Suppressor of apoptosis. Modulates unfolded protein response signaling. Modulates ER calcium homeostasis by acting as a calcium-leak channel. Negatively regulates autophagy and autophagosome formation, especially during periods of nutrient deprivation, and reduces cell survival during starvation. Interacts with BCL2. Interacts with BCL2L1. The intra-membrane loop at the C-terminus acts as a calcium pore, mediating calcium leak from the ER into the cytosol. Belongs to the BI1 family.