IDENTIFICATION OF A CDNA PROTEIN LEADING TO AN INCREASED P-I-UPTAKE IN XENOPUS-LAEVIS OOCYTES/

In a previous report we documented an increased Na+-dependent transpor t of inorganic phosphate (P-i) in Xenopus laevis oocytes injected with mRNA isolated from rabbit duodenum (Yagci et al., Pfluegers Al-ch. 42 2:211-216, 1992; ref 24). In the present study we have used expression cloning in oocytes to search for the cDNA/mRNA involved in this effec t. The identified cDNA (provisionally named PiUS; for P-i-uptake stimu lator) lead to a 3-4-fold stimulation of Na+-dependent P-i-uptake (10n g cRNA injected, 3-5 days of expression). Na+-independent uptake of P- i was also affected but transport of sulphate and L-arginine (in the p resence or absence of sodium) remained unchanged. The apparent K-m-val ues for the induced Na+-dependent uptake were 0.26 +/- 0.04 mM for P-i and 14.8 +/- 3.0 mM for Na+. The 1796 bp cDNA codes for a protein of 425 amino acids. Hydropathy analysis suggests a lack of transmembrane segments. In vitro translation resulted in a protein of 60 kDa and pro vided no evidence of glycosylation. In Northern blots a mRNA of simila r to 2 kb was recognized in various tissues including different intest inal segments, kidney cortex, kidney medulla, liver and heart. Homolog y searches showed no similarity to proteins involved in membrane trans port and its control. In conclusion, we have cloned from a rabbit smal l intestinal cDNA library a novel cDNA encoding a protein stimulating P-i-uptake into Xenopus laevis oocytes, but which is not a P-i-transpo rter itself.