Applied and Environmental Microbiology vol:73 issue:14 pages:4602-4608
Endo-beta-1,4-xylanases (EC 184.108.40.206, endoxylanases), key enzymes in the degradation of xylan, are considered to play an important role in phytopathogenesis, as they occupy a prominent position in the arsenal of hydrolytic enzymes secreted by phytopathogens to breach the cell wall and invade the plant tissue. Plant endoxylanase inhibitors are increasingly being pinpointed as part of a counterattack mechanism. To understand the surprising XIP-type endoxylanase inhibitor insensitivity of endoxylanases XylA and XylB from the phytopathogen Fusarium graminearum, an extensive mutational study of these enzymes was performed. Using combinatorial and site-directed mutagenesis, the XIP-insensitivity of XylA as well as XylB was proven to be solely due to amino acid sequence adaptations in the "thumb" structural region. While XylB residues Cys(141), Asp(148) and Cys(149) were shown to prevent XIP-interaction, the XIP-insensitivity of XylA could even be ascribed to the occurrence of only one aberrant residue, i.e. Val(151). In addition to providing a thorough explanation for the XIP-insensitivity of both F. graminearum endoxylanases at the molecular level, XylA and XylB mutants with altered inhibition specificities and pH optima were generated in the course of this study. As this is the first experimental elucidation of the molecular determinants dictating the interaction specificity between endoxylanases from phytopathogenic origin and a plant inhibitor, this work sheds more light on the ongoing evolutionary arms race between plants and phytopathogenic fungi involving recognition of endoxylanases.