A new type of endo-xyloglucan transferase devoted to xyloglucan hydrolysis in the cell wall of azuki bean epicotyls

A new type of xyloglucan-degrading enzyme was isolated from the cell wall of azuki bean (Vigna angularis Ohwi et Ohashi cv, Takara) epicotyls and its characteristics were determined, The enzyme was purified to apparent homogeneity by Concanavalin A (Con A)-Sepharose, cation exchange, and gel filtration columns from a cell wall protein fraction extracted with 1 M sodium chloride, The purified enzyme gave a single protein band of 33 kDa on SDS-PAGE, The enzyme specifically cleaved xyloglucans and showed maximum activity at pH 5.0 when assayed by the iodine-staining method. An increase in reducing power in xyloglucan solution was clearly detected after treatment with the purified enzyme. Xylogluclans with molecular masses of 500 and 25 kDa were gradually hydrolyzed to 5 kDa for 96 h without production of any oligo- or monosaccharide with the purified enzyme. The purified enzyme did not show an endo-type transglycosylation reaction, even in the presence of xyloglucan oligosaccharides, Partial amino acid sequences of the enzyme shared an identity with endo-xyloglucan transferase (EXGT) family, especially with xyloglucan endotransglycosylase (XET) from nasturtium. These results suggest that the enzyme is a new member of EXGT devoted solely to xyloglucan hydrolysis.