CyLoP-1: a novel cysteine-rich cell-penetrating peptide for cytosolic delivery of cargoes.

Cell-penetrating peptides (CPPs) may have impli-cations in biomedical sciences by improving the delivery of a wide variety of drugs through the membrane barrier. CPPs are generally taken up by endocytotic pathways, and vesicular encapsulation is a limiting factor in the area of intracellular targeting. A novel, cationic cysteine-rich CPP, CyLoP-1, has been developed exhibiting distinguished diffused cytosolic distribution along with endosomal uptake at low micromolar concentrations. Comparative uptake analysis with known CPPs showed CyLoP-1 as a promising delivery vector to access the cytosol in a variety of cell types. In addition to the positively charged residues, the presence of cysteines and tryptophans proved to be essential to maintain its functionality. Also, the oxidation status of the cysteines played an important role for the uptake efficiency of CyLoP-1, with the disulfide-containing form being more effective. The distinct feature of CyLoP-1 to enter the cytosol was further explored by the covalent attachment of cargoes of different nature and sizes. In particular, induction of caspase-3 activity (indicating apoptosis) by a CyLoP-1-SmacN7 conjugate proved successful delivery of the pro-apoptotic cargo to its site of action in the cytosol. Efficient intracellular delivery into the entire cytosol already at low micromolar concentrations makes CyLoP-1 a promising candidate for cytosolic delivery of cargoes of small sizes. Thus, this peptide might prove to be useful for efficient transmembrane delivery of agents directed to cytosolic targets.