Potent inhibitor of cell death. Inhibits activation of caspases. Appears to regulate cell death by blocking the voltage-dependent anion channel (VDAC) by binding to it and preventing the release of the caspase activator, CYC1, from the mitochondrial membrane. Also acts as a regulator of G2 checkpoint and progression to cytokinesis during mitosis. Isoform Bcl-X(L) also regulates presynaptic plasticity, including neurotransmitter release and recovery, number of axonal mitochondria as well as size and number of synaptic vesicle clusters. During synaptic stimulation, increases ATP availability from mitochondria through regulation of mitochondrial membrane ATP synthase F(1)F(0) activity and regulates endocytic vesicle retrieval in hippocampal neurons through association with DMN1L and stimulation of its GTPase activity in synaptic vesicles (By similarity). May attenuate inflammation impairing NLRP1-inflammasome activation, hence CASP1 activation and IL1B release (By similarity). Isoform Bcl-X(S) promotes apoptosis. Homodimer. Interacts with BAD. Interacts with PGAM5. Interacts with HEBP2. Interacts with p53/TP53 and BBC3; interaction with BBC3 disrupts the interaction with p53/TP53. Interacts with ATP5F1A and ATP5F1B; the interactions mediate the association of isoform Bcl-X(L) with the mitochondrial membrane ATP synthase F(1)F(0) ATP synthase (By similarity). Interacts with VDAC1 (By similarity). Interacts with BCL2L11 (via BH3) (PubMed:14499110, PubMed:27013495). Interacts with RNF183 (By similarity). Interacts with GIMAP3/IAN4 and GIMAP5/IAN5 (PubMed:16509771). Interacts with GIMAP5 and HSPA8/HSC70; the interaction between HSPA8 and BCL2L1 is impaired in the absence of GIMAP5 (PubMed:21502331). Interacts with isoform 4 of CLU; this interaction releases and activates BAX and promotes cell death (By similarity). Forms heterodimers with BAX, BAK or BCL2; heterodimerization with BAX does not seem to be required for anti-apoptotic activity (By similarity). Interacts with isoform 1 of SIVA1; the interaction inhibits the anti-apoptotic activity (By similarity). Interacts with IKZF3 (By similarity). Interacts with RTL10/BOP (By similarity). Interacts with DNM1L and CLTA; DNM1L and BCL2L1 isoform BCL-X(L) may form a complex in synaptic vesicles that also contains clathrin and MFF (By similarity). Interacts (via the loop between motifs BH4 and BH3) with NLRP1 (via LRR repeats), but not with NLRP2, NLRP3, NLRP4, PYCARD, nor MEFV (By similarity). Interacts with BECN1 (By similarity). Localizes to the centrosome when phosphorylated at Ser-49. After neuronal stimulation, translocates from cytosol to synaptic vesicle and mitochondrion membrane in a calmodulin-dependent manner. Widely expressed, with highest levels in the brain, thymus, bone marrow, and kidney. Bcl-X(L) and Bcl-X(delta-TM) expression is enhanced in B- and T-lymphocytes that have been activated. Bcl-X(beta) is expressed in both embryonal and postnatal tissues, whereas Bcl-X(L) is predominantly found in postnatal tissues. The BH4 motif is required for anti-apoptotic activity. The BH1 and BH2 motifs are required for both heterodimerization with other Bcl-2 family members and for repression of cell death. The loop between motifs BH4 and BH3 is required for the interaction with NLRP1. Proteolytically cleaved by caspases during apoptosis. The cleaved protein, lacking the BH4 motif, has pro-apoptotic activity. Phosphorylated on Ser-62 by CDK1. This phosphorylation is partial in normal mitotic cells, but complete in G2-arrested cells upon DNA-damage, thus promoting subsequent apoptosis probably by triggering caspases-mediated proteolysis. Phosphorylated by PLK3, leading to regulate the G2 checkpoint and progression to cytokinesis during mitosis. Phosphorylation at Ser-49 appears during the S phase and G2, disappears rapidly in early mitosis during prometaphase, metaphase and early anaphase, and re-appears during telophase and cytokinesis (By similarity). Ubiquitinated by RNF183 during prolonged ER stress, leading to degradation by the proteosome. Belongs to the Bcl-2 family.