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Engineering Cell-Permeable Proteins through Insertion of Cell-Penetrating Motifs into Surface Loops

Effective delivery of proteins into the cytosol and nucleus of mammalian cells would open the door to a wide range of applications including treatment of many currently intractable diseases. However, despite great efforts from numerous investigators and the development of a variety of innovative methods, effective protein delivery in a clinical setting is yet to be accomplished. Herein we report a potentially general approach to engineering cell-permeable proteins by genetically grafting a short cell-penetrating peptide to an exposed loop region of a protein of interest. The grafted peptide is conformationally constrained by the protein structure, sharing the structural features of cyclic cell-penetrating peptides and exhibiting enhanced proteolytic stability and cellular entry efficiency. Insertion of an amphipathic motif, Arg-Arg-Arg-Arg-Trp-Trp-Trp, into the loop regions of enhanced green fluorescent protein (EGFP), protein-tyrosine phosphatase 1B (PTP1B), and purine nucleoside phosphorylase (PNP) rendered all three proteins cell-permeable and biologically active in cellular assays. When added into growth medium, the engineered PTP1B dose-dependently reduced the phosphotyrosine levels of intracellular proteins, while the modified PNP protected PNP-deficient mouse T lymphocytes (NSU-1) against toxic levels of deoxyguanosine, providing a potential enzyme replacement therapy for a rare genetic disease.