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. Isoform 2 has antiapoptotic activity. Interacts with HIF3A isoform 2 (via C-terminus domain) (PubMed:21546903). Interacts with BAD, BOK, BIK, BAX, BAK1, and TPT1. Interacts with BBC3, BMF and PMAIP1 (PubMed:15550399, PubMed:18589438). Interacts with BOP. Interacts with BCL2L11; this interaction may sequester BCL2L11 and prevent its pro-apoptotic activity (PubMed:16543145, PubMed:27013495, PubMed:15550399). Interacts with GIMAP5 and HSPA8/HSC70; the interaction between HSPA8 and MCL1 is impaired in the absence of GIMAP5 (PubMed:21502331). Cytoplasmic, associated with mitochondria. Up-regulated by IL3 and CSF2. Up-regulated in murine embryonal carcinoma cells in response to retinoic acid treatment. Levels reach a maximum after 4 hours, are decreased after 8 hours and are back to maximum after 12 hours. Levels are decreased after 24 hours and back to basal levels after 48 hours. Expression remains constant in retinoic acid-treated embryonic stem cells. 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-140, by GSK3, in response to IL3/interleukin-3 withdrawal. Phosphorylation at Ser-140 induces ubiquitination and proteasomal degradation, abrogating the anti-apoptotic activity. Treatment with taxol or okadaic acid induces phosphorylation on additional sites. Ubiquitinated. Ubiquitination is induced by phosphorylation at Ser-140 (By similarity). This isoform is more stable than isoform 1 in cells undergoing apoptosis. Belongs to the Bcl-2 family.