ACETYL XYLAN ESTERASE FROM TRICHODERMA-REESEI CONTAINS AN ACTIVE-SITESERINE RESIDUE AND A CELLULOSE-BINDING DOMAIN

The axe1 gene encoding acetyl xylan esterase was isolated from an expr ession library of the filamentous fungus Trichoderma reesei using anti bodies raised against the purified enzyme. Apparently axe1 codes for t he two forms, pI 7 and pI 6.8, of acetyl xylan esterase previously cha racterized. The axe1 encodes 302 amino acids including a signal sequen ce and a putative propeptide. The catalytic domain has no amino acid s imilarity with the reported acetyl xylan esterases but has a clear sim ilarity, especially in the active site, with fungal cutinases which ar e serine esterases. Similarly to serine esterases, the axe1 product wa s 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, threon ine and proline. The binding domain can be separated from the catalyti c domain by limited proteolysis without affecting the activity of the enzyme towards acetylated xylan, but abolishing its capability to bind cellulose.