Rangel, RobertoGuzman-Rojas, Lilianale Roux, Lucia G.Staquicini, Fernanda I.Hosoya, HitomiBarbu, E. MagdaOzawa, Michael G.Nie, JingLangley, Robert R.Sage, E. HeleneKoivunen, ErkkiGelovani, Juri G.Lobb, Roy R.Pasqualini, RenataArap, WadihSidman, RichardDunner, Kenneth, Jr.2013-03-012012Rangel, Roberto, Liliana Guzman-Rojas, Lucia G. le Roux, Fernanda I. Staquicini, Hitomi Hosoya, E. Magda Barbu, Michael G. Ozawa, et al. 2012. Combinatorial targeting and discovery of ligand-receptors in organelles of mammalian cells. Nature Communications 3:788.2041-1723http://nrs.harvard.edu/urn-3:HUL.InstRepos:10357579Phage display screening allows the study of functional protein–protein interactions at the cell surface, but investigating intracellular organelles remains a challenge. Here we introduce internalizing-phage libraries to identify clones that enter mammalian cells through a receptor-independent mechanism and target-specific organelles as a tool to select ligand peptides and identify their intracellular receptors. We demonstrate that penetratin, an antennapedia-derived peptide, can be displayed on the phage envelope and mediate receptor-independent uptake of internalizing phage into cells. We also show that an internalizing-phage construct displaying an established mitochondria-specific localization signal targets mitochondria, and that an internalizing-phage random peptide library selects for peptide motifs that localize to different intracellular compartments. As a proof-of-concept, we demonstrate that one such peptide, if chemically fused to penetratin, is internalized receptor-independently, localizes to mitochondria, and promotes cell death. This combinatorial platform technology has potential applications in cell biology and drug development.en-USJournal Article2013-03-0110.1038/ncomms1773