From the transcript for this lecture:
There are other members of this family that are pro-death factors: three examples are bak, bax, and bim. In a normal healthy cell, bak and bax are normally present; however bak is bound by bcl-2, which neutralizes it, and bax is in the cytoplasm, where it can’t do much in terms of mitochondria. Bim is present in low levels. When a cell gets a signal to cause death, bim levels go up, bax and bim move into the mitochondrion, and bim binds to bcl-2, displacing bak and neutralizing bcl-2. And now bak binds to bax, when bax and bak bind to one another they form a pore and this pore permits the release of cytochrome c from the mitochondrion. This then binds to APAF1, which binds to caspase 9, which then activates caspase 2, causing death.