Heat-induced dimerization of BCL-xL through alpha-helix swapping.

The dimerization of anti-apoptotic BCL-xL by three-dimensional domain swapping has recently been discovered at alkaline pH; however, the high energetic barrier between the dimer and monomer forms of BCL-xL prevents them from interconverting at room temperature and neutral pH. Here, we demonstrate that BCL-xL dimers can be easily prepared by heating concentrated protein above 50 degrees C. The 38 kDa BCL-xL dimer was fully characterized by multi-resonance nuclear magnetic resonance (NMR) spectroscopy, and the mechanism of dimerization by alpha-helix swapping was confirmed. Dimerization strongly affects the NMR signals from the turn between helices alpha5 and alpha6 of BCL-xL and a portion of the long loop between helices alpha1 and alpha2. Measurements of residual dipolar couplings demonstrate that the solution structure of the BCL-xL dimer is very close to the crystal structure. Dimer formation does not prevent tight binding of ligands to the hydrophobic cleft of BCL-xL; however, binding of a BID BH3-peptide or a polyphenol drug, gossypol, to BCL-xL significantly slowed monomer-dimer interconversion and is an example of the control of BCL protein oligomerization by ligand binding.