Probing the complex interaction between the pH-low insertion peptide and membrane bilayers

The pH-low insertion peptide (pHLIP) is a small pH responsive peptide with applications as a cancer therapeutic agent. pHLIP is the C-helix of bacteriorhodopsin (bR), which was found to be soluble in buffer yet interacts with model membranes. Moving forward, advances have been made to further understand the complex interactions that occur between pHLIP and the membrane using a myriad of biophysical techniques. From these studies, it was found that pHLIP interacts with the membrane in three distinct states: soluble in solution at neutral pH, associates with membranes as a random coil at neutral pH, and finally a transmembrane alpha helix at acidic pH. Taking advantage of the pH responsive properties, pHLIP has been utilized as a cancer therapeutic agent.Although considerable work has been conducted with pHLIP, the precise understanding of the peptide-lipid interactions that occur are not completely understood. The plasma membrane, the primary target of pHLIP, is a diverse entity. Studies have shown that the plasma membrane consists of a multitude of lipid species that are distributed in an asymmetric fashion between the two leaflets. The lipids in the plasma membrane are not only diverse; they are also quite mobile. Lipids within the membrane can move both individually as well as collectively. pHLIP has been studied quite selectively in single component model membranes that do not include the characteristics described above.Here, we studied how changes in the membrane composition affect the interactions between pHLIP and the lipids in the membrane. Using model membranes, we investigated how a symmetric distribution of a phospholipid with a phosphatidylserine (PS) headgroup influences the insertion of pHLIP. Taking the next step, we further mimicked the plasma membrane by distributing PS in an asymmetric fashion between the two leaflets. We found that both distributions affect the acidity needed for insertion of pHLIP. We also investigated how pHLIP influences membrane dynamics by using neutron scattering techniques to probe collective membrane dynamics. We found that the inserted state of pHLIP changes collective membranes dynamics. Overall, this work advances our understanding of how pHLIP interacts with the lipid bilayer.