Arginine rich cell-penetrating peptides are short cationic peptides capable of traversing the plasma membranes of eukaryotic cells. While successful intracellular delivery of many biologically active macromolecules has been accomplished using these peptides, their mechanisms of cell entry are still under investigation. Ionic interactions between the highly cationic peptides and the anionic cell membrane and other anionic molecules on the cell surface are believed to be the initial step in the internalization process.

We examined the interactions of TAT peptide with prototypical cell membranes using confocal microscopy and synchrotron small angle x-ray scattering (SAXS) and studied the effect of membrane charge and intrinsic curvature. We find that the TAT peptide induces negative Gaussian (‘saddle-splay’) membrane curvature, which is topologically required for pore formation. TAT peptide drastically remodels vesicles into a porous ‘sponge-like’ bicontinuous manifold. By applying ideas from coordination chemistry, soft condensed matter physics and differential geometry, we propose a geometric mechanism facilitated by both electrostatics and bidentate hydrogen bonding.

We also examined the interactions of other arginine rich cell-penetrating peptides, including Antp and oligoarginine, with model cell membranes, and find that the transduction activity correlates with induction of negative Gaussian curvature. The negative Gaussian membrane curvature is broadly enabling and its induction can lower the free energy barriers for a range of different entry mechanisms, such as direct translocation as well as endocytotic pathways. Furthermore, we show that the TAT peptide interacts strongly with actin cytoskeleton, which enhances membrane deformation and cytoskeleton reorganization necessary for endocytotic processes. We propose a mechanism that explains how a relatively simple molecule, like the TAT peptide, facilitates direct entry and multiple endocytotic mechanisms.