The use of membrane-permeable peptides as carrier vectors for the intracellular delivery of various proteins and macromolecules for modifying cellular function is well documented. Arginine-rich peptides, including those derived from human immunodeficiency virus 1 Tat protein, are among the representative classes of these vectors. The internalization mechanism of these vector peptides and their protein conjugates was previously regarded as separate from endocytosis, but more recent reevaluations have concluded that endocytosis is involved in their internalization. In this report, we show that the uptake of octa-arginine (R8) peptide by HeLa cells was significantly suppressed by the macropinocytosis inhibitor ethylisopropylamiloride (EIPA) and the F-actin polymerization inhibitor cytochalasin D, suggesting a role for macropinocytosis in the uptake of the peptide. In agreement with this we observed that treatment of the cells with R8 peptide induced significant rearrangement of the actin cytoskeleton. The internalization efficiency and contribution of macropinocytosis were also observed to have a dependency on the chain length of the oligoarginine peptides. Uptake of penetratin, another representative peptide carrier, was less sensitive to EIPA and penetratin did not have such distinct effects on actin localization. The above observations suggest that penetratin and R8 peptides have distinct internalization mechanisms.

A basic peptide derived from human immunodeficiency virus (HIV)-1 Tat protein (positions 48 -60) has been reported to have the ability to translocate through the cell membranes and accumulate in the nucleus, the characteristics of which are utilized for the delivery of exogenous proteins into cells. Based on the fluorescence microscopic observations of mouse macrophage RAW264.7 cells, we found that various arginine-rich peptides have a translocation activity very similar to Tat-(48 -60). These included such peptides as the D-amino acid-and arginine-substituted Tat-(48 -60), the RNA-binding peptides derived from virus proteins, such as HIV-1 Rev, and flock house virus coat proteins, and the DNA binding segments of leucine zipper proteins, such as cancer-related proteins c-Fos and c-Jun, and the yeast transcription factor GCN4. These segments have no specific primary and secondary structures in common except that they have several arginine residues in the sequences. Moreover, these peptides were able to be internalized even at 4°C. These results strongly suggested the possible existence of a common internalization mechanism ubiquitous to arginine-rich peptides, which is not explained by a typical endocytosis. Using (Arg) n (n ‫؍‬ 4 -16) peptides, we also demonstrated that there would be an optimal number of arginine residues (n ϳ 8) for the efficient translocation.Recently, methods have been developed for the delivery of exogenous proteins into living cells with the help of membranepermeable carrier peptides such as HIV-1 1 Tat-(48 -60) and Antennapedia-(43-58) (1-11). By genetically or chemically hybridizing these carrier peptides, the efficient intracellular delivery of various oligopeptides and proteins was achieved. One of the most amazing examples is the Tat-␤-galactosidase fusion protein (4), which has a molecular mass as high as 120 kDa.Intraperitoneal injection of the protein resulted in delivery of the protein with ␤-galactosidase activity to various tissues in mice, including the brain. The peptide-mediated approaches would allow the incorporation of peptides containing unnatural amino acids or nonpeptide molecules such as fluorescence probes. These methods would become powerful tools not only for therapeutic purposes as an alternative to gene delivery, but also for the understanding of the mechanisms behind fundamental cellular events, such as signal transduction and gene transcription.Besides the potential of Tat-(48 -60) as a protein carrier, the internalization mechanism of the peptide attracted our interest. For example, Tat-(48 -60) (GRKKRRQRRRPPQ) is a highly basic and hydrophilic peptide, which contains 6 arginine and 2 lysine residues in its 13 amino acid residues. However, the peptide was reported to be translocated through the cell membranes in 5 min at a concentration of 0.1 M (2). Internalization of the peptide was not inhibited even at 4°C. The peptide is less toxic to cells than other basic membrane-interacting agents. The above features suggested that the internalization mechanism of Tat-...

Spintronics is a research field that aims to understand and control spins on the nanoscale and should enable next-generation data storage and manipulation. One technological and scientific key challenge is to stabilize small spin textures and to move them efficiently with high velocities. For a long time, research focused on ferromagnetic materials, but ferromagnets show fundamental limits for speed and size. Here, we circumvent these limits using compensated ferrimagnets. Using ferrimagnetic Pt/GdCo/TaO films with a sizeable Dzyaloshinskii-Moriya interaction, we realize a current-driven domain wall motion with a speed of 1.3 km s near the angular momentum compensation temperature (T) and room-temperature-stable skyrmions with minimum diameters close to 10 nm near the magnetic compensation temperature (T). Both the size and dynamics of the ferrimagnet are in excellent agreement with a simplified effective ferromagnet theory. Our work shows that high-speed, high-density spintronics devices based on current-driven spin textures can be realized using materials in which T and T are close together.