[Transduction peptides, the useful face of a new signaling mechanism]

Transduction peptides that cross the plasma membrane of live cells are commonly used for the in vitro and in vivo targeting of hydrophilic drugs into the cell interior. Although this family of peptides has recently increased and will probably continue to do so, the two mainly used peptides are derived from transcription factors. Indeed, TAT is a 12 amino acid long arginine-rich peptide present in the HIV transcription factor, and penetratin - or its variants - corresponds to 16 amino acids that define the highly conserved third helix of the DNA-binding domain (homeodomain) of homeoprotein transcription factors. In this review, we shall recall the different steps that have led to the discovery of transduction peptides and present the most likely hypotheses concerning the mechanisms involved in their internalization. At the risk of being incomplete or, even, biased, we shall concentrate on penetratins and TAT. The reason is that these peptides have been studied for over ten years leading to the edification of robust knowledge regarding their properties. This attitude will not preclude comparisons with other peptides, if necessary. Our goal is to describe the mode of action of these transduction peptides, their range of activity in term of cell types that accept them and cargoes that they can transport, and, also, some of the limitations that one can encounter in their use. Finally, based on the idea that peptide transduction is the technological face of a physiological property of some transcription factors, we shall discuss the putative physiological function of homeoprotein transduction, and, as a consequence, the possibility to use these factors as therapeutic proteins.