Background: Fungal swollenins (SWOs) constitute a class of accessory proteins that are homologous to canonical plant expansins. Expansins and expansin-related proteins are well known for acting in the deagglomeration of cellulose structure by loosening macrofibrils. Consequently, SWOs can increase the accessibility and efficiency of the other enzymes involved in the saccharification of cellulosic substrates. Thus, SWOs are promising targets for improving the hydrolysis of plant biomass and for use as an additive to enhance the efficiency of an enzyme cocktail designed for the production of biofuels.
Results: Here, we report the initial characterization of an SWO from Trichoderma harzianum (ThSwo) that was successfully produced using Escherichia coli as a host. Initially, transcriptome and secretome data were used to compare swo gene expression and the amount of secreted ThSwo. The results from structural modeling and phylogenetic analysis of the ThSwo protein showed that ThSwo does preserve some structural features of the plant expansins and family-45 glycosyl hydrolase enzymes, but it evolutionarily diverges from both of these protein classes. Recombinant ThSwo was purified at a high yield and with high purity and showed secondary folding similar to that of a native fungal SWO. Bioactivity assays revealed that the purified recombinant ThSwo created a rough and amorphous surface on Avicel and displayed a high synergistic effect with a commercial xylanase from T. viride, enhancing its hydrolytic performance up to 147 ± 7%.
Conclusions: Many aspects of the structure and mechanism of action of fungal SWOs remain unknown. In the present study, we produced a recombinant, active SWO from T. harzianum using a prokaryotic host and confirmed its potential synergistic role in biomass degradation. Our work paves the way for further studies evaluating the structure and function of this protein, especially regarding its use in biotechnology.
Keywords: Biomass degradation; Expansin; Overexpression; Swollenin; Trichoderma harzianum.