High xylanase activity and stability toward alkaline pH is strongly desired for pulping and bleaching processes. We previously enhanced thermal stability of Bacillus circulans xylanase (BCX) by inserting into a thermophilic maltodextrin-binding protein from Pyrococcus furiosus (PfMBP) (the resulting complex named as PfMBP-BCX165). In the present study, we aimed to evolve the inserted BCX domain within PfMBP-BCX165 for greater xylanase activity toward alkaline pH while maintaining enhanced thermal stability. No BCX sequence variation was required for the thermal stabilization, thus allowing us to explore the entire BCX sequence space for the evolution. Specifically, we randomized the BCX sequence within PfMBP-BCX165 and then screened the resulting libraries to identify a PfMBP-BCX165 variant, PfMBP-BCX165. The T50R mutation enhanced xylanase activity of PfMBP-BCX165 toward alkaline pH without compromising thermal stability. When compared to PfMBP-BCX165, the corresponding unfused BCX mutant, BCX, exhibited similar pH dependence of xylanase activity, yet suffered from limited thermal stability. In summary, we showed that one can improve thermal stability and xylanase activity of BCX toward alkaline pH by inserting into PfMBP followed by sequence variation of the BCX domain. Our study also suggested that insertional fusion to PfMBP would be a useful stabilizing platform for evolving many proteins.

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High thermostability of an enzyme is critical for its industrial application. While many engineering approaches such as mutagenesis have enhanced enzyme thermostability, they often suffer from reduced enzymatic activity. A thermally stabilized enzyme with unchanged amino acids is preferable for subsequent functional evolution necessary to address other important industrial needs. In the research presented here, we applied insertional fusion to a thermophilic maltodextrin-binding protein from Pyrococcus furiosus (PfMBP) in order to improve the thermal stability of Bacillus circulans xylanase (BCX). Specifically, we used an engineered transposon to construct a combinatorial library of randomly inserted BCX into PfMBP. The library was then subjected to functional screening to identify successful PfMBP-BCX insertion complexes, PfMBP-BCX161 and PfMBP-BCX165, displaying substantially improved kinetic stability at elevated temperatures compared to unfused BCX and other controls. Results from subsequent characterizations were consistent with the view that lowered aggregation of BCX and reduced conformational flexibility at the termini was responsible for increased thermal stability. Our stabilizing approach neither sacrificed xylanase activity nor required changes in the BCX amino acid sequence. Overall, the current study demonstrated the benefit of combinatorial insertional fusion to PfMBP as a systematic tool for the creation of enzymatically active and thermostable BCX variants.