Translated anti-sense product of the Na/phosphate co-transporter (NaPi-II)

The homeostasis of P_i in marine teleosts is maintained by renal P_i secretion as well as by P_i reabsorption. A Na/P_i co-transport system belonging to the NaPi-II protein family is instrumental in tightly controlled renal P_i handling in mammals and fish. We have isolated an NaPi-II related cDNA from winter flounder. It was cloned from a female gonad cDNA library and is 624 bp long. The transcript is expressed in female and male flounder gonads as well as in kidney and intestine, although at very low levels. RNase H digestion experiments revealed an opposite orientation of the transcript with regard to NaPi-II-related mRNA. The anti-sense orientation was confirmed by genomic sequence analysis and Southern blotting. Alluding to the sense transcript, the anti-sense transcript was denoted IPAN. The open reading frame of IPAN encodes a basic protein of 68 amino acid residues. Immunohistochemistry confined the anti-sense related protein, Ipan, to a submembranous compartment of immature oocytes, suggesting a role in oocyte development. In kidney and intestine Ipan is partly co-localized with the Na/P_i co-transporter, implying a regulatory function for the anti-sense protein. However, direct protein–protein interaction could not be established. The existence of a putative open reading frame in other species extends the biological significance of the novel protein.