Cell-Penetrating Peptide
A short peptide (typically 5-30 amino acids) capable of crossing cell membranes and facilitating intracellular delivery of attached cargo. Cell-penetrating peptides are used as drug delivery tools to transport peptide therapeutics, nucleic acids, or other molecules into cells that would otherwise be inaccessible.
Technical Context
CPPs are typically 5-30 amino acid cationic or amphipathic peptides that cross cell membranes through: direct penetration (creating transient pores or inverted micelles in the lipid bilayer), macropinocytosis (stimulating cellular uptake through membrane ruffling), or endocytosis (receptor-mediated or adsorptive internalisation). Classic CPPs: Tat peptide (GRKKRRQRRRPPQ, from HIV Tat protein), penetratin (RQIKIWFQNRRMKWKK, from Antennapedia homeodomain), and polyarginine (R8-R12). CPPs can deliver diverse cargo: peptide therapeutics, proteins, nucleic acids, nanoparticles, and imaging agents. Limitations: poor selectivity (CPPs enter all cell types, not just target cells), endosomal trapping (cargo may be degraded in lysosomes after endocytic uptake), and potential membrane toxicity at high concentrations. Targeted CPPs (conjugated to cell-specific ligands) address the selectivity limitation.