Selective localization of Bcl-2 to the inner mitochondrial and smooth endoplasmic reticulum membranes in mammalian cells

Bcl-2, an anti-apoptotic protein, is believed to be localized in the outer mitochondrial membrane, endoplasmic reticulum, and nuclear envelope, Howeve r, Bcl-2 has also been suggested as playing a role in the maintenance of mi tochondrial membrane potential, indicating its possible association with th e inner mitochondrial membrane. We therefore further examined the exact loc alization of Bcl-2 in mitochondria purified from wild-type and bcl-2-transf ected PC12 cells and pre- and postnatal rat brains. Double immunostaining d emonstrated that Bcl-5 was co-localized with subunit beta of F(1)F(0)ATPase in the inner mitochondrial membrane. Biochemical analysis of isolated mito chondria using digitonin and trypsin suggests an association of Bcl-2 with the inner mitochondrial membrane, More interestingly, the majority of Bcl-2 disappeared from the inner membrane of mitochondria when cultured under se rum deprivation. These results suggest that Bcl-2 acts as an anti-apoptotic regulator by localizing mainly to the inner mitochondrial and smooth ER me mbranes.