Protein kinase C consensus sites and the regulation of renal Na/Pi-cotransport (NaPi-2) expressed in XENOPUS laevis oocytes

Renal brush border membrane sodium/phosphate (Na/Pi)-cotransport activity is inhibited by hormonal mechanisms involving activation of protein kinases A and C. The recently cloned rat renal Na/Pi-cotransporter (NaPi-2) contains several protein kinase C but no protein kinase A consensus sites [17, 20]. In the present study we have expressed wild type and polymutant (protein kinase C consensus sites removed) NaPi-2-transporters in Xenopus laevis oocytes. The expression of transport function as well as the basic transport properties were unaffected by the removal of the consensus sites. Pharmacological activation of protein kinase C with phorbol 12,13-didecanoate (PDD) led to a time-dependent inhibition of expressed wild type Na/Pi-cotransport function; simultaneous exposure to staurosporine (0.3) prevented the PDD induced (50 nM) inhibition. The kinase-C-mediated inhibition was not prevented by the removal of the protein kinase C consensus sites. Pharmacological activation of protein kinase A (dibutyryl adenosine 3':5':cyclic monophosphate (cAMP)/forskolin) had no effect on wild type NaPi-2-induced oocyte Na/Pi-cotransport. It is concluded that the protein-kinase-C-mediated regulation of expressed Na/Pi-cotransport does not involve the predicted consensus sites. The involvement of "cryptic" phosphorylation sites and/or of a phosphorylated "regulatory" protein is discussed.