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224 Cell-Penetrating Peptides: Processes and Applications
10.10 Speculations on the Mechanisms of the CPP Translocation ....................240
Acknowledgment ...................................................................................................241
References..............................................................................................................241
10.1 INTRODUCTION
Cell-penetrating peptides, CPPs, are water-soluble but have the ability to translocate
through various cell membranes with high efficiency and low lytic activity. The
transport is nonendocytoic, does not require a chiral receptor, and is non-cell-type
specific. 1-3 When covalently linked to “cargos” (polypeptides, oligonucleotides, particles,
etc.) many times their own molecular mass, CPPs still retain their translocating
ability.
Examples of CPPs are so-called Tat-derived peptides, as well as peptides based
on sequences from certain transcription factors, signal sequences, chimeras between
biological sequences, or completely synthetic (artificial) peptides. The most studied
are those with sequences derived from homeodomains, where “penetratin” peptide
is an archetype among the CPPs. Homeodomain (homeobox) proteins belong to a
class of transcription factors involved in multiple morphological processes. 3 The
sequences of penetratin, transportan, and a few other CPPs are shown in Table 10.1.
The mechanisms behind translocation of the CPPs are still mostly unknown,
although it is clear that the molecular details of peptide–membrane interaction are
fundamentally important for the translocation process. The role of any particular
secondary structure in the process, though, remains disputed.
Background for the present review is the overall question of translocation mechanism
of CPPs; however, with the lack of unambiguous experimental evidence
regarding this question, we will mainly focus on membrane interaction properties
of certain CPPs and analogues. Membrane model systems will be presented and the
various biophysical methods used will be briefly described. We will also make a
short comparison with studies on other classes of bioactive, membrane-interacting
peptides in which more is known about the mechanisms of action.
10.2 SEQUENCES AND GENERAL PROPERTIES OF SELECTED CPPS
The sequences of selected CPPs discussed in this review are shown in Table 10.1,
together with some variant sequences reported to have lost their translocating property.
Penetratin, denoted pAntp, has a sequence corresponding to the 16 residues of
the third α-helix (residues 43–58) from the Antennapedia homeodomain protein of
Drosophila. 1,4-6 This third helix was found to be responsible not only for interaction
with DNA by binding specifically to cognate sites in the genome, but also for
translocation of the entire protein across cell membranes. 4,5 The pAntp peptide alone
retains the membrane translocation properties and has therefore been proposed as a
universal intercellular delivery vector. 1 A homologous peptide, pIsl, derived from
the Islet-1 homeodomain, also behaves as an efficient CPP. 7
Transportan is a chimeric 27-amino acid peptide with a sequence derived from
the 1–12 residues of galanin and linked with a Lys to the sequence of the wasp