Despite continuing advances in the development of macromolecules, including peptides, proteins, and oligonucleotides, for therapeutic purposes, the successful application of these hydrophilic molecules has so far been hampered by their inability to efficiently traverse the plasma membrane. The discovery of a class of peptides (cell-penetrating peptides, CPPs) with the ability to mediate the non-invasive and efficient import of a whole host of cargoes, both in vitro and in vivo, has provided a new means by which the problem associated with cellular delivery can be circumvented.
A complete understanding of the translocation mechanism(s) of CPPs has so far proven elusive. Initial studies indicated an ATP-independent, non-endocytotic mechanism, dependent on direct peptide-membrane interactions, making it an enticing challenge from a biophysical point of view. To gain an insight into this mechanism, we identified three new CPP sequences, one corresponding to the third helix of the Islet-1 homeodomain, the other two corresponding to the unprocessed N-termini of the mouse and bovine PrPs, denoted mPrPp and bPrPp, respectively. We then investigated the membrane interactions of these peptides, comparing them to two well-characterized CPPs, the Antennapedia homeodomain-derived pAntp, and the chimeric transportan, in a variety of model membrane systems, using several spectroscopic techniques.
Both pAntp and transportan were found to reside in the headgroup region of the bilayer, oriented along the surface (perpendicular to the bilayer normal). However, differences were observed between the peptides – with the homeodomain-derived peptides, pAntp and pIsl, on the one hand, and transportan and the prion-derived peptides on the other – in terms of their membrane interactions, in particular their membrane perturbation effects. These differences suggest that the peptides belong to two classes of CPPs that translocate through different mechanisms. This hypothesis was given further substance by the recent re-evaluation of the translocation mechanism, which led to the conclusion that many peptides, including pAntp, translocate by an energy-dependent, endocytotic mechanism.
Interesting structural behaviour was observed for the homeodomain-derived CPPs, where they readily underwent an α → β structural conversion, depending on experimental conditions. High peptide concentration and/or high negative membrane surface charge was found to promote β-sheet structure. This structural conversion characteristic was shared by the prion-derived peptides, which along with their CPP property and their membrane perturbation effects, may have implications for the infectivity and toxicity associated with prion diseases.
Stockholm: Institutionen för biokemi och biofysik , 2004. , 68 p.
2004-10-08, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 14:00