Most chloroplast and mitochondrial proteins are imported from the cytosol through distinct routes that ensure specificity in targeting. However, several proteins are targeted to both organelles because they have an ambiguous signal that can be recognized by both import systems (Silva-Filho, 2003). The contribution of the receptors and translocation systems of the organelles to this sort of dual distribution is essentially unknown. However, several studies have analysed the structure and function of such ambiguous targeting signals. Comparison of sequences of known ambiguous signals shows an overall similarity to mitochondria- and chloroplast-specific signals. However, ambiguous signals contain additional leucine and phenylalanine residues, which result in an overall increase in hydrophobicity compared with either mitochondrial-specific or chloroplast-specific signals (Peeters & Small, 2001). The relative significance of particular amino acids in the ambiguous signal of pea glutathione reductase (GR) was examined by mutation analysis. The mutants were studied in vitro using import assays and in vivo by tracing the location of GR-signal-GFP (green fluorescent protein) fusions (Chew et al, 2003). The results suggest that hydrophobic residues are important for targeting both mitochondria and chloroplasts. Single positively charged residues seem to be more important for mitochondrial targeting, but when a basic residue was co-mutated with a hydrophobic one, the mutations also had a significant effect on import into chloroplasts (Chew et al, 2003). Thus, dual targeting by ambiguous signals appears to be achieved by combining distinct targeting instructions in a single N-terminal peptide. Nevertheless, as several amino-acid residues are crucial for targeting to both organelles, this is not a simple superimposition of two separate signals.
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