Molecular cloning of endogenous β-glucosidase from common Japanese brackish water clam Corbicula japonica

Studies on the cellulose utilization by animals have been conducted in keeping with the recent developments in molecular biology. In mollusks, endogenous cellulases have been reported from blue mussel, abalone, and freshwater snail. We previously reported the possibility of cellulose assimilation by Corbicula japonica, a representative bivalve dominant in brackish water environments in Japan, and the cloning of its endogenous cellulase (β-1,4-glucanase) gene (Sakamoto, K., Touhata, K., Yamashita, M., Kasai, A. and Toyohara, H., 2007. Cellulose digestion by common Japanese freshwater clam Corbicula japonica. Fish. Sci. 73, 675–683). However, the gene of β-glucosidase, another enzyme essential for the complete cellulose decomposition to glucose units, has not yet been isolated from the mollusk. Therefore, we attempted the molecular cloning of endogenous β-glucosidase from C. japonica and succeeded in the isolation of a cDNA with a 2832-bp open reading frame (ORF) encoding 943 amino acid residues (CjCel1A). CjCEL1A has 2 repeated GHF-1(Glycosyl Hydrolase family 1)-like domains and showed high similarity with known insect β-glucosidases and mammalian lactase-phlorizin-hydrolases. Reverse transcription (RT)-PCR analysis and in situ hybridization revealed that CjCEL1A is likely to be produced in the secretory cells in the digestive gland, suggesting that CjCEL1A is a digestive β-glucosidase of C. japonica and is not derived from symbionts.