Acetyl xylan esterase from Trichoderma reesei contains an active-site serine residue and a cellulose-binding domain

The axe1 gene encoding acetyl xylan esterase was isolated from an expression library of the filamentous fungus Trichoderma reesei using antibodies raised against the purified enzyme. Apparently axe1 codes for the two forms, pI 7 and pI 6.8, of acetyl xylan esterase previously characterized. The axe1 encodes 302 amino acids including a signal sequence and a putative propeptide. The catalytic domain has no amino acid similarity with the reported acetyl xylan esterases but has a clear similarity, especially in the active site, with fungal cutinases which are serine esterases. Similarly to serine esterases, the axe1 product was inactivated with phenylmethylsulfonyl fluoride. At its C-terminus it carries a cellulose binding domain of fungal type, which is separated from the catalytic domain by a region rich in serine, glycine, threonine and proline. The binding domain can be separated from the catalytic domain by limited proteolysis without affecting the activity of the enzyme towards acetylated xylan, but abolishing its capability to bind cellulose.