Cellular Uptake Of Antennapedia Penetratin Peptides Is A Two-Step Process In Which Phase Transfer Precedes A Tryptophan-Dependent Translocation

Several homeodomains and homeodomain-containing proteins enter live cells through
a receptor- and energy-independent mechanism. Translocation through biological
membranes is conferred by the third alpha-helix of the homeodomain, also known as
Penetratin. Biophysical studies demonstrate that entry of Penetratin into cells
requires its binding to surface lipids but that binding and translocation are
differentially affected by modifications of some physico-chemical properties of
the peptide, like helical amphipathicity or net charge. This suggests that the
plasma membrane lipid composition affects the internalization of Penetratin and
that internalization requires both lipid binding and other specific properties.
Using a phase transfer assay, it is shown that negatively charged lipids promote
the transfer of Penetratin from a hydrophilic into a hydrophobic environment,
probably through charge neutralization. Accordingly, transfer into a hydrophobic
milieu can also be obtained in the absence of negatively charged lipids, by the
addition of DNA oligonucleotides. Strikingly, phase transfer by charge
neutralization was also observed with a variant peptide of same charge and
hydrophobicity in which the tryptophan at position 6 was replaced by a
phenylalanine. However, Penetratin, but not its mutant version, is internalized
by live cells. This underscores that charge neutralization and phase transfer
represent only a first step in the internalization process and that further
crossing of a biological membrane necessitates the critical tryptophan residue at
position 6.