Recognition of xyloglucan by the crystalline cellulose-binding site of a family 3a carbohydrate-binding module

• CtCBM3a can bind to xyloglucan in addition to crystalline cellulose.
• Five point mutations at the CtCBM3a crystalline cellulose binding site abolished recognition of both crystalline cellulose and xyloglucan.
• XG polysaccharides, but not XG-derived oligosaccharides, in solution can be bound by CtCBM3a.

Type A non-catalytic carbohydrate-binding modules (CBMs), exemplified by CtCBM3acipA, are widely believed to specifically target crystalline cellulose through entropic forces. Here we have tested the hypothesis that type A CBMs can also bind to xyloglucan (XG), a soluble β-1,4-glucan containing α-1,6-xylose side chains. CtCBM3acipA bound to xyloglucan in cell walls and arrayed on solid surfaces. Xyloglucan and cellulose were shown to bind to the same planar surface on CBM3acipA. A range of type A CBMs from different families were shown to bind to xyloglucan in solution with ligand binding driven by enthalpic changes. The nature of CBM-polysaccharide interactions is discussed.