title: Roles of carboxyl groups in the transmembrane insertion of peptides.
abstract: We have used pHLIP® [pH (low) insertion peptide] to study the roles of carboxyl groups in transmembrane (TM) peptide insertion. pHLIP binds to the surface of a lipid bilayer as a disordered peptide at neutral pH; when the pH is lowered, it inserts across the membrane to form a TM helix. Peptide insertion is reversed when the pH is raised above the characteristic pK(a) (6.0). A key event that facilitates membrane insertion is the protonation of aspartic acid (Asp) and/or glutamic acid (Glu) residues, since their negatively charged side chains hinder membrane insertion at neutral pH. In order to gain mechanistic understanding, we studied the membrane insertion and exit of a series of pHLIP variants where the four Asp residues were sequentially mutated to nonacidic residues, including histidine (His). Our results show that the presence of His residues does not prevent the pH-dependent peptide membrane insertion at ~pH 4 driven by the protonation of carboxyl groups at the inserting end of the peptide. A further pH drop leads to the protonation of His residues in the TM part of the peptide, which induces peptide exit from the bilayer. We also find that the number of ionizable residues that undergo a change in protonation during membrane insertion correlates with the pH-dependent insertion into the lipid bilayer and exit from the lipid bilayer, and that cooperativity increases with their number. We expect that our understanding will be used to improve the targeting of acidic diseased tissue by pHLIP.
authors: Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA.
authors: Francisco N
authors: FN
authors: Barrera
authors: Dhammika
authors: D
authors: Weerakkody
authors: Michael
authors: M
authors: Anderson
authors: Oleg A
authors: OA
authors: Andreev
authors: Yana K
authors: YK
authors: Reshetnyak
authors: Donald M
authors: DM
authors: Engelman
mesh_terms: D000595:Amino Acid Sequence; D002264:Carboxylic Acids / Q000737:chemistry*; D002462:Cell Membrane / Q000378:metabolism*; D057846:Cell-Penetrating Peptides / Q000378:metabolism*; D002942:Circular Dichroism; D016503:Drug Delivery Systems; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D008051:Lipid Bilayers / Q000378:metabolism*; D008081:Liposomes; D008565:Membrane Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000378:metabolism*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence; D014461:Ultracentrifugation
chemical_list: D002264:Carboxylic Acids; D057846:Cell-Penetrating Peptides; D008051:Lipid Bilayers; D008081:Liposomes; D008565:Membrane Proteins; D010446:Peptide Fragments