Characterization of Ruminiclostridium josui arabinoxylan arabinofuranohydrolase, RjAxh43B, and RjAxh43B-containing xylanolytic complex

A novel gene (axh43B) from Ruminiclostridium josui encoding a cellulosomal enzyme consisting of a catalytic module of subfamily GH43_10, a family-6 carbohydrate-binding module, and a dockerin module, was expressed using Escherichia coli. RjAxh43 B released only arabinose from arabinoxylan and 23,33-di-α-l-arabinofuranosyl xylotriose, but not 32-α-l-arabinofuranosyl xylobiose or 23-α-l-arabinofuranosyl xylotriose, strongly suggesting that RjAxh43 B is an arabinoxylan α-l-1,3-arabinofuranohydrolase capable of cleaving α-1,3-linked arabinose residues of doubly arabinosylated xylan. When Axh43 B was mixed with the recombinant scaffolding protein RjCipA of R. josui at a molar ratio of 6:1, the activity of the RjAxh43B-RjCipA complex (6:1) toward insoluble wheat arabinoxylan was similar to that of RjAxh43 B alone, suggesting that RjAxh43 B does not show a proximity effect, which is defined as an activity enhancement effect caused by the presence of plural catalytic subunits adjoining each other. When RjAxh43A was mixed with xylanase RjXyn10C, they acted synergistically toward insoluble wheat arabinoxylan and rice straw powder in the absence of RjCipA. Furthermore, the RjAxh43B-RjXyn10C-RjCipA (3:3:3) complex had higher activity toward insoluble wheat arabinoxylan than a mixture of RjAxh43 B and RjXyn10C without RjCipA, suggesting that incorporation of a xylanase and an α-l-arabinofuranosidase into a cellulosome is beneficial for more efficiently degrading arabinoxylan.