Triticum aestivum L. endoxylanase inhibitor (TAXI) consists of two inhibitors, TAXI I and TAXI II, with different specificities

Abstract

The Triticum aestivum L. endoxylanase inhibitor (TAXI) discovered by Debyser and Delcour [(1997) Eur. Pat. filed April 1997, published as WO 98/49278] and Debyser, Derdelinckx and Delcour [(1997) J. Am. Soc. Brew. Chem. 55, 153Ő156] seems to be a mixture of two different endoxylanase inhibitors, called TAXI I and TAXI II. By using Aspergillus niger as well as Bacillus subtilis endoxylanases for assaying inhibition activity, both inhibitors could be purified to homogeneity from wheat (Triticum aestivum L., var. Soissons). TAXI I and TAXI II have similar molecular structures. They both have a molecular mass of approx. 40.0kDa, are not glycosylated and occur in two molecular forms, i.e. a non-proteolytically processed one and a proteolytically processed one. However, the pI of TAXI II (at least 9.3) is higher than that of TAXI I (8.8). TAXI I and TAXI II clearly show different inhibition activities towards different endoxylanases. The N-terminal amino acid sequences of both inhibitors show a high degree of identity, which might indicate that there is an evolutionary relationship between them.

The Dual Nature of the Wheat Xylanase Protein Inhibitor XIP-I

“…This could also explain why two different classes of xylanase inhibitors with different specificities and expression patterns occur in cereals (29,34). TAXI is the other type of xylanase inhibitor isolated from wheat and is specific for GH11 xylanases (35). Both inhibitors show a common inhibition strategy by binding to the active site region (Sansen et al) (38).…”

A Wheat Xylanase Inhibitor Gene,Xip-I, but NotTaxi-I, Is Significantly Induced by Biotic and Abiotic Signals That Trigger Plant Defense

“…1,2) XIP-I competitively inhibits xylanases belonging to glycoside hydrolase family 10 (GH10) and family 11 (GH11). 3) TAXI-I and TAXI-II differ in specificity for GH11 microbial xylanases.…”

“…3) TAXI-I and TAXI-II differ in specificity for GH11 microbial xylanases. 2) The structural basis of their inhibition has been elucidated by X-ray crystallographic analysis of XIP-I 3) and TAXI-I 4) in complexes with fungal xylanases. Since neither type of inhibitor has the ability to inhibit wheat endogenous xylanases, 5) it has been speculated that they are involved in plant defense against fungal pathogens.…”

Specific Characterization of Substrate and Inhibitor Binding Sites of a Glycosyl Hydrolase Family 11 Xylanase fromAspergillus niger

“…They are high pI, nonglycosylated proteins with molecular masses of ϳ40 kDa. At least two inhibitors of this type (TAXI I and TAXI II) with different pI values (8.8 and ϳ9.3, respectively) and varying specificities toward different endoxylanases have been identified in wheat (46). The N-terminal amino acid sequences of TAXI-I and TAXI-II showed a high degree of identity, but there was no similarity to XIP-I.…”

“…The N-terminal amino acid sequences of TAXI-I and TAXI-II showed a high degree of identity, but there was no similarity to XIP-I. The TAXI-like inhibitors are believed to be active against bacterial and fungal family 11 endoxylanases but not against family 10 endoxylanases (46,47). XIP-I inhibited both family-10 and 11 fungal xylanases apart from the family 10 Aspergillus aculeatus xylanase with K i values ranging from 3.4 to 610 nM, but bacterial family 10 and 11 xylanases were not inhibited (48).…”