ABSTRACT
Fluorescence .............................................................................................. 129
7.6.1 Determining the Insertion Depth Using Brominated Lipids ........ 130
7.6.2 Evaluating Membrane Interactions of Cationic CPPs from
Characteristic Tryptophan Fluorescence Parameters .................... 131
7.7 Concluding Remarks ................................................................................ 132
References .......................................................................................................... 134
While cell studies repeatedly provide evidence that cell-penetrating peptides
(CPPs) can deliver biochemically functional cargo into cells, the molecular
mechanisms by which the import occurs are still shrouded in mystery. The initial
belief that CPP-cargo constructs managed to enter cells via direct lipid interactions
has somewhat come to nought after the discovery that studies using fixed cells may
have caused artifactual results due to the redistribution of peptide.1,2 The
reevaluation of cell uptake has pointed to the importance of endocytosis in CPP
cell entry, especially when CPPs are conjugated to cargoes.3-6 Still, the concept of
peptide translocation through a lipid membrane remains important because
subsequent studies on live cells have proven that entry can occur when endocytosis
is shut down.7 Furthermore, CPP-cargo constructs that internalize via endocytosis
must indeed be able to escape degradation (hence cross the endosomal membrane)
in order to assert biological effects. CPPs that are reported to use direct membrane
translocation mechanisms are usually rich in arginines or sufficiently hydrophobic
to form pores (e.g., transportan). However, the archetypal CPP penetratin seems to
enter cells entirely by endocytosis.
