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HA2-TAT is a peptide-based delivery agent that combines the pH-sensitive HA2 fusion peptide from Influenza and the cell-penetrating peptide TAT from HIV. This chimeric peptide is engineered to induce the cellular uptake of macromolecules into endosomes via the TAT moiety and to respond to the acidifying lumen of endosomes to cause membrane leakage and release of macromolecules into cells via the HA2 moiety. The question of how HA2 and TAT affect the properties of one another remains, however, unanswered and the behavior of the peptide inside endosomes is mostly uncharacterized. To address these issues, the binding and membrane leakage activity of a glutamic acid-enriched analogue E5-TAT was assessed with red blood cells and giant unilamellar vesicles as membrane models for endosomes. Hemolysis and microscopy assays reveal that E5-TAT binds to membranes in a pH-dependent manner and causes membrane leakage by inducing the formation of pores through which macromolecules can escape. The TAT moiety contributes to this activity by causing a shift in the pH response of E5 and by binding to negatively charged phospholipids. On the other hand, TAT binding to glycosaminoglycans reduces the lytic activity of E5-TAT. Addition of TAT to the C-terminus of E5 can therefore either increase or inhibit the activity of E5 depending on the cellular components present at the membrane. Taken together, these results suggest a model for the endosomolytic activity of the peptide and provide the basis for the molecular design of future delivery agents.Cell-penetrating peptides (CPPs) provide general and useful tools to deliver macromolecular cargos into live cells. A prototypical CPP is the TAT peptide derived from HIV TAT.(1,2) TAT has been used to deliver peptides, proteins, DNA, liposomes and nanoparticles. (3,4) Multiple mechanisms are involved in cellular penetration mediated by TAT. TAT has been shown to directly translocate across the lipid bilayer of the plasma membrane when it is both labeled with a small molecule and present above a threshold concentration.(5) On the other hand, TAT is internalized by endocytosis at low concentration or when conjugated to large hydrophilic molecules.(6) Multiple endocytic mechanisms have been shown to be involved in the uptake of TAT-conjugates. TAT-conjugates can for instance enter cells by clathrin or caveolin-dependent endocytosis. In addition, TAT and other CPPs have also been shown to induce macropinocytosis, an effect that leads to the increase in uptake of fluid-phase endocytosis markers.(6-8) Induction of macropinocytosis appears to be mediated by the interaction of the positively charged peptide with membrane-associated heparan sulfate proteoglycans.(9) However, recent evidence suggests that HSPG do not act as receptors but † This work was supported by Award Number R01GM087227 from the National Institute of General Medical Sciences. G.J. was partially supported by the NIH molecular biophysics training grant (T32GM065088) * To whom correspondence should be addressed. Phone: 979 ...

We describe the synthesis and cellular delivery properties of multivalent and branched delivery systems consisting of cell-penetrating peptides assembled onto a peptide scaffold using Native Chemical Ligation. A trimeric delivery system presenting three copies of the prototypical cellpenetrating peptide TAT shows an endosomolytic activity much higher than its monomeric and dimeric counterparts. This novel reagent promotes the endosomal release of macromolecules internalized into cells by endocytosis and, as a result, it can be used to achieve cytosolic delivery of bioactive but cell-impermeable macromolecules in either cis (covalent conjugation) or trans (simple co-incubation).Cell-penetrating peptides (CPPs) can be conjugated to cell-impermeable macromolecules and carry their cargo into mammalian cells.(1-3) A prototypical CPP is the cationic TAT peptide (GRKKRRQRRR). TAT-macromolecule conjugates interact with cell-surface proteoglycans and are internalized into cells by TAT-induced macropinocytosis and other endocytosis mechanisms. TAT and its cargo then enter the cytosolic space of the cell by escaping from endosomes.(4) This endosomolytic activity is however only modest and a large fraction of macromolecules remain trapped inside endocytic organelles and unable to reach their intracellular targets.(4,5) In order to increase delivery of cargo to the cytoplasm, the challenge is to enhance the endosomolytic activity of TAT. This is particularly difficult given that the exact mechanisms involved in this critical step of the delivery process are unclear. A potential solution to this problem however is to create multivalent delivery agents displaying multiples copies of the peptide.(6) This is based on the principle that multivalency results in an increase of the CPP's local concentration at the sites where the peptide interacts with cellular components. It has been clearly established that such an approach can increase the endocytic uptake of cationic peptides, and that branched molecules can function and multitask in ways their linear homologues cannot. (7-9) Whether multivalency enhances the endosomolytic activity of CPPs remains however uncertain. In this report, we hypothesize that branched TAT derivatives would also have an increased endosomolytic activity as compared to TAT and that these reagents could induce the release of macromolecules trapped inside endocytic organelles into the cytosolic space of cells. To test this hypothesis, we report a simple and modular synthetic route to generate branched pellois@tamu.edu,. # these two authors have contributed equally to this work. We show that dimerization of TAT does not improved its delivery properties while species with more than 3 branches lead to unproductive membrane binding. In contrast, a trimeric TAT translocates into live cells with much higher efficiency than its monomeric counterpart. Importantly the endosomolytic activity of the trimeric peptide is sufficiently increased that it is able to deliver a bioactive peptide in trans. Together, the...