Conformational switching associated with the conversion of a protein structure from a water-soluble to a membrane-inserted form is a critical step in several cellular processes. A notable example of such switching is a tumor-targeting pH low insertion peptide (pHLIP). It is well established that pHLIP can selectively target cancer cells; however, the exact molecular mechanism of the process is not fully understood. Numerous experimental studies demonstrated that pH, lipid composition, and the presence of divalent cations modulate membrane interactions of pHLIP. While previous computational studies have explored pHLIP either in its unstructured soluble conformation or in its final transmembrane helical conformation, the interfacial state of pHLIP, responsible for insertion modulation by various factors, has received little attention. Here, we investigate how pHLIP interacts with the membrane interfaces of two lipid compositions (POPC and 3POPS:1POPC). For each lipid environment, we further characterize pHLIP behavior under different protonation states of acidic residues and in the presence or absence of Ca 2+ . Our study is carried out using microsecond-timescale molecular dynamics simulations at physiological temperature with atomic resolution, with each condition repeated across multiple peptide starting configurations. Results show that at neutral pH and in the absence of Ca 2+ , pHLIP in POPC membranes positions its acidic residues closer to the membrane surface, resulting in a flatter configuration and greater residue penetration beyond the phosphate headgroups compared to the 3POPS:1POPC membranes. In the mixed POPS/POPC systems, protonation brings acidic residues closer to the membrane core. Similarly, adding 0.1 M Ca 2+ to unprotonated systems produces the same effect on acidic residues as protonation. Future work supported by NIH (R01 GM145991) and Anton (MCB190070P) will examine conformational switching observed in some simulated systems and the effects of pHLIP on membrane properties, such as bilayer thickness and lipid arrangement.
Nguyen et al. (Sun,) studied this question.