Phosphatidylinositol 4,5-bisphosphate (PIP2) as well as diacylglycerol (DG) activate protein kinase C (PKC) in the presence of calcium and phosphatidylserine. The pH at half-activation (pK) is 6.2 for DG.PKC and 7.7 for PIP2.PKC. Since the second monophosphate proton in position 5 of the PIP2 inositol (i.e., the last ionizable proton) has a pK of 7.7 (Van Paridon et al., (1986) Biochim. Biophys. Acta. 877, 216), the active effector is a fully deprotonated PIP2. Activation of PKC by PIP2 thus may follow intracellular alkalinization and be tied to the down-regulation of the Na+/H+ antiport mechanism. Since alkalinization is obligatory for cell proliferation, PIP2(5-).Ca.PKC may also be the gate that opens the pathways toward this and connected cellular reactions. A PIP2 analog in which inositol carbons 2-4 and the 4-phosphate have been removed, 1-phosphatidyl-rac-glycerol-3-phosphate (PGP), is completely inactive as PKC effector; this suggests that both 4-and 5-phosphate are engaged in the PIP2(5-).Ca.PKC complex. A model of the activated kinase takes this into account.