Conformational Switching on Lipid Interfaces: Lessons from Refolding and Membrane Insertion of Diphtheria Toxin,...

The conformational switching of a protein from its water-soluble conformation to its membrane-inserted conformation is critical for many cellular processes: cellular entry of bacterial toxins via escape from the endosome, apoptotic regulation by the Bcl-2 protein family to preserve or disrupt the outer mitochondrial membrane, and tumor targeting by pH-Low-Insertion-P eptide pHLIP. Here, several empirical mechanisms of conformational switching are described, illustrating how the formation of membrane-competent conformation and subsequent membrane insertion can be triggered and modulated by pH and divalent cations, abided by changes in lipid composition of the target membranes. Despite structural similarities of the water-soluble conformations of the diphtheria toxin translocation (T) domain with that of proapoptotic BAX and antiapoptotic Bcl-xL, their corresponding insertion pathways and modes of conformational switching are markedly different. The conformational switching of the T-domain is initiated by protonation of key histidine residues in solution, while in the Bcl-2 proteins BAX and Bcl-xL, it occurs in the membrane proximity. Consequently, changes in the composition of the lipid bilayer (e.g., local enrichment in cardiolipin) can affect insertion and functioning of Bcl-2 proteins. Conformational switching of pHLIP occurs on membrane interfaces via pH-dependent and Ca2+-dependent mechanisms, which is strongly influenced by lipid composition.