The N-terminal end of Bax contains a mitochondrial-targeting signal.

The translocation of Bax alpha, a pro-apoptotic member of the BCL-2 family from the cytosol to mitochondria, is a central event of the apoptotic program. We report here that the N-terminal (NT) end of Bax alpha, which contains its first alpha helix (Eta alpha 1), is a functional mitochondrial-addressing signal both in mammals and in yeast. Similar results were obtained with a newly described variant of Bax called Bax psi, which lacks the first 20 amino acids of Bax alpha and is constitutively associated with mitochondria. Deletion of Eta alpha 1 impairs the binding of Bax psi to mitochondria, whereas a fusion of the N terminus of Bax alpha, which contains Eta alpha 1 with a cytosolic protein, results in the binding of the chimeric proteins to mitochondria both in a cell-free assay and in vitro. More importantly, the mitochondria-bound chimeric proteins inhibit the interaction of Bax psi with mitochondria as well as Bax-apoptogenic properties. The mutations of the Eta alpha 1, which inhibit Bax alpha and Bax psi translocation to mitochondria, also block the subsequent activation of the execution phase of apoptosis. Conversely, a deletion of the C terminus does not appear to influence Bax alpha and Bax psi mitochondrial addressing. Taken together, our results suggest that Bax is targeted to mitochondria by its NT and thus through a pathway that is unique for a member of the BCL-2 family.