Homeoproteins and Homeoprotein-derived Peptides: Going in and Out

Since the initial evidence that antennapedia homeobox [1] can cross cell membranes and internalize into cells, numerous peptides with similar translocation properties have been described. These peptides are referred to as cell-penetrating peptides (CPPs) or protein- transduction domains (PTDs). Reviews on reported CPP sequences have been recently published [2,3], together with reviews on their mechanisms of internalization [2,4-6]. In this review, we will focus on natural homeoproteins and homeoprotein-derived peptides and describe results that have been obtained among different laboratories to unravel the different pathways by which these molecules reach the cell cytosol and nucleus or transfer from one cell to another. Using homeoproteins as a paradigm, we will also summarize recent evidences of the physiological functions of endogenous protein translocation.