Ca2+-dependent protein phosphorylation in brush border membranes of rat kidney proximal tubules

Ca2+-dependent protein phosphorylation was studied in isolated brush border membrane vesicles from rat kidney cortex. Phosphorylation of about 20 target proteins with gamma-32P-ATP was observed after opening the vesicles transiently by detergent treatment or by hypotonic shock indicating that phosphorylation takes place from the cytoplasmic side. Five of these polypeptides (Mr 12, 19, 29, 97 and 138 kD) showed increased phosphorylation in the presence of micromolar calcium. Addition of exogenous calmodulin did not enhance the calcium induced phosphorylation significantly, nor did trifluoperazine depress it, suggesting that Ca2+-calmodulin-dependent protein kinase is not involved in the studied Ca2+-induced phosphorylation. 12-O-tetradecanoylphorbol-13-acetate (TPA) minimized the Ca2+ dependency for the 12 and 97 kD polypeptides. Cytotoxin I inhibited the incorporation of phosphate into the 12 and 97 kD polypeptides in a dose-dependent manner. Excess phosphatidylserine could reverse this inhibition but stimulated also the phosphorylation of the 19 kD polypeptide. These findings suggest that at least the 12 and the 97 kD polypeptides are substrates for an endogenous protein kinase C. When studied under conditions where Ca2+ increases membrane phosphorylation, no effects could be observed on the kinetic parameters (Km and Vmax) of the sodium-dependent phosphate uptake.