Basic peptides derived from the HIV-1 1 Tat protein (Tat-(48 -60)) and Drosophila Antennapedia protein (Antp-(43-58)) have been reported to have the ability to translocate through the cell membranes and to carry exogenous molecules into the cytoplasm and nucleus (1-13). A 119-kDa protein, -galactosidase, genetically fused with the former peptide segment, was successfully carried into various tissues in mice including the brain via intraperitoneal injection (6). The 5-bromo-4-chloro-3-indolyl -D-galactopyranoside (X-gal) staining of the tissues indicated that the fusion protein was delivered in its active form. Oligo-DNAs and metal chelates were also brought into cells using the Tat-derived peptide (4, 7). Such a method to deliver bioactive molecules into cells using membrane-permeable peptides has a great potential for therapeutic fields.We have recently demonstrated that not only Tat-(48 -60) and Antp-(43-58) but also various arginine-rich RNA-or DNAbinding peptides such as HIV-1 Rev-(34 -50) and flock house virus (FHV) coat-(35-49) were membrane-permeable and have the ability to bring exogenous protein into cells (14). Even octaarginine (Arg 8 ) gave similar results based on the fluorescence microscopic observation of the fluorescein-labeled peptides (14, 15). These peptides seem to have other similarities in translocation, namely facile internalization within 5 min, little uptake inhibition at 4°C, and localization in the nucleus and cytosol. The above results suggested the possible existence of a ubiquitous mechanism for the internalization of the argininerich peptides. Since there were no sequence similarities among these peptides except that they had several arginine residues, arginine seemed to be the key amino acid for membrane permeability.Despite the great potential of the arginine-rich peptides as carriers of proteins, nucleic acids, and other bioactive compounds, little is known about the mechanism of their internalization. Involvement of the cell surface heparan sulfate (HS) and low density lipoprotein receptor-related protein (LRP) was suggested in the translocation of the full-length Tat protein (16,17). The addition of HS and the inhibitor of LRP to the culture medium produced a significant decrease in the cellular uptake of the protein. However, the uptake of the full-length Tat protein suffered a certain decrease at 4°C (16), and some energydependent endocytosis pathway seemed to play a significant role in the internalization of the Tat protein. These results suggested that the mechanisms of internalization of the Tat-(48 -60) peptide and the full-length Tat protein may not be completely parallel. Actually, the importance of the "core" domain (Tat-(37-48)) of the Tat protein has been claimed for the LRP-dependent internalization pathway (16).We have pointed out that many arginine-rich peptides showed very similar characteristics in translocation with HIV-1 Tat-(48 -60) (14). In addition to the translocation mechanisms of these arginine-rich peptides, it is unclear whether these peptides share...