We have isolated two cDNA clones, NaPi-2 and NaPi-3, by screening rat kidney cortex and human kidney cortex cDNA libraries, respectively, for expression of sodium-dependent phosphate transport in Xenopus laevis oocytes. Substrate specificity and a detailed kinetic analysis (Na, Pi, H+ concentrations) suggested that expressed uptake activities relate to proximal tubular brush border membrane Na/Pi cotransport. NaPi-2 cDNA contains 2464 bp encoding a protein of 637 aa; NaPi-3 cDNA contains 2573 bp encoding a protein of 639 aa. NaPi-2- and NaPi-3-deduced protein sequences show high homology to each other but are different from the protein sequence deduced from the previously cloned NaPi-1 cDNA (from rabbit proximal tubules). Hydropathy profile predictions suggest at least eight membrane-spanning regions in NaPi-2/3-related proteins. In vitro translation results in proteins of the expected size and suggests glycosylation. Northern blot analysis shows corresponding mRNA species (approximately 2.7 kb) in kidney cortex of various species but no hybridization with RNAs isolated from a variety of other tissues (including intestinal segments); a hybridization signal (approximately 4.8 kb) was observed only in the lung (human). We conclude that we have structurally identified two closely related proteins most likely involved in human and rat renal brush border Na/Pi cotransport.

Na+-phosphate (Pi) cotransport across the renal brush border membrane is the rate limiting step in the overall reabsorption of filtered Pi. Murine and human renal-specific cDNAs (NaPi-7 and NaPi-3, respectively) related to this cotransporter activity (type II Na+-Pi cotransporter) have been cloned. We now report the cloning and characterization of the corresponding mouse (Npt2) and human (NPT2) genes. The genes were cloned by screening mouse genomic and human chromosome 5-specific libraries, respectively. Both genes are approximately 16 kb and are comprised of 13 exons and 12 introns, the junctions of which conform to donor and acceptor site consensus sequences. Putative CAAT and TATA boxes are located, respectively, at positions -147 and -40 of the Npt2 gene and -143 and -51 of the NPT2 gene, relative to nucleotide 1 of the corresponding cDNAs. The translation initiation site is within exon 2 of both genes. The first 220 bp of the mouse and human promoter regions exhibit 72% identity. Two transcription start sites (at positions -9 and - 10 with respect to nucleotide 1 of NaPi-7 cDNA) and two polyadenylylation signals were identified in the Npt2 gene by primer extension, 5' and 3' rapid amplification of cDNA ends (RACE). A 484-bp 5' flanking region of the Npt2 gene, comprising the CAAT box, TATA box, and exon 1, was cloned upstream of a luciferase reporter gene; this construct significantly stimulated luciferase gene expression, relative to controls, when transiently transfected into OK cells, a renal cell line expressing type II Na+ -Pi cotransporter activity. The present data provide a basis for detailed analysis of cis and trans elements involved in the regulation of Npt2/NPT2 gene transcription and facilitate screening for mutations in the NPT2 gene in patients with autosomally inherited disorders of renal Pi reabsorption.

>sp|Q06495|NPT2A_HUMAN Sodium-dependent phosphate transport protein 2A OS=Homo sapiens GN=SLC34A1 PE=1 SV=1
MLSYGERLGSPAVSPLPVRGGHVMRGTAFAYVPSPQVLHRIPGTSAYAFPSLGPVALAEHTCPCGEVLERHEPLPAKLAL
EEEQKPESRLVPKLRQAGAMLLKVPLMLTFLYLFVCSLDMLSSAFQLAGGKVAGDIFKDNAILSNPVAGLVVGILVTVLV
QSSSTSTSIIVSMVSSGLLEVSSAIPIIMGSNIGTSVTNTIVALMQAGDRTDFRRAFAGATVHDCFNWLSVLVLLPLEAA
TGYLHHITRLVVASFNIHGGRDAPDLLKIITEPFTKLIIQLDESVITSIATGDESLRNHSLIQIWCHPDSLQAPTSMSRA
EANSSQTLGNATMEKCNHIFVDTGLPDLAVGLILLAGSLVLLCTCLILLVKMLNSLLKGQVAKVIQKVINTDFPAPFTWV
TGYFAMVVGASMTFVVQSSSVFTSAITPLIGLGVISIERAYPLTLGSNIGTTTTAILAALASPREKLSSAFQIALCHFFF
NISGILLWYPVPCTRLPIRMAKALGKRTAKYRWFAVLYLLVCFLLLPSLVFGISMAGWQVMVGVGTPFGALLAFVVLINV
LQSRSPGHLPKWLQTWDFLPRWMHSLKPLDHLITRATLCCARPEPRSPPLPPRVFLEELPPATPSPRLALPAHHNATRL