Molecular characterization, immunofluorescent localization, and expression levels of two bicarbonate anion transporters in the whitish mantle of the giant clam, Tridacna squamosa, and the implications for light-enhanced shell formation.

ABSTRACT

Giant clams conduct light-enhanced shell formation, which requires the increased transport of Ca2+ and inorganic carbon (Ci) from the hemolymph through the shell-facing epithelium of the whitish inner mantle to the extrapallial fluid where CaCO3 deposition occurs. The major form of Ci in the hemolymph is HCO3-, but the mechanisms of HCO3- transport through the basolateral and apical membranes of the shell-facing epithelial cells remain unknown. This study aimed to clone from the inner mantle of Tridacna squamosa the complete coding cDNA sequences of electrogenic Na+-HCO3-cotransporter 1 homolog (NBCe1-like-b) and electrogenic Na+-HCO3-cotransporter 2 homolog (NBCe2-like). NBCe1-like-b comprised 3360 bp, encoding a 125.7 kDa protein with 1119 amino acids. NBCe1-like-b was slightly different from NBCe1-like-a of the ctenidium reported elsewhere, as it had a serine residue (Ser1025), which might undergo phosphorylation leading to the transport of Na+ HCO3- at a ratio of 1 2 into the cell. NBCe1-like-b was localized at the basolateral membrane of the shell-facing epithelial cells, and its gene and protein expression levels increased significantly in the inner mantle during illumination, indicating a role in the light-enhanced uptake of HCO3- from the hemolymph. The sequence of NBCe2-like obtained from the inner mantle was identical to that reported previously for the outer mantle. In the inner mantle, NBCe2-like had an apical localization in the shell-facing epithelial cells, and its protein abundance was upregulated during illumination. Hence, NBCe2-like might take part in the light-enhanced transport of HCO3- through the apical membrane of these cells into the extrapallial fluid.