Characterization of a unique polysaccharide monooxygenase from the plant pathogen <i>Magnaporthe oryzae</i>.

Abstract

Blast disease in cereal plants is caused by the fungus <i>Magnaporthe oryzae</i> and accounts for a significant loss in food crops. At the outset of infection, expression of a putative polysaccharide monooxygenase (<i>Mo</i>PMO9A) is increased. <i>Mo</i>PMO9A contains a catalytic domain predicted to act on cellulose and a carbohydrate-binding domain that binds chitin. A sequence similarity network of the <i>Mo</i>PMO9A family AA9 showed that 220 of the 223 sequences in the <i>Mo</i>PMO9A-containing cluster of sequences have a conserved unannotated region with no assigned function. Expression and purification of the full length and two <i>Mo</i>PMO9A truncations, one containing the catalytic domain and the domain of unknown function (DUF) and one with only the catalytic domain, were carried out. In contrast to other AA9 polysaccharide monooxygenases (PMOs), <i>Mo</i>PMO9A is not active on cellulose but showed activity on cereal-derived <u>m</u>ixed (1→3, 1→4)-<u>β</u>-D-<u>g</u>lucans (MBG). Moreover, the DUF is required for activity. <i>Mo</i>PMO9A exhibits activity consistent with C4 oxidation of the polysaccharide and can utilize either oxygen or hydrogen peroxide as a cosubstrate. It contains a predicted 3-dimensional fold characteristic of other PMOs. The DUF is predicted to form a coiled-coil with six absolutely conserved cysteines acting as a zipper between the two α-helices. <i>Mo</i>PMO9A substrate specificity and domain architecture are different from previously characterized AA9 PMOs. The results, including a gene ontology analysis, support a role for <i>Mo</i>PMO9A in MBG degradation during plant infection. Consistent with this analysis, deletion of <i>Mo</i>PMO9A results in reduced pathogenicity.