X-pep, a novel cell-penetrating peptide motif derived from the hepatitis B virus

• The N-terminus of the X-protein contains a novel cell-penetrating peptide (X-pep).
• X-pep does not resemble known cell-penetrating peptides.
• X-pep penetrates liver cancer cells, and displays cell-selective uptake.
• The active core of X-pep is represented by the pentapeptide MAARL.
• Cell uptake of X-pep is chiral selective indicating it is receptor-mediated.

Cell-penetrating peptides (CPPs) are able to penetrate the plasma membrane and gain access to the interior of any replicating or non-replicating cell, and are being considered as drug delivery agents. Here we describe the serendipitous discovery of a novel CPP motif (MAARLCCQ), designated X-pep, located at the extreme N-terminus of the X-protein of the hepatitis B virus. X-pep, and a C-terminally truncated form of the peptide (MAARL), readily penetrated HepG2 cells. Further truncation by removal of the terminal leucine residue impaired the cell-penetrating activity of peptide, indicating that MAARL is the active core of the peptide. X-pep is located adjacent to another CPP, namely Xentry, and like Xentry is unable to penetrate unactivated resting lymphocytes suggesting selective cell uptake. A d-isomeric form of the MAARL peptide was not cell-permeable, indicating that the cell-penetrating function of the peptide involves stereoselective interaction with a chiral receptor. The discovery of X-pep, which bears no resemblance to known CPPs, allows studies to be undertaken to determine additional characteristics of this novel CPP.