Structural biological studies on the sugar-binding mechanism of two sugar-binding domains having different physiological functions

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We investigated the sugar-binding mechanism of the two sugar-binding domains belonging to the R-type lectin family, one is xylan binding domain (XBD) in Stnaptamyees olivaasoviridis xylanase and another is the C-terminal domain of a novel 29-kDa lectin from earthworm (EW29Ch) having hemagglutinating activity, in order to clarify why the R-type lectins can have various functions. By the NMR titration method, we determined the binding activities of the two sugar-binding domains for the following sugars: lactose, galactose, xylose, xylobiose, xylobiose, xylotetraose, and xylohexaose for XBD; lactose, melibiose, galactose, methyl-α-galactopyranoside, and metbyl-β-galactopyranoskle. The results showed that XBD had three binding sites in the three subdomains α, β, and γ, and that each of the three binding sites had different sugarbinding specificities for kind of sugars and all of the binding sites had high binding activities for xylotetraose and xylohexaose. Thus, the three binding sites of … More XBD may bind different xylan chains at the same time as effective substrate binding in the presence of an insoluble xylan. The NMR titration experiments of EW29Ch with sugars showed that EW29Ch had two binding sites in the two subdomains, α and γ, and the sugar-binding activities of the α binding site are much higher than those of γ binding site. Further, the α binding site had a β-ranomer preference among the sugars. These results indicate that each of the two sugar-binding sites of EW29Ch has a distinct sugar-binding mode. STD-NMR experiments for the mixture of lactose with EW29Ch demonstrated that the galactose residue of the lactose mainly interacts with EW29Ch. Furthermore, the conformational changes of EW29Ch by binding with sugars differed among kind of sugars, which may relate with its hemagglutinating activity. Finally, we suggest that the sugar-binding activities and specificities of the sugar-binding sites in R-type lectins are closely related with their physiological functions. Less