An Introduction to Cell Penetrating Peptides:
Due to the hydrophilic nature of the majority of proteins and peptides, they are largely impermeable to cell membranes. Cell Penetrating Peptides (CPP’s) therefore possess a pivotal role in terms of their ability to successfully facilitate cellular uptake of molecular cargos across the cell or plasma membrane. They typically possess a high relative abundance of the positively charged amino acids Arginine and Lysine or possess an alternating pattern of polar and non-polar residues. Various mechanisms by which CPP’s traverse cell membranes have been postulated including endocytosis and direct translocation but their exact mode of action remains unclear and is, to this day, widely investigated.
Focus on Arg9
Arg9 is a synthetic homopolyarginine nona-peptide (R9) cell penetrating peptide (CPP). The arginine amino acids play an important role in efficient cellular uptake. The guanidinium group is a critical structural determinant for tight and rapid interactions with cell membrane. The cationic guanidinium group can form electrostatic interactions with anionic cell membranes components such as phospholipids and sulphated proteoglycans. This interaction can trigger the activation of specific intracellular signalling and cell internalization via various pathways.
The hydrophilic arginine amino acids destabilize and disrupt the phospholipid bilayer and nucleate transient pores for passive diffusion across the membrane. In addition, the extended 3 carbon linear chain of arginine facilitates insertion and cell permeation. In the presence of different types of phospholipids Arg9 exhibits structural polymorphism with no interfacial properties or specific secondary structure, which implicates endocytosis as another mechanism of cellular uptake. However, the specific endocytosis pathways utilized is dependent on multiple factors such as peptide concentration, environmental conditions and membrane composition. Cell viability and membrane integrity studies of Arg9 peptide intracellular delivery were shown to have low cytotoxicity, no leakage or liposome bilayer entrapment.
Oligo-arginine peptides have been used to deliver a variety of functionally active cargos such as peptides, small interfering RNA, oligonucleotides, plasmid DNA and liposomes into mammalian cells and plant cells with high translocation efficiency. Non-covalent and covalent strategies have been used to assess cargo delivery. Click Arg9 can be used to form chemical linkages with the cargo and act as a carrier peptide. Understanding the translocation mechanisms of Arg9 can provide an insight in the development of efficient molecular delivery vehicles of drugs and other agents into the cytosol and nucleus of cells.
Various Arg9 peptides are available via the new Discovery Peptides Catalogue:
Click on the corresponding catalogue number to be taken to the Discovery webpage:
Standard unmodified Peptide: Arg9 – Catalogue Number: crb1000182
5-carboxyfluoroscein labelled Arg9: [5-Fam]-Arg9 – Catalogue Number: crb1100289
4-pentynyl “Click” functionalised Arg9: Click Arg9 – Catalogue Number: crb1000111