Differential proteomic analysis of the secretome of Irpex lacteus and other white-rot fungi during wheat straw pretreatment

Abstract

Background: Identifying new high-performance enzymes or enzyme complexes to enhance biomass degradation is the key for the development of cost-effective processes for ethanol production. Irpex lacteus is an efficient microorganism for wheat straw pretreatment, yielding easily hydrolysable products with high sugar content. Thus, this fungus was selected to investigate the enzymatic system involved in lignocellulose decay, and its secretome was compared to those from Phanerochaete chrysosporium and Pleurotus ostreatus which produced different degradation patterns when growing on wheat straw. Extracellular enzymes were analyzed through 2D-PAGE, nanoLC/MS-MS, and homology searches against public databases.

Results: In wheat straw, I. lacteus secreted proteases, dye-decolorizing and manganese-oxidizing peroxidases, and H2O2 producing-enzymes but also a battery of cellulases and xylanases, excluding those implicated in cellulose and hemicellulose degradation to their monosaccharides, making these sugars poorly available for fungal consumption. In contrast, a significant increase of β-glucosidase production was observed when I. lacteus grew in liquid cultures. P. chrysosporium secreted more enzymes implicated in the total hydrolysis of the polysaccharides and P. ostreatus produced, in proportion, more oxidoreductases.

Conclusion: The protein pattern secreted during I. lacteus growth in wheat straw plus the differences observed among the different secretomes, justify the fitness of I. lacteus for biopretreatment processes in 2G-ethanol production. Furthermore, all these data give insight into the biological degradation of lignocellulose and suggest new enzyme mixtures interesting for its efficient hydrolysis.

Figure 1

2D-gels from I. lacteus secretomes released under SSF and SmF conditions. (a-c) 7, 14 and 21-d wheat straw SSF cultures, respectively; (d) 21-d wheat straw SSF cultures with Mn⁺² supplementation; and (e) 21-d SmF cultures in CSS medium. The spots analyzed are numbered on the gels and the proteins identified detailed in Table 2.

Figure 2

Functional classification of the lignocellulose-degrading enzymes found in the secretomes, according to Uniprot and JGI searches. Groups of proteins released by I. lacteus grown on wheat straw (a-b) or in submerged cultures (c-d), and in SSF cultures P. chrysosporium (e-f) and P. ostreatus (g-h). The total number of protein matches from JGI and Uniprot databases (Additional file 1: Tables S2-S9) is shown in parenthesis. Basidiomycota databases were used for I. lacteus secretome searches. The results from P. chrysosporium and P. ostreatus were searched against their own databases. SSF= solid state fermentation on wheat straw; SmF= submerged cultures in CSS; GH= glycosil hydrolases.