The interaction of BAD (Bcl-2/Bcl-XL-antagonist, causing cell death) with Bcl-2/Bcl-XL is thought to neutralize the anti-apoptotic effects of the latter proteins, and may represent one of the mechanisms by which BAD promotes apoptosis. A variety of survival signals are reported to induce the phosphorylation of BAD at Ser112 or Ser136, triggering its dissociation from Bcl-2/Bcl-XL. Ser136 is thought to be phosphorylated by protein kinase B (PKB, also called Akt), which is activated when cells are exposed to agonists that stimulate phosphatidylinositol 3-kinase (PI3K). In contrast, Ser112 is reported to be phosphorylated by mitogen-activated protein (MAP) kinase-activated protein kinase-1 (MAPKAP-K1, also called RSK) and by cAMP-dependent protein kinase (PKA). Here we identify Ser155 as a third phosphorylation site on BAD. We find that Ser155 is phosphorylated preferentially by PKA in vitro and is the only residue in BAD that becomes phosphorylated when cells are exposed to cAMP-elevating agents. The phosphorylation of BAD at Ser155 prevents it from binding to Bcl-XL and promotes its interaction with 14-3-3 proteins. We also provide further evidence that MAPKAP-K1 mediates the phosphorylation of Ser112 in response to agonists that activate the classical MAP kinase pathway. However insulin-like growth factor 1, a potent activator of PI3K and PKB does not increase the phosphorylation of Ser136 in BAD-transfected HEK-293 cells, and nor is the basal level of Ser136 phosphorylation suppressed by inhibitors of PI3K.
The insulin-directed phosphorylation site on ATP-citrate lyase is identical with the site phosphorylated by the cAMP-dependent protein kinase in vitro.
