Synergism between cucumber α-expansin, fungal endoglucanase and pectin lyase

Several recombinant fungal enzymes (endoglucanase and pectinase) were studied for their interactions with α-expansin in cell wall extension and polysaccharide degradation. Both Cel12A and Cel5A were able to hydrolyze cellulose CMC-Na and mixed-linkage β-glucan. In contrast to Cel5A, Cel12A could also hydrolyze xyloglucan and induce wall extension of cucumber hypocotyls in an in vitro assay. Combining α-expansin, even at high concentrations, with Cel12A did not enhance the maximum/final wall extension rate induced by Cel12A alone. These results strongly suggest that modification/degradation of the xyloglucan molecule/network is the key for cell wall extension, and α-expansin and Cel12A may share the same acting site in the substrate. Pectinase (Pel1, a pectin lyase) enhanced α-expansin-induced wall extension in a concentration-dependent manner, suggesting that the pectin network may normally regulate accessibility of expansin to the xyloglucan–cellulose complex. α-Expansin enhanced Cel12Aʹs hydrolytic activity on cellulose CMC-Na but not on xyloglucan and β-glucan. Expansin did not affect Cel5Aʹs hydrolytic activity. Interestingly, expansin also enhanced Pel1ʹs activity on degrading high esterified pectin. A potential explanation for why expansin could synergistically interact with only certain enzymes on specific polysaccharides is discussed. Additional results also suggested that cell wall swelling may not be a significant event during the action of expansin and hydrolases.