Modulation of Protein-Membrane Interactions by Lipids and Divalent Cations

The post-translational insertion of proteins into membranes is common in a number of biologically important processes, such as the cellular entry of bacterial toxins or regulation of apoptosis by the Bcl-2 family of proteins, as well as in targeted drug delivery. While our ability to target or manipulate these processes can be beneficial for human health, the basic knowledge of the mechanism of membrane modulation of conformational switching is lacking. To provide a more complete picture of protein-membrane interactions in the cell, this manuscript explores the effects of lipid composition and physiological [Ca2+/Mg2+] on the membrane insertion and refolding of peptides and proteins. The main focus is on the peptide pHLIP (pH-Low Insertion Peptide), a promising cancer targeting system, and the apoptotic inhibitor Bcl-xL. The results presented here demonstrate that physiological concentrations of Ca2+ and Mg2+ induce the membrane insertion and refolding of both pHLIP and Bcl-xL. In both cases, the divalent cation-dependent effects are further modulated by lipid composition. These results highlight the importance of physiological conditions on the study of protein membrane interactions and shows that their omission can lead to false assumptions of unfavorable interactions