Involved in the regulation of apoptosis versus cell survival, and in the maintenance of viability but not of proliferation. Mediates its effects by interactions with a number of other regulators of apoptosis (By similarity). Interacts with HIF3A (via C-terminus domain) (By similarity). Interacts with BOK, BIK, BAX, BAK1, and TPT1. Interacts with unphosphorylated BAD (By similarity). Interacts with BMF, BBC3 and PMAIP1 (By similarity). Interacts with BOP (By similarity). Interacts with BCL2L11; may sequester BCL2L11 to prevent its pro-apoptotic activity (By similarity). Interacts with GIMAP5 and HSPA8/HSC70; the interaction between HSPA8 and MCL1 is impaired in the absence of GIMAP5 (By similarity). Cytoplasmic, associated with mitochondria. Cleaved by CASP3 during apoptosis, yielding a pro-apoptotic C-terminal fragment. Rapidly degraded in the absence of phosphorylation in the PEST region. Phosphorylated on Ser-159, by GSK3, in response to IL3/interleukin-3 withdrawal. Phosphorylation at Ser-159 induces ubiquitination and proteasomal degradation, abrogating the anti-apoptotic activity. Treatment with taxol or okadaic acid induces phosphorylation on additional sites (By similarity). Ubiquitinated. Ubiquitination is induced by phosphorylation at Ser-159 (By similarity). Belongs to the Bcl-2 family.