MOLECULAR MECHANISM OF POLYPEPTIDE INSERTION INTO BILAYER AND EXIT

Abstract

The pH-low insertion peptide (pHLIP) and pH-low insertion cycle (pHLIC) have been shown to target cancer cells and inflammation due to the acidic environment present at those sites. It has been demonstrated that pHLIP’s and pHLIC’s pH dependent behavior stems from the protonation and deprotonation of aspartic acid (Asp) and glutamic acid (Glu) residues. A decrease in pH leads to the protonation of Asp/Glu located in membrane-inserting part of peptides, which increases the overall hydrophobicity of pHLIP and pHLIC and triggers the insertion across a lipid bilayer. Despite similarity of pHLIP and pHLIC ability to sense pH at cell surfaces the mechanisms of peptides insertion into membrane is different. pHLIP, which is a flexible polymer in solution at high pH, undergoes pH-triggered folding in membrane to transition from coil to transmembrane helix. pHLIC, which is a rigid cyclic peptide, undergoes pH-triggered partition into membrane without changes of its structure. pHLIP peptide insertion occurs in several steps, with a rapid interfacial helix formation (folding) completed within 100 ms followed by the rate limiting step of peptide insertion across membrane to form a transmembrane helix. Exit from the bilayer and unfolding is triggered by deprotonation of Asp/Glu residues induced by pH raise. The reverse process of unfolding and exit proceeds through different intermediate states. The detailed kinetics study of pHLIP variants pH-triggered insertion and exit from the membrane of liposomes allowed to elucidate the molecular mechanism of membrane-associated folding and unfolding, and design and test new pHLIP variants with tunable pH-dependent properties. Biophysical investigation of several pH-sensitive and pH-insensitive cyclic peptides led to the selection of best pHLIC candidate for targeting and imaging of neuroinflammation, which is associated with development of variety of neurodegenerative diseases.