The interaction of the cell-penetrating peptide penetratin with heparin, heparansulfates and phospholipid vesicles investigated by ESR spectroscopy

An ESR investigation of the interaction of spin-labelled penetratin with heparin, heparansulfates and several phospholipid vesicle formulations is reported. Penetratin is a 16-aa peptide corresponding to the third helix of the Antennapedia homeodomain and belonging to the cell-penetrating peptide family. The present study shows that ESR spectroscopy can provide specific and reliable information about the mechanism of interaction of penetratin with polysaccharides and lipids, at a molecular level. The study showed that: (i) heparin and heparansulfates specifically interact with spin-labelled penetratin and promote peptide aggregation and concentration on their molecular surface; (ii) penetratin does not interact with neutral lipids, whereas it enters negatively charged lipid bilayers; (iii) cholesterol plays a negative effect on the insertion of penetratin into the lipid membrane; (iv) the interaction of penetratin with lipid vesicles is strongly dependent on lipid concentration. In a low lipid regime, penetratin associates with the polar heads of phospholipids and aggregates on the membrane surface; once the lipid concentration attains a threshold, the peptide enters the lipid bilayer. This step is characterized by reduced peptide mobility and partial disaggregation.It has been shown that ESR spectroscopy is a valuable investigation tool in studies related to the still unclear mechanism of the internalization process.