Monomeric and oligomeric forms of Bcl-xL in direct and indirect inhibition of apoptosis

The Bcl-2 protein Bcl-xL is a well-known inhibitor of apoptosis which can either directly inhibit the pore-forming Bcl-2 proteins, as Bax or Bak, or indirectly inhibit pore formation by sequestering the proapoptotic BH3-only proteins, as cBid or Bim. The structural basis of the inhibition of pore formation in the outer mitochondrial membrane by Bcl-xL is still largely unknown. Up to now, challenges in obtaining full-length Bcl-xL have hampered the full understanding of the inhibitory interactome at the membrane. Here we present an optimized protocol for the purification of full-length Bcl-xL, which allows separation of monomeric Bcl-xL from large oligomers which could not be otherwise eliminated in the purification procedure. The obtained monomeric Bcl-xL successfully inhibited pore formation via both modes of action, based on assays performed on large unilamellar vesicles in the presence of a minimal Bcl-2 interactome constituted by Bcl-xL, cBid and Bax. In contrast, the oligomeric fraction retained only the ability to indirectly inhibit pore formation by sequestering cBid. The presented protocol will pave the way towards biophysical studies aimed to unveil the structural details of apoptosis inhibition.