Growth Factors Inactivate the Cell Death Promoter BAD by Phosphorylation of Its BH3 Domain on Ser155

The Bcl-2 family protein BAD promotes apoptosis by binding through its BH3 domain to Bcl-xL and related cell death suppressors. When BAD is phosphorylated on either Ser112 or Ser136, it forms a complex with 14-3-3 in the cytosol and no longer interacts with Bcl-xLat the mitochondria. Here we show that phosphorylation of a distinct site Ser155, which is at the center of the BAD BH3 domain, directly suppressed the pro-apoptotic function of BAD by eliminating its affinity for Bcl-xL. Protein kinase A functioned as a BAD Ser155 kinase both in vitro and in cells. BAD Ser155 was found to be a major site of phosphorylation induced following stimulation by growth factors and prevented by protein kinase A inhibitors but not by inhibitors of the phosphatidylinositol 3-kinase/Akt pathway. Growth factors inhibited BAD-induced apoptosis in both a Ser112/Ser136- and a Ser155-dependent fashion. Thus, growth factors engage an anti-apoptotic signaling pathway that inactivates BAD by direct modification of its BH3 cell death effector domain.