Proteinaceous Xylanase Inhibitors: Structure, Function and Evolution

Endo-(1,4)-β-D-xylanases of plant and microbial origin play an important role in the degradation of arabinoxylan from plant cell wall. To date, two distinct types of xylanase inhibitors, the TAXI (Triticum aestivum xylanase inhibitor) and XIP (Xylanase inhibitor protein) types have been identified in cereals (rye, barley, maize, rice, durum and bread wheat). TAXI inhibits fungal and bacterial xylanases from glycoside hydrolase (GH) family 11 (GH11) whereas XIP inhibitors display species selectivity for both GH10 and GH11 xylanases. The evolution and biological role of the xylanase inhibitors are discussed in the light of the features which have recently become available by resolution of the crystal structures of the inhibitors isolated from wheat, XIP-I and TAXI-I, free and in complex with target xylanases.