The opossum kidney cell type IIa Na/P(i) cotransporter is a phosphoprotein
M Jankowski1, H Hilfiker, J Biber
1Institute of Physiology, University of Zurich, Zurich, Switzerland.
Background/Aim:
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.
Conclusion:
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.
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