Cell penetrating peptides (CPPs) are peptides displaying the ability to cross cell membranes and transport cargo molecules inside cells. Several uptake mechanisms (endocytic or direct translocation through the membrane) are being considered, but the interaction between the CPP and the cell membrane is certainly a preliminary key point to the entry of the peptide into the cell. In this study, we used three basic peptides: RL9 (RRLLRRLRR-NH(2)), RW9 (RRWWRRWRR-NH(2)) and R9 (RRRRRRRRR-NH(2)). While RW9 and R9 were internalised into wild type Chinese Hamster Ovary cells (CHO) and glycosaminoglycan-deficient CHO cells, at 4°C and 37°C, RL9 was not internalised into CHO cells. To better understand the differences between RW9, R9 and RL9 in terms of uptake, we studied the interaction of these peptides with model lipid membranes. The effect of the three peptides on the thermotropic phase behaviour of a zwitterionic lipid (DMPC) and an anionic lipid (DMPG) was investigated with differential scanning calorimetry (DSC). The presence of negative charges on the lipid headgroups appeared to be essential to trigger the peptide/lipid interaction. RW9 and R9 disturbed the main phase transition of DMPG, whereas RL9 did not induce significant effects. Isothermal titration calorimetry (ITC) allowed us to study the binding of these peptides to large unilamellar vesicles (LUVs). RW9 and R9 proved to have about ten fold more affinity for DSPG LUVs than RL9. With circular dichroism (CD) and NMR spectroscopy, the secondary structure of RL9, RW9 and R9 in aqueous buffer or lipid/detergent conditions was investigated. Additionally, we tested the antimicrobial activity of these peptides against Escherichia coli and Staphylococcus aureus, as CPPs and antimicrobial peptides are known to share several common characteristics. Only RW9 was found to be mildly bacteriostatic against E. coli. These studies helped us to get a better understanding as to why R9 and RW9 are able to cross the cell membrane while RL9 remains bound to the surface without entering the cell.

How the membrane environment informs when and where actin is polymerized in clathrin-mediated endocytosis is unclear. Daste et al. show that high membrane curvature stimulates PI(3,4)P2 dephosphorylation by INPP4A and that PI(3)P recruits SNX9 in conjunction with both PI(4,5)P2 and high membrane curvature. Furthermore, they find that Lowe syndrome mimics this membrane microenvironment with the aberrant formation of a PI(4,5)P2/PI(3)P intermediate, giving rise to actin comets.