Genetic and functional characterization of a novel GH10 endo-β- 1,4-xylanase with a ricin-type β-trefoil domain-like domain from Luteimicrobium xylanilyticum HY-24

The gene (1488-bp) encoding a novel GH10 endo-β-1,4-xylanase (XylM) consisting of an N-terminal catalytic GH10 domain and a C-terminal ricin-type β-trefoil lectin domain-like (RICIN) domain was identified from Luteimicrobium xylanilyticum HY-24. The GH10 domain of XylM was 72% identical to that of Micromonospora lupini endo-β-1,4-xylanase and the RICIN domain was 67% identical to that of Actinospica robiniae hypothetical protein. The recombinant enzyme (rXylM: 49 kDa) exhibited maximum activity toward beechwood xylan at 65 °C and pH 6.0, while the optimum temperature and pH of its C-terminal truncated mutant (rXylM△RICIN: 35 kDa) were 45 °C and 5.0, respectively. After pre-incubation of 1 h at 60 °C, rXylM retained over 80% of its initial activity, but the thermostability of rXylM△RICIN was sharply decreased at temperatures exceeding 40 °C. The specific activity (254.1 U mg−1) of rXylM toward oat spelts xylan was 3.4-fold higher than that (74.8 U mg−1) of rXylM△RICIN when the same substrate was used. rXylM displayed superior binding capacities to lignin and insoluble polysaccharides compared to rXylM△RICIN. Enzymatic hydrolysis of β-1,4-d-xylooligosaccharides (X3-X6) and birchwood xylan yielded X3 as the major product. The results suggest that the RICIN domain in XylM might play an important role in substrate-binding and biocatalysis.