TAXI-I inhibits degradation of xylan in the cell wall.
Xylanase inhibitor-I (TAXI-I) is a member of potent TAXI-type inhibitors of fungal and bacterial family 11 xylanases. Plants developed a diverse battery of defense mechanisms in response to continual challenges by a broad spectrum of pathogenic microorganisms. Their defense arsenal includes inhibitors of cell wall-degrading enzymes, which hinder a possible invasion and colonization by antagonists. Xylanases of fungal and bacterial pathogens are the key enzymes in the degradation of xylan in the cell wall. Plants secrete proteins that inhibit these degradation glycosidases, including xylanase. Surprisingly, TAXI-I displays structural homology with the pepsin-like family of aspartic proteases but is proteolytically nonfunctional, because one or more residues of the essential catalytic triad are absent. The structure of the TAXI-inhibitor, Aspergillus niger xylanase I complex, illustrates the ability of tight binding and inhibition with subnanomolar affinity and indicates the importance of the C-terminal end for the differences in xylanase specificity among different TAXI-type inhibitors. This family also contains pepsin-like aspartic proteinases homologous to TAXI-I. Unlike TAXI-I, they have active site aspartates and are functionally active. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).