Targeting Signals for Protein Import into Mitochondria and Other Subcellular Organelles

The current picture of the mitochondrial targeting peptide displayed in this chapter against a background of what is known about two other kinds of related sorting signals, namely secretory signal peptides and chloroplast transit peptides. Picture of the mitochondria1 matrix-targeting signal is based on the concept of the positively charged amphiphilic α-helix. This model is supported by a substantial body of evidence: statistical, biophysical, and genetical. The amphililic helix is required not only for correct targeting, but apparently also for correct cleavage, where it might serve to roughly position the cleavage enzyme in relation to the rather loosely defined cleavage site pattern. It is still unclear how the targeting peptide is recognized. Biophysical studies have demonstrated that targeting peptides have strong lipid-interacting properties, suggesting that direct protein-lipid interaction may be important at some stage. Both cytoplasmic and mitochondrial receptors that bind targeting peptides have been identified, and it is recently suggested that mitochondrial hsp70 also binds to targeting peptides as they emerge in to the matrix space.