Abstract
Giant clams display light-enhanced inorganic phosphate (P i ) absorption, but how the absorbed P i is translocated to the symbiotic dinoflagellates living extracellularly in a tubular system is unknown. They can accumulate P i in the kidney, but the transport mechanism remains enigmatic. This study aimed to elucidate the possible functions of sodium-dependent phosphate transporter protein 1-homolog (PiT1-like), which co-transport Na + and H 2 PO 4 – , in these two processes. The complete cDNA coding sequence of PiT1-like , which comprised 1,665 bp and encoded 553 amino acids (59.3 kDa), was obtained from the fluted giant clam, Tridacna squamosa . In the kidney, PiT1-like was localized in the plasma membrane of nephrocytes, and could therefore absorb P i from the hemolymph. As the gene and protein expression levels of PiT1-like were up-regulated in the kidney during illumination, PiT1-like could probably increase the removal of P i from the hemolymph during light-enhanced P i uptake. In the ctenidial epithelial cells, PiT1-like had a basolateral localization and its expression was also light-dependent. It might function in P i sensing and the absorption of P i from the hemolymph when P i was limiting. In the outer mantle, PiT1-like was localized in the basolateral membrane of epithelial cells forming the tertiary tubules. It displayed light-enhanced expression levels, indicating that the host could increase the translocation of P i from the hemolymph into the tubular epithelial cells and subsequently into the luminal fluid to support increased P i metabolism in the photosynthesizing dinoflagellates. Taken together, the accumulation of P i in the kidney of giant clams might be unrelated to limiting the availability of P i to the symbionts to regulate their population.