title: The opossum kidney cell type IIa Na/P(i) cotransporter is a phosphoprotein.
abstract: OBJECTIVE
  Parathyroid hormone (PTH)-dependent inhibition of proximal tubular P(i) reabsorption is mediated by protein kinase A and/or C and is associated with reduced border membrane expression of the type IIa Na/P(i) cotransporter. The aim of this study was to analyze phosphorylation of the type IIa cotransporter protein.
  
  
  METHODS
  Opossum kidney cells were used as a 'proximal tubular' cell model. Protein phosphorylation was determined by immunoprecipitation of the type IIa Na/P(i) cotransporter, followed by autoradiography. The transporter protein content was evaluated by Western blotting and transport activity by tracer P(i) uptake.
  
  
  RESULTS
  Under control conditions (no PTH) the transporter was phosphorylated; upon treatment with PTH, a decrease in phosphorylation was observed. A protein phosphatase inhibitor (okadaic acid) was unable to prevent PTH-induced Na/P(i) cotransporter inhibition but reduced transporter degradation.
  
  
  CONCLUSIONS
  The type IIa Na/P(i) cotransporter is a phosphoprotein, but alterations in its phosphorylation seem not to be involved in P(i) transport inhibition.
authors.affiliation: Institute of Physiology, University of Zurich, Zurich, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Jankowski
authors.forename: H
authors.initials: H
authors.lastname: Hilfiker
authors.forename: J
authors.initials: J
authors.lastname: Biber
authors.forename: H
authors.initials: H
authors.lastname: Murer
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000032:analysis / Q000378:metabolism*; D002478:Cells, Cultured; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D009893:Opossums; D010750:Phosphoproteins / Q000032:analysis / Q000378:metabolism*; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters
chemical_list: D002352:Carrier Proteins; D010750:Phosphoproteins; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters