Targeted Subcellular Protein Delivery Using Cleavable Cyclic Cell-Penetrating Peptides.

The delivery of entire functional proteins into living cells is a long-sought goal in science. Cyclic cell-penetrating peptides (cCPPs) have proven themselves to be potent delivery vehicles to carry proteins upon conjugation into the cytosol of living cells with immediate bioavailability via a non-endosomal uptake pathway. With this strategy, we pursue the cytosolic delivery of mCherry, a medium-sized fluorescent protein. Afterward, we achieve subcellular delivery of mCherry to different intracellular loci by genetic fusion of targeting peptides to the protein sequence. We show efficient transport into a membrane-bound compartment, the nucleus, as well as targeting of the actin cytoskeleton, marking one of the first ways to label actin fluorescently in genetically unmodified living cells. Furthermore, we demonstrate that only by conjugation of cCPPs via a disulfide bond, is flawless localization to the target area achieved. This finding underlines the importance of using a cCPP-based delivery vehicle that is cleaved inside cells, for the precise intracellular localization of a protein of interest.