title: SAP(E) - A cell-penetrating polyproline helix at lipid interfaces.
abstract: Cell-penetrating peptides (CPPs) are short membrane-permeating amino acid sequences that can be used to deliver cargoes, e.g. drugs, into cells. The mechanism for CPP internalization is still subject of ongoing research. An interesting family of CPPs is the sweet arrow peptides - SAP(E) - which are known to adopt a polyproline II helical secondary structure. SAP(E) peptides stand out among CPPs because they carry a net negative charge while most CPPs are positively charged, the latter being conducive to electrostatic interaction with generally negatively charged membranes. For SAP(E)s, an internalization mechanism has been proposed, based on polypeptide aggregation on the cell surface, followed by an endocytic uptake. However, this process has not yet been observed directly - since peptide-membrane interactions are inherently difficult to monitor on a molecular scale. Here, we use sum frequency generation (SFG) vibrational spectroscopy to investigate molecular interactions of SAP(E) with differently charged model membranes, in both mono- and bi-layer configurations. The data suggest that the initial binding mechanism is accompanied by structural changes of the peptide. Also, the peptide-model membrane interaction depends on the charge of the lipid headgroup with phosphocholine being a favorable binding site. Moreover, while direct penetration has also been observed for some CPPs, the spectroscopy reveals that for SAP(E), its interaction with model membranes remains limited to the headgroup region, and insertion into the hydrophobic core of the lipid layer does not occur.
authors.affiliation: Max Planck Institute for Polymer Research, Molecular Spectroscopy Department, 55128 Mainz, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Franz
authors.affiliation: Max Planck Institute for Polymer Research, Molecular Spectroscopy Department, 55128 Mainz, Germany.
authors.forename: Marco
authors.initials: M
authors.lastname: Lelle
authors.affiliation: Max Planck Institute for Polymer Research, Molecular Spectroscopy Department, 55128 Mainz, Germany; Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University, 07743 Jena, Germany.
authors.forename: Kalina
authors.initials: K
authors.lastname: Peneva
authors.affiliation: Max Planck Institute for Polymer Research, Molecular Spectroscopy Department, 55128 Mainz, Germany.
authors.forename: Mischa
authors.initials: M
authors.lastname: Bonn
authors.affiliation: Max Planck Institute for Polymer Research, Molecular Spectroscopy Department, 55128 Mainz, Germany; Department of Chemistry, Aarhus University, Århus C DK-8000, Denmark. Electronic address: weidner@mpip-mainz.mpg.de.
authors.forename: Tobias
authors.initials: T
authors.lastname: Weidner
mesh_terms: D057846:Cell-Penetrating Peptides / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D008956:Models, Chemical; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D055672:Static Electricity
chemical_list: D057846:Cell-Penetrating Peptides; D008051:Lipid Bilayers; D010455:Peptides; C011083:polyproline
keywords: Cell-penetrating peptide; Peptide-membrane interaction; Polyproline; Sum frequency generation