Crystal structure of a recombinant Agaricus bisporus mushroom mannose-binding protein with a longer C-terminal region.

A lectin-like protein was discovered in Agaricus bisporus as part of the mushroom tyrosinase complex. The protein has a β-trefoil fold, which is typical of the ricin B-like-type lectin family. The structure of the recombinant protein has been elucidated, and its specific sugar-binding affinity towards mannose and mannitol has also been reported; therefore, the protein was named A. bisporus mannose-binding protein (Abmb). Although the sugar-binding site of Abmb is predicted to be close to the C-terminus, the sugar-binding site has not yet been determined. In this study, a variant of recombinant Abmb with a longer C-terminal region including a 6×His-tag was constructed and its structure was solved at 1.51 and 2.34 Å resolution in an orthorhombic and a monoclinic space group, respectively. The overall structure showed a β-trefoil fold as previously reported; however, several surface loop regions including the C-terminal region showed high flexibility. In addition, a glycan-search assay of this variant showed weak binding affinity towards β-D-galactose but no affinity towards α-D-mannose. The plasticity of the C-terminal tail could be related to the differences in the carbohydrate-binding affinity of Abmb.