A fungal galectin from Agrocybe cylindracea (ACG) exhibits broad binding specificity for b-galactose–
containing glycans. We determined the crystal structures of wild-type ACG and the N46A mutant,
with and without glycan ligands. From these structures and a saccharide-binding analysis of the
N46A mutant, we revealed that a conformational change of a unique insertion sequence containing
Asn46 provides two binding modes for ACG, and thereby confers broad binding specificity. We propose
that the unique sequence provides these two distinct glycan-binding modes by an induced-fit
mechanism.