Bcl-xL is the dominant regulator of apoptosis, or active cell suicide. It is known as the survival protein because the long form of Bcl-xL has cell death repressor activity. It is found in the mitochondrial membranes of cells that are long-lived and postmitotic, such as adult brain cells. On the other hand, Bak accelerates programmed cell death by binding to and antagonizing the repressor Bcl-2. It is a membrane-bound protein found mainly in the heart and skeletal muscle.
The Bcl-2 family of proteins are able to form heterodimers, and this is an important event in the regulation of apoptosis. Here, Bcl-xL(yellow) has formed a heterodimer with the death-promoting region of the Bcl-2 related protein Bak (shown by residue). It has been found that Bak takes on an amphipathic alpha helical conformation that interacts with Bcl-xL via hydrophobic and electrostatic interactions.
Bcl-xL is 181 amino acid residues long. It consists of 10 alpha helices, 2 of which are 310 alpha helices. The Bak peptide is 16 residues long and is as follows: Gly-Gln-Val-Gly-Arg-Gln-Leu-Ala-Ile-Ile-Gly-Asp-Asp-Ile-Asn-Arg.