The trimeric solution structure and fucose-binding mechanism of the core fucosylation-specific lectin PhoSL

The core α1–6 fucosylation-specific lectin from a mushroom Pholiota squarrosa (PhoSL) is a potential tool for precise diagnosis of cancers. This lectin consists of only 40 amino acids and can be chemically synthesized. We showed here that a synthesized PhoSL peptide formed a trimer by gel filtration and chemical cross-linking assays, and determined a structure of the PhoSL trimer by NMR. The structure possesses a β-prism motif with a three-fold rotational symmetry, where three antiparallel β-sheets are tightly connected by swapping of β-strands. A triad of Trp residues comprises the structural core, forming NH–π electrostatic interactions among the indole rings. NMR analysis with an excess amount of fucose revealed the structural basis for the molecular recognition. Namely, fucose deeply enters a pocket formed at a junction of β-sheet edges, with the methyl group placed at the bottom. It forms a number of hydrophobic and hydrogen-bonding interactions with PhoSL residues. In spite of partial similarities to the structures of other functionally related lectins, the arrangement of the antiparallel β-sheets in the PhoSL trimer is novel as a structural scaffold, and thus defines a novel type of lectin structure.