Novel sugar-binding specificity of the type XIII xylan-binding domain of afamily F/10 xylanase from Streptomyces olivaceoviridis E-86

The type XIII xylan-binding domain (XBD) of a family F/10 xylanase (FXYN) f rom Streptomyces olivaceoviridis E-86 was found to be structurally similar to the ricin B chain which recognizes the non-reducing end of galactose and specifically binds to galactose containing sugars. The crystal structure o f XBD [Fujimoto, Z. et al. (2000) J. Mol. Biol. 300, 575-585] indicated tha t the whole structure of XBD is very similar to the ricin B chain and the a mino acids which form the galactose-binding sites are highly conserved betw een the XBD and the ricin B chain. However, our investigation of the bindin g abilities of wt FXYN and its truncated mutants towards xylan demonstrated that the XBD bound xylose-based polysaccharides, Moreover, it was found th at the sugar-binding unit of the XBD was a trimer, which was demonstrated i n a releasing assay using sugar ranging in size from xylose to xyloheptaose . These results indicated that the binding specificity of the XBD was diffe rent from those of the same family lectins such as the ricin B chain. Somew hat surprisingly, it was found that lactose could release the XBD from inso luble xylan to a level half of that observed for xylobiose, indicating thai the XBD also possessed the same galactose recognition site as the ricin B chain. It appears that the sugar-binding pocket of the XBD has evolved from the ancient ricin super family lectins to bind additional sugar targets, r esulting in the differences observed in the sugar-binding specificities bet ween the lectin group (containing the ricin B chain) and the enzyme group. (C) 2000 Federation of European Biochemical Societies. Published by Elsevie r Science B.V. All rights reserved.