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guadinium ion can form with a phosphate, sulphate or carboxylate group on its
counterion. Mono- (resp. di-) methylation of the guadinium group of a CPP pre-
vents formation of one (resp two) hydrogen bond between the guadinium and its
counterion. For the fluorescinated arginine octamer it entailed a 80% (resp. 95%)
decrease of its uptake into Jurkat cells (treatment : 5 min, 50 mM). In the framework
of the solubilizing counterion mechanism, the strength of the non covalent bonds
between a CPP and its counterions would therefore prove important. Interestingly
this may give a rationale to the reported lower internalization efficiency of CPPs for
which arginine residues were replaced by lysines (Mitchell et al.
2000
).
Other proposed mechanisms (Fig.
1
) are those that maintain, at least partly, the
CPP in a polar environment (aqueous solution or polar layer of the membrane).
Crossing the membrane then requires a transient reorganization of the bilayer such
as the formation of a pore or the encapsulation of CPPs in an inverted micelle.
Both options entail high local curvature of the lipids (on the rim of the pore or in/
around the inverted micelle embedded in the bilayer) and a possible mismatch
between the lipids (“void” on the rim of the inverted micelle). From an energetic
point of view, the saving of the Born energy is balanced to some extent by the cost
of the deformation of the bilayer that can amount to tens of kT (Siegel
1993
; Glaser
et al.
1988
). Within these models, the transition is driven by the interaction of sev-
eral peptides with the surface of the membrane and its consequences on the curva-
ture and stability of the bilayer.
The inverted micelle model (Fig.
1
) has been mainly proposed for penetratin
(Derossi et al.
1996
). In this model, cationic residues of penetratins interact with
negatively charged phospholipids in the plasma membrane and subsequent interac-
tion of Trp in the peptide with the hydrophobic membrane is thought to induce an
invagination in the plasma membrane. The concomitant reorganization of the
neighboring lipids results in formation of an inverted micelle, followed by release
of peptide and cargo upon micelle disruption. This explanation for the penetratin
translocation is supported by
31
P-NMR and differential scanning calorimetry
experiments showing that penetratin favors the lamellar to hexagonal inverse transi-
tion for certain lipid compositions (Berlose et al.
1996
; Alves et al.
2008
).
A pore formation could occur through different paths (Fig.
1
). One is referred to
as the “carpet model”. It consists of the binding of numerous peptides in the polar
region of the membrane. These peptides destabilize the lipid assembly and lead to
disruption of the bilayer and formation of pores. This model was first proposed for
some AMPs (Shai
1999
). But molecular dynamic simulations have suggested that it
applies to TAT and arginine nonamer (Herce and Garcia
2007
; Herce et al.
2009
).
The proposed mechanism was that these CPPs bound strongly to the phosphate and
carbonyl groups of the phospholipids deep in the membrane just above the hydro-
phobic core. This destabilized the membrane and lead to the crossing of few CPPs
immediately followed by the opening of aqueous pores. This later event has been
experimentally confirmed by Herce and collaborators in the case of arginine nona-
mer by electrophysiological measurements on planar bilayers and cells. However the
simulations have been criticized by Yesylevskyy and collaborators who found no
pore formation evidenced by similar simulations (Yesylevskyy et al.
2009
).
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