Cell Penetrating Peptides
One of the major problems in cell biology is the inefficient transfer of peptides or proteins across cellular membranes. In order to deliver a peptide or a protein from the outside to the inside of intact cells, a carrier is generally required. Cell Penetrating Peptides (CPPs, also known as Cell Permeable Peptides or as Protein Transduction Domains, PTDs), are carriers with small peptide domains (generally less than 40 amino acids) that can easily cross cell membranes. Multiple cell permeable peptides have been identified that facilitate cellular uptake of various molecular cargo, ranging from nanosize particles to small chemical molecules. Cell penetrating sequences can be used as extensions to peptide sequences thereby making them more permeable to cell membranes, or cell penetrating peptides can be attached to other cargo molecules to enhance their cellular uptake.
The process of introducing drugs into cells has always proved a major challenge for scientists. However, cell-penetrating peptides (CPPs) have the ability to enter the plasma membrane of a cell independent of a membrane receptor. They are usually small peptides 10–30 residues in length with positively charged amino acid sequences.
CPPs are capable of delivering biologically active cargo to the cell interior. The desired therapeutic cargo could be attached to a CPP and then delivered to an intracellular target, thereby overcoming the entry restrictions set by the plasma membrane.
CPPs have gained a widespread popularity as very promising non-viral transmembrane delivery vectors. Although they have been used successfully for carrying different cargoes of varying size and nature (plasmid DNA, peptides, proteins, nanoparticles, and quantum dots, etc.), the most rapid progress has been made with the delivery of oligonucleotides (ONs).
How to Make Your Peptide Cell Permeable?
Cys(Npys)-(Arg)9: C(Npys)RRRRRRRRR - NH2
Your peptide will be conjugated to the Cys(Npys)-(Arg)9. The peptide is synthesized with an activated cysteine residue C(Npys). The 3-nitro-2-pyridinesulfenyl (Npys) is a novel selective protecting group which can be activated for peptide bond formation.
The S(Npys) group of this cysteine-containing R9 peptide can selectively react with the free thiol group of your cysteine-containing peptide. They will form an unsymmetrical disulfide bond. The final conjugate will effectively deliver the target peptide into cells and then release the therapeutics such as siRNA or your target peptide in the reducing environment. This conjugation will make your peptide cell permeable.