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B-cell lymphoma-extra large (Bcl-xl), encoded by the BCL2-like 1 gene, is a transmembrane molecule in the mitochondria. It is a member of the Bcl-2 family of proteins, and acts as an anti-apoptotic protein by preventing the release of mitochondrial contents such as cytochrome c, which leads to caspase activation and ultimately, programmed cell death.

Function
It is a well-established concept in the field of apoptosis that relative amounts of pro- and anti-survival Bcl-2 family of proteins determine whether the cell will undergo cell death: if more Bcl-xL is present, then pores are non-permeable to pro-apoptotic molecules and the cell survives. However, if Bax and Bak become activated, and Bcl-xL is sequestered away by gatekeeper BH3-only factors (e.g. Bim), causing a pore to form, cytochrome c is released leading to initiation of caspase cascade leading to apoptotic events.

While the exact signaling pathway of Bcl-XL is still not known, it is believed that Bcl-XL differs highly from Bcl-2 in the their mechanism of inducing apoptosis. A study was done where Bcl-XL was about ten times more functional than Bcl-2 when induced by the chemotherapy drug, Doxorubicin. NMR stidues show that Bcl-XL can specifically bind to cytochrome C, with interactions of residues, preventing apoptosis.



Clinical Significance
Bcl-xl dysfunction in mice can cause ineffective production of red blood cells, sever anemia, hemolysis, and death. This protein has also been shown as a requirement for heme production and in erythroid lineage, Bcl-xL is a major survival factor responsible for an estimated half of the total survival "signal" proerythroblasts must receive in order to survive and become red cells. Bcl-xL promoter contains GATA-1 and Stat5 sites. This protein accumulates throughout the differentiation, ensuring the survival of erythroid progenitors. Because iron metabolism and incorporation into hemoglobin occurs inside the mitochondria, Bcl-xL was suggested to play additional roles in regulating this process in erythrocytes which could lead to a role in polycythemia vera, a disease where there is an overproduction of erythrocytes.

Effects
Similar to Bcl-2, Bcl-xL has been implicated in the survival of cancer cells. Bcl-xL is known to be over-expressed in hematopoietic disorders such as polycythemia vera, where Jak2 mutations lead to over-activation of intracellular signaling molecules, such as Stat5, which lead to transcription of Bcl-xL gene.

Related proteins
Other Bcl-2 proteins include Bcl-2, Bcl-w, Bcl-xs, and Mcl-1.