The carbohydrate-binding module of <i>Fragaria</i> × <i>ananassa</i> expansin 2 (CBM-FaExp2) binds to cell wall polysaccharides and decreases cell wall enzyme activities "in vitro"

Abstract

A putative carbohydrate binding module (CBM) from strawberry (<i>Fragaria</i> × ananassa</i> Duch.) expansin 2 (CBM-FaExp2) was cloned and the encoding protein was over-expressed in <i>Escherichia coli</i> 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 ± 0.44 mL g⁻¹, K_ad = 11.37 ± 0.87 mL g⁻¹, K_ad = 10.4 ± 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.