A new phosphatidylinositol 4,5-bisphosphate-binding site located in the C2 domain of protein kinase Cα

In view of the interest shown in phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P-2) as a second messenger, we studied the activation of protein kinase Calpha by this phosphoinositide. By using two double mutants from two different sites located in the C2 domain of protein kinase Calpha we have determined and characterized the PtdIns(4,5)P-2-binding site in the protein, which was found to be important for its activation. Thus, there are two distinct sites in the C2 domain: the first, the lysine-rich cluster located in the beta3- and beta4-sheets and which activates the enzyme through direct binding of PtdIns(4,5)p2; and the second, the already well described site formed by the Ca2+-binding region, which also binds phosphatidylserine and a result of which the enzyme is activated. The results obtained in this work point to a sequential activation model, in which protein kinase Ca needs Ca2+ before the PtdIns(4,5)P-2-dependent activation of the enzyme can occur.