Protein transduction domains and their utility in gene therapy

Protein transduction domains (PTDs, sometimes termed cell permeable proteins (CPP) or membrane translocating sequences (MTS)) are small peptides that are able to ferry much larger molecules into cells independent of classical endocytosis. This property makes PTDs ideal tools to transfer proteins and other molecules into living cells for research purposes. The mechanism by which this internalization takes place is poorly understood. It is evident, however, that many known PTDs bind to the same surface molecules (Heparan Sulphate Proteoglycans, HSPG) before internalization, and that internalization is dependent on these molecules. PTDs, although at this moment mainly used for the chemical or bacterial production of membrane permeable proteins can become powerful tools for gene therapy. By incorporating a PTD in the therapeutic gene product, the protein produced in the transfected cell might be enabled to spread to non-transfected cells, thereby creating an increased therapeutic effect. In this review, we give an overview of PTDs that may be useful for gene therapy applications, and discuss some of the problems that can be expected when incorporating PTDs in gene therapy approaches.