Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes that play a key role in the oxidative degradation of various biopolymers such as cellulose and chitin. While hunting for new LPMOs, we identified a new family of proteins, defined here as X325, in various fungal lineages. The three-dimensional structure of X325 revealed an overall LPMO fold and a His brace with an additional Asp ligand to Cu(II). Although LPMO-type activity of X325 members was initially expected, we demonstrated that X325 members do not perform oxidative cleavage of polysaccharides, establishing that X325s are not LPMOs. Investigations of the biological role of X325 in the ectomycorrhizal fungus Laccaria bicolor revealed exposure of the X325 protein at the interface between fungal hyphae and tree rootlet cells. Our results provide insights into a family of copper-containing proteins, which is widespread in the fungal kingdom and is evolutionarily related to LPMOs, but has diverged to biological functions other than polysaccharide degradation.

裂解多糖单加氧酶 (LPMO) 是一种含铜酶，在纤维素和几丁质等各种生物聚合物的氧化降解中起关键作用。在寻找新的 LPMO 时，我们在各种真菌谱系中发现了一个新的蛋白质家族，在此定义为 X325。X325 的三维结构揭示了一个整体的 LPMO 折叠和一个带有附加 Asp 配体到 Cu(II) 的 His 大括号。尽管最初预计 X325 成员的 LPMO 型活性，但我们证明 X325 成员不执行多糖的氧化裂解，从而确定 X325 不是 LPMO。X325在外生菌根真菌Laccaria bicolor中的生物学作用研究揭示了在真菌菌丝和树根细胞之间的界面处暴露了 X325 蛋白。我们的研究结果提供了对含铜蛋白质家族的见解，该家族广泛存在于真菌界，在进化上与 LPMO 相关，但已分化为多糖降解以外的生物学功能。