Novel carbohydrate-binding module of beta-1,3-xylanase from a marine bacterium, Alcaligenes sp. strain XY-234.

A beta-1,3-xylanase gene (txyA) from a marine bacterium, Alcaligenes sp. strain XY-234, has been cloned and sequenced. txyA consists of a 1,410-bp open reading frame that encodes 469 amino acid residues with a calculated molecular mass of 52,256 Da. The domain structure of the beta-1,3-xylanase (TxyA) consists of a signal peptide of 22 amino acid residues, followed by a catalytic domain which belongs to family 26 of the glycosyl hydrolases, a linker region with one array of DGG and six repeats of DNGG, and a novel carbohydrate-binding module (CBM) at the C terminus. The recombinant TxyA hydrolyzed beta-1,3-xylan but not other polysaccharides such as beta-1,4-xylan, carboxymethylcellulose, curdlan, glucomannan, or beta-1,4-mannan. TxyA was capable of binding specifically to beta-1,3-xylan. The analysis using truncated TxyA lacking either the N- or C-terminal region indicated that the region encoding the CBM was located between residues 376 and 469. Binding studies on the CBM revealed that the K(d) and the maximum amount of protein bound to beta-1,3-xylan were 4.2 microM and 18.2 micromol/g of beta-1,3-xylan, respectively. Furthermore, comparison of the enzymatic properties between proteins with and without the CBM strongly indicated that the CBM of TxyA plays an important role in the hydrolysis of beta-1,3-xylan.