Phosphorylation of BCL-2 regulates ER Ca2+ homeostasis and apoptosis

Phosphorylation of BCL-2 within an unstructured loop inhibits its antiapoptotic effect. We found that phosphorylated BCL-2 predominantly localized to the endoplasmic reticulurn (ER) and tested whether phosphorylation would control its activity at this organelle, where Ca2+ dynamics serve as a critical control point for apoptosis. Phosphorylation greatly inhibits the ability of BCL-2 to lower [Ca2+](er) and protect against Ca2+-dependent death stimuli. Cells expressing nonphosphorylatable BCL-2(AAA) exhibited increased leak of Ca2+ from the ER and further diminished steady-state [Ca2+](er) stores when compared to cells expressing BCL-2(wt). Consequently, when BCL-2 is phosphorylated, Ca2+ discharge from the ER is increased, with a secondary increase in mitochondrial Ca2+ uptake. We also demonstrate that phosphorylation of BCL-2 inhibits its binding to proapoptotic family members. This inhibitory mechanism manifested at the ER, where phosphorylated BCL-2 was unable to bind proapoptotic members. [Ca2+](er) proved coordinate with the capacity of BCL-2 to bind proapoptotic BH3-only members, further integrating the apoptotic pathway and Ca2+ modulation. Unexpectedly, the regulation of ER Ca2+ dynamics is a principal avenue whereby BCL-2 phosphorylation alters susceptibility to apoptosis.