PROTEIN-KINASE-C CONSENSUS SITES AND THE REGULATION OF RENAL NA P-I-COTRANSPORT (NAPI-2) EXPRESSED IN XENOPUS-LAEVIS OOCYTES/

Renal brush border membrane sodium/phosphate (Na/P-i)-cotransport acti vity is inhibited by hormonal mechanisms involving activation of prote in kinases A and C. The recently cloned rat renal Na/P-i-cotransporter (NaPi-2) contains several protein kinase C but no protein kinase A co nsensus sites [17, 20]. In the present study we have expressed wild ty pe and poly-mutant (protein kinase C consensus sites removed) NaPi-2-t ransporters in Xenopus laevis oocytes. The expression of transport fun ction 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/P-i-cotransport function; simult aneous exposure to staurosporine (0.3) prevented the PDD induced (50 n M) inhibition. The kinase-C-mediated inhibition was not prevented by t he removal of the protein kinase C consensus sites. Pharmacological ac tivation of protein kinase A (dibutyryl adenosine 3': 5':cyclic monoph osphate (cAMP)/forskolin) had no effect on wild type NaPi-2-induced oo cyte Na/P-i-cotransport. It is concluded that the protein-kinase-C-med iated regulation of expressed Na/P-i-cotransport does not involve the predicted consensus sites. The involvement of ''cryptic'' phosphorylat ion sites and/or of a phosphorylated ''regulatory'' protein is discuss ed.