Identification and characterization of multiple β-glucan binding proteins in the Pacific oyster, Crassostrea gigas

The present study reports on the characterization of two cDNAs coding β-glucan binding proteins (βGBPs), designated as Cg-βGBP-1 and Cg-βGBP-2, from the Pacific oyster, Crassostrea gigas. Cg-βGBP-1 consists of 555 amino acid residues and possesses two possible integrin recognition sites. The other protein, Cg-βGBP-2, is composed of 447 amino acid residues without integrin recognition sites. Domain structures of both Cg-βGBPs are similar to other invertebrate βGBPs, but phylogenetic positions and major expression tissues for these proteins are different. Cg-βGBP-1 is expressed in circulatory hemocytes and Cg-βGBP-2 in digestive glands. Functional assays using recombinant proteins revealed that Cg-βGBP-2 enhanced the phenoloxidase (PO) activity of hemocyte suspensions under the presence of laminarin, but Cg-βGBP-1 did not show this enhancement. It is suggested that Cg-βGBPs in the Pacific oyster have evolved to obtain different immunological functions. Cg-βGBP-1 possibly evolved for hemocyte-related functions through integrin, and Cg-βGBP-2 for the PO activation system.