An alpha-L-arabinofuranosidase from Trichoderma reesei containing a noncatalytic xylan-binding domain

L-Sorbose, an excellent cellulase and xylanase inducer from Trichoderma ree sei PC-3-7, also induced alpha-L-arabinofuranosidase (alpha-AF) activity. A n alpha-AF induced by L-sorbose was purified to homogeneity, and its molecu lar mass was revealed to be 35 kDa (AF35), which was not consistent with th at of the previously reported alpha-AF, Another species, with a molecular m ass of 53 kDa (AF53), which is identical to that of the reported alpha-AF, was obtained by a different purification procedure. Acid treatment of the a mmonium sulfate-precipitated fraction at pH 3.0 in the purification steps o r pepsin treatment of the purified AF53 reduced the molecular mass to 35 kD a. Both purified enzymes have the same enzymological properties, such as:pH , and temperature effects on activity and kinetic parameters for p-nitrophe nyl-alpha-L-arabinofuranoside (pNPA), Moreover, the N-terminal amino acid s equences of these enzymes were identical with that of the reported alpha-AF . Therefore, it is obvious that AF35 results from the proteolytic cleavage of the C-terminal region of AF53. Although AF35 and AF53 showed the same ca talytic constant with pNPA, the former showed drastically reduced specific activity against oat spelt xylan compared to the latter. Furthermore, AF53 was bound to xylan rather than to crystalline cellulose (Avicel), but AF35 could not be bound to any of the glycans, These results suggest that AF53 i s a modular glycanase, which consists of an N-terminal catalytic domain and a C-terminal noncatalytic xylan-binding domain.