Abstract
A pivotal step toward apoptosis is oligomerization of the Bcl-2 relative Bak. We recently reported that its oligomerization initiates by insertion of an exposed BH3 domain into the groove of another Bak monomer. We now report that the resulting BH3:groove dimers can be converted to the larger oligomers that permeabilize mitochondria by an interface between α6 helices. Cysteine residues placed in α6 could be crosslinked only after apoptotic signaling. Cysteines placed at both interfaces established that the BH3:groove dimer is symmetric and that the α6:α6 interface can link these dimers into homo-oligomers containing at least 18 Bak molecules. A putative zinc-binding site in α6 was not required to form the α6:α6 interface, and its mutation in full-length Bak did not affect Bak conformation, oligomerization, or function. We conclude that α6:α6 interaction occurs during Bak oligomerization and proapoptotic function, but we find no evidence that zinc binding to that interface regulates apoptosis.