Na+-dependent phosphate cotransporters: the NaPi protein families.
In vertebrates, the level of inorganic phosphate (Pi) is tightly balanced both inside the cell and in the whole organism. A number of different Na+-dependent Pi cotransport systems involved in Pi homeostasis have been identified and characterized at the molecular level in the past 7 years. The transporters constitute three different protein families denoted NaPi-I, NaPi-II and NaPi-III. NaPi-I from the rabbit was the first member of this family to be cloned. However, it still resists efforts to unravel its physiological role and a clear-cut functional identity: is it a Cl- channel, a Na+/Pi cotransporter, a regulator, or does it perform a combination of these functions? These questions provide a slight taste of the problems associated with orphan genes derived from sequencing projects. The members of the NaPi-II protein family are crucially involved in tightly controlled renal Pi excretion and, as recently discovered, intestinal Pi absorption. The expression and the cellular distribution of NaPi-II in the proximal tubular epithelium are affected by hormonal and metabolic factors known to influence extracellular fluid Pi homeostasis. Recently, the expression of NaPi-II has been demonstrated in osteoclasts and brain; however, the physiological roles of NaPi-II in these tissues remain to be established. The members of the third protein family, NaPi-III, have been identified on the basis of their function as viral receptors. The widespread expression of this family suggests that NaPi-III is involved in supplying the basic cellular metabolic needs for Pi.
Werner
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Max-Planck-Institute for Molecular Physiology, Rheinlanddamm 201, Germany. andreas.werner@mpi-dortmund.mpg.de.
Dehmelt
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Nalbant
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D000818:Animals; D002352:Carrier Proteins / Q000145:classification / Q000235:genetics / Q000502:physiology*; D003001:Cloning, Molecular; D006224:Cricetinae; D004734:Energy Metabolism; D019143:Evolution, Molecular; D005399:Fishes / Q000378:metabolism; D006801:Humans; D007668:Kidney / Q000378:metabolism; D008322:Mammals / Q000378:metabolism; D051379:Mice; D005810:Multigene Family; D009928:Organ Specificity; D010802:Phylogeny; D011487:Protein Conformation; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology*; D011817:Rabbits; D051381:Rats; D050608:Sodium-Phosphate Cotransporter Proteins; D013312:Stress, Physiological / Q000378:metabolism; D027981:Symporters
D002352:Carrier Proteins; D020033:Protein Isoforms; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters