The carbohydrate-binding module of Fragaria x ananassa expansin 2 (CBM-FaExp2) binds to cell wall polysaccharides and decreases cell wall enzyme activities in vitro

A putative carbohydrate binding module (CBM) from strawberry (Fragaria x ananassa Duch.) expansin 2 (CBM-FaExp2) was cloned and the encoding protein was over-expressed in Escherichia coli and purified in order to evaluate its capacity to bind different cell wall polysaccharides in vitro. The protein CBM-FaExp2 bound to microcrystalline cellulose, xylan and pectin with different affinities (K-ad = 33.6 +/- A 0.44 mL g(-1), K-ad = 11.37 +/- A 0.87 mL g(-1), K-ad = 10.4 +/- A 0.19 mL g(-1), respectively). According to in vitro enzyme assays, this CBM is able to decrease the activity of cell wall degrading enzymes such as polygalacturonase, endo-glucanase, pectinase and xylanase, probably because the binding of CBM-FaExp2 to the different substrates interferes with enzyme activity. The results suggest that expansins would bind not only cellulose but also a wide range of cell wall polymers.