The third helix of the Antennapedia homeodomain translocates through biological membranes.

TLDR

The Antennapedia homeodomain can cross biological membranes independently of energy, a property lost by mutating its C‑terminal region. The authors examined membrane uptake of synthetic peptides derived from the third helix of the homeodomain. Both 16‑ and 20‑residue peptides from the third helix translocate across membranes at 4 °C without energy, are recovered intact inside cells, and are not lysosomal, whereas a 15‑residue peptide does not internalize; replacing two tryptophans with phenylalanines markedly reduces translocation, indicating that internalization depends on specific residues rather than general hydrophobicity.

Abstract

The 60-amino acid long homeodomain of Antennapedia crosses biological membranes by an energy-independent mechanism, a phenomenon abolished by directed mutagenesis within the polypeptide C-terminal region. This finding led us to study the internalization of several chemically synthesized peptides derived from the third helix of the homeodomain. We report here that a polypeptide of 16 amino acids in length corresponding to the third helix of the homeodomain deleted of its N-terminal glutamate is still capable of translocating through the membrane. A longer peptide of 20 amino acids also translocates, whereas shorter peptides (15 amino acids) are not internalized by the cells. As is also the case for the entire homeodomain, the 20- and 16-amino acid long peptides are internalized at 4 degrees C, suggesting an energy-independent mechanism of translocation not involving classical endocytosis. The two translocated peptides can be recovered, intact, within the cells, strongly suggesting that they are not targeted to the lysosomal compartment. Finally, substitution of two tryptophans by two phenylalanines strongly diminishes translocation, raising the possibility that the internalization of the third helix is not solely based on its general hydrophobicity.