Efficient delivery of cell impermeable phosphopeptides by a cyclic peptide amphiphile containing tryptophan and arginine

Phosphopeptides are valuable reagent probes for studying protein-protein and protein-ligand interactions. The cellular delivery of phosphopeptides is challenging because of the presence of the negatively charged phosphate group. The cellular uptake of a number of fluorescent-labeled phosphopeptides, including F′-GpYLPQTV, F′-NEpYTARQ, F′-AEEEIYGEFEAKKKK, F′-PEpYLGLD, F′-pYVNVQN-NH₂, and F′-GpYEEI (F′ = fluorescein), was evaluated in the presence or absence of a [WR]₄, a cyclic peptide containing alternative arginine (R) and tryptophan (W) residues, in human leukemia cells (CCRF-CEM) after 2 h incubation using flow cytometry. [WR]₄ improved significantly the cellular uptake of all phosphopeptides. PEpYLGLD is a sequence that mimics the pTyr1246 of ErbB2 that is responsible for binding to the Chk SH2 domain. The cellular uptake of F′-PEpYLGLD was enhanced dramatically by 27-fold in the presence of [WR]₄ and was found to be time-dependent. Confocal microscopy of a mixture of F′-PEpYLGLD and [WR]₄ in live cells exhibited intracellular localization and significantly higher cellular uptake compared to that of F′-PEpYLGLD alone. Transmission electron microscopy (TEM) and isothermal calorimetry (ITC) were used to study the interaction of PEpYLGLD and [WR]₄. TEM results showed that the mixture of PEpYLGLD and [WR] ₄ formed noncircular nanosized structures with width and height of 125 and 60 nm, respectively. ITC binding studies confirmed the interaction between [WR]₄ and PEpYLGLD. The binding isotherm curves, derived from sequential binding models, showed an exothermic interaction driven by entropy. These studies suggest that amphiphilic peptide [WR]₄ can be used as a cellular delivery tool of cell-impermeable negatively charged phosphopeptides.