Novel carbohydrate-recognition mode of the invertebrate C-type lectin SPL-1 fromSaxidomus purpuratusrevealed by the GlcNAc-complex crystal in the presence of Ca²⁺

Abstract

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 Ca 2+ -independent manner. Unlike ordinary C-type lectins, which require Ca 2+ ions for carbohydrate recognition, these lectins recognize specific carbohydrates mainly through interactions with the acetamido group without Ca 2+ ions, even though Ca 2+ 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 Ca 2+ revealed that the binding of SPL-1 to GlcNAc is stabilized by hydrogen bonds to the water molecule(s) coordinating Ca 2+ , whereas in ordinary C-type lectins Ca 2+ 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. research communications Acta Cryst. (2020). F76, 271-277 Unno et al. Carbohydrate recognition by SPL-1 277

Functional Diversity of Novel Lectins with Unique Structural Features in Marine Animals

“…In fact, the QPD/EPN motifs are replaced by RPD (Arg-Pro-Asp) or KPD (Lys-Pro-Asp) in the subunits of SPL-1 and SPL-2, which are not directly involved in carbohydrate recognition. While Ca 2+ is not essential for carbohydrate binding, its presence can enhance the binding affinity of SPL-1 and SPL-2 by coordinating two water molecules that form hydrogen bonds with the 3-and 4-hydroxy groups of the carbohydrates [56]. These lectins likely function in recognizing the acetamido groups present on the surface molecules of microorganisms.…”