Phosphatidylinositol 4,5-Bisphosphate Decreases the Concentration of Ca(2+), Phosphatidylserine and Diacylglycerol Required for Protein Kinase C α to Reach Maximum Activity

The C2 domain of PKCα possesses two different binding sites, one for Ca(2+) and phosphatidylserine and a second one that binds PIP(2) with very high affinity. The enzymatic activity of PKCα was studied by activating it with large unilamellar lipid vesicles, varying the concentration of Ca(2+) and the contents of dioleylglycerol (DOG), phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphadidylserine (POPS) in these model membranes. The results showed that PIP(2) increased the V(max) of PKCα and, when the PIP(2) concentration was 5 mol% of the total lipid in the membrane, the addition of 2 mol% of DOG did not increase the activity. In addition PIP(2) decreases K(0.5) of Ca(2+) more than 3-fold, that of DOG almost 5-fold and that of POPS by a half. The K(0.5) values of PIP(2) amounted to only 0.11 µM in the presence of DOG and 0.39 in its absence, which is within the expected physiological range for the inner monolayer of a mammalian plasma membrane. As a consequence, PKCα may be expected to operate near its maximum capacity even in the absence of a cell signal producing diacylglycerol. Nevertheless, we have shown that the presence of DOG may also help, since the K(0.5) for PIP(2) notably decreases in its presence. Taken together, these results underline the great importance of PIP(2) in the activation of PKCα and demonstrate that in its presence, the most important cell signal for triggering the activity of this enzyme is the increase in the concentration of cytoplasmic Ca(2+).