Identification and Characterization of a Novel Quadruple-Domain Galectin from the Hong Kong Oyster Crassostrea hongkongensis

Abstract

Galectins, i.e. β-galactoside-binding animal lectins, play essential roles in the innate immunity systems of vertebrates and invertebrates. In the present study, a full-length cDNA coding for a galectin from the Hong Kong oyster Crassostrea hongkongensis (designated ChGal) was cloned and characterized. The complete cDNA of ChGal included an open reading frame (ORF) of 1668 bp, as well as 5′- and 3′-untranslated regions (UTRs) of 75 bp and 302 bp, respectively. The ORF coded for a putative protein of 555 amino acids with an estimated molecular mass of 63.4 kDa and a theoretical isoelectric point of 5.0. Sequence analysis revealed that ChGal contains four carbohydrate recognition domains (CRDs), each containing the conserved carbohydrate-binding motifs H-NPR and WG-ER. Phylogenetic analysis produced an unrooted tree with four clades: single CRDs, C-terminal CRDs with tandem-repeat galectin clade, N-terminal CRDs with tandem-repeat galectin clade, and quadruple-CRD clusters. ChGal clustered in the quadruple-domain galectins, which are divided into four subgroups on the basis of the position of each individual CRD. Recombinant ChGal exhibited strong agglutinating activity against Escherichia coli, Vibrio alginolyticus, and Bacillus thuringiensis. ChGal mRNA was constitutively expressed in all tissues tested, with the highest level detected in the digestive gland. The expression of ChGal significantly increased in haemocytes in response to V. alginolyticus challenge. These results suggest that ChGal play an important role in the innate immune responses of the Hong Kong oyster.