Novel carbohydrate-recognition mode of the invertebrate C-type lectin SPL-1 from Saxidomus purpuratus revealed by the GlcNAc-complex crystal in the presence of Ca²⁺

The C-type lectins SPL-1 and SPL-2 from the bivalve Saxidomus purpuratus are composed of A and B chains and of two B chains, respectively. They bind specific carbohydrates containing acetamido groups, such as N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc), in a Ca²⁺-independent manner. Unlike ordinary C-type lectins, which require Ca²⁺ ions for carbohydrate recognition, these lectins recognize specific carbohydrates mainly through interactions with the acetamido group without Ca²⁺ ions, even though Ca²⁺ enhances the binding affinity of these lectins, especially SPL-1. In the present study, the crystal structure of the SPL-1–GlcNAc complex in the presence of Ca²⁺ revealed that the binding of SPL-1 to GlcNAc is stabilized by hydrogen bonds to the water molecule(s) coordinating Ca²⁺, whereas in ordinary C-type lectins Ca²⁺ directly forms coordinate bonds to the hydroxy groups of carbohydrates. These differences may also allow SPL-1 and SPL-2 to recognize both GlcNAc and GalNAc, which have different orientations of the 4-hydroxy group.