Molecular Characterization and Dietary Regulation of the AlaSerCys Transporter 2 in Grass Carp, Ctenopharyngodon idella

AlaSerCys Transporter 2 (ASCT2), a glutamine/amino acid transporter, plays an important role in the absorption of neutral amino acids. In this study, we cloned the full-length cDNA of ASCT2 from grass carp, Ctenopharyngodon idella, which encodes a 541-amino-acid protein. Phylogenetic analysis revealed that the ASCT2 sequence of grass carp clustered with the ASCT2 from Danio rerio. ASCT2 mRNA was differentially expressed in different tissues, with a gradient expression from high to low in tissues of the liver, gill, muscle, midgut, hypophysis, hindgut, kidney, heart, foregut, brain, spleen, and gonad. Meanwhile, the quantitative real-time polymerase chain reaction results showed that 22% crude protein diets could significantly stimulate ASCT2 gene expression. In addition, ASCT2 was expressed in higher quantities in the soybean meal group than in the fishmeal group. Furthermore, 7.5g/kg glutamine and 2.5g/kg glutamine dipeptide remarkably improved the expression of ASCT2. These results suggested that ASCT2 expression is regulated by the dietary protein levels and sources, as well as appropriate concentration of glutamine and glutamine dipeptide additives.