A galectin with quadruple-domain from bay scallop Argopecten irradians is involved in innate immune response

Galectins are a family of β-galactoside-binding lectins that specifically bind to β-galactoside residues and play crucial roles in innate immune responses of invertebrates and vertebrates. The cDNA of bay scallop Argopecten irradians galectin (designated as AiGal2) was cloned by rapid amplification of cDNA ends (RACE) method based on the expressed sequence tag (EST). The full-length cDNA of AiGal2 was of 2137 bp. The open reading frame encoded a polypeptide of 555 amino acids containing four carbohydrate-recognition domains. The deduced amino acid sequence and multi-domain organization of AiGal2 were highly similar to those of mollusk galectins. A typical galectin fold in β-sandwich arrangement was identified in the potential tertiary structure of all the four CRDs in AiGal2. The mRNA transcripts of AiGal2 were found to be constitutively expressed in a wide range of tissues and mainly in hepatopancreas, adductor muscle and kidney. After scallops were challenged by Vibrio anguillarum or Micrococcus luteus, the mRNA expression level of AiGal2 was up-regulated significantly, while it did not changed remarkably after Pichia pastoris challenge. The recombined AiGal2 (rAiGal2) exhibited strong activity to agglutinate E. coli, V. anguillarum, Vibrio fluvialis, Edwardsiella tarda and M. luteus, and the agglutinating activities could be inhibited by both d-galactose and lactose. The in vitro encapsulation assay revealed that rAiGal2 could bind to hemocytes and enhanced its encapsulation of agarose beads. These results collectively suggested that AiGal2 functioned as a pattern recognition receptor in immune defense and contributed to the non-self recognition and elimination in cellular immune response of bay scallop.