An unusual cell penetrating peptide identified using a plasmid display-based functional selection platform.

Cell penetrating peptides (CPPs) have tremendous potential for use in gene and drug delivery applications. The selection of new CPPs with desired capabilities from randomized peptide libraries is challenging, since the CPP phenotype is a complex selection target. Here we report the discovery of an unusual new CPP from a randomized peptide library using a functional selection system based on plasmid display (PD). After four rounds of screening of a 14-mer peptide library over PC12 cells, several peptides were identified and tested for their ability to deliver the green fluorescent protein (GFP). One peptide (SG3) exhibited a cell penetrating phenotype; however, unlike other well-known CPPs such as TAT or Penetratin, the newly identified peptide was not highly cationic. The PD protocol necessitated the addition of a cationic lipid (Lipofectamine2000), and in the presence of this compound, the SG3 peptide significantly outperformed the well-known TAT CPP in the delivery of GFP to PC12 cells and primary astrocytes. When the SG3 peptide was fused to the pro-apoptotic BH3 peptide from the Bak protein, significant cell death was induced in cultured primary astrocytes, indicating relevant, intracellular delivery of a functional cargo. The PD platform is a useful method for identifying functional new CPPs from randomized libraries with unique delivery capabilities.