The involvement of zinc transporters in the zinc accumulation in the Pacific oyster Crassostrea gigas

Zinc transporters play vital roles in regulating zinc content and localization by mobilizing zinc across cellular and intracellular membranes. Pacific oyster Crassostrea gigas is one of the most zinc-rich animals, which has been regarded as an excellent food for zinc supplement. But the information about zinc transporters and their involvements in zinc accumulation in oysters is still limited. In the present study, a total of 28 zinc transporter genes, including nine Zinc transporter genes (CgZnTs) and 19 Zrt/Irt-like protein genes (CgZIPs), were identified in C. gigas genome using a genome-wide search strategy. There were five ZIP10 homologs in C. gigas, which were much more than those in mammals, fish and other mollusks. Among oyster zinc transporters, immense variations were detected in their gene structure, protein length and physicochemical properties. Phylogenetic analysis showed that most of these transporters were distinctly clustered with their homologs from Homo sapiens, Danio rerio and other mollusks, and the most closely related transporters shared similar motif compositions. The highest zinc content was detected in the oyster mantle and gill, while the lowest level was found in the adductor muscle. The mRNA of all tested CgZnTs and CgZIPs were constitutively expressed in oyster tissues, and most of them were highly expressed in the gill or hepatopancreas. The analysis of RNA-seq data from gill and hepatopancreas showed that all the transporters exhibited divergent response patterns under zinc stress, except for CgZIP4 whose expression was almost undetectable in the two tissues. The results indicated that zinc transporters played important roles in the regulation of zinc homeostasis in C. gigas, which provided a solid foundation for further functional analysis of zinc transporters in oysters and other mollusks.