.

Extensive evidence showed that the efficiency of fungal wood degradation is closely dependent on their ability to cope with the myriad of putative toxic compounds called extractives released during this process. By analysing global gene expression of Phanerochaete chrysosporium after short oak extractive treatment (1, 3 and 6 h), we show that the early molecular response of the fungus concerns first mitochondrial stress rescue followed by the oxidation and finally conjugation of the compounds. During these early responses, the lignolytic degradative system is not induced, rather some small secreted proteins could play an important role in cell protection or signaling. By focusing on the functional characterization of an hitherto uncharacterized glutathione transferase, we show that this enzyme interacts with wood molecules suggesting that it could be involved in the detoxification of some of them, or act as a scavenger to prevent their cytosolic toxicity and favour their transport.

Secretion of small proteins is species-specific within Aspergillus sp.

Small secreted proteins (SSP) have been defined as proteins containing a signal peptide and a sequence of less than 300 amino acids. In this analysis, we have compared the secretion pattern of SSPs among eight aspergilli species in the context of plant biomass degradation and have highlighted putative interesting candidates that could be involved in the degradative process or in the strategies developed by fungi to resist the associated stress that could be due to the toxicity of some aromatic compounds or reactive oxygen species released during degradation. Among these candidates, for example, some stress-related superoxide dismutases or some hydrophobic surface binding proteins (HsbA) are specifically secreted according to the species . Since these latter proteins are able to recruit lytic enzymes to the surface of hydrophobic solid materials and promote their degradation, a synergistic action of HsbA with the degradative system may be considered and need further investigations. These SSPs could have great applications in biotechnology by optimizing the efficiency of the enzymatic systems for biomass degradation.