Targeting of Foreign Proteins to the Chloroplast

The chloroplast is the product of two genomes with the majority of its polypeptide complement encoded by nuclear DNA and synthesized on cytosolic ribosomes. Proteins destined for the chloroplast must therefore contain targeting information to allow their correct localization. It has been clear now for a number of years that nuclear-encoded chloroplast proteins are synthesized as precursors with a cleavable N-terminal signal sequence (1 ). This N-terminal extension, termed the transit sequence, is removed during or just after translocation into the organelle by a soluble peptidase resident in the chloroplast stroma (2 ). The transit sequence has been shown to be necessary and sufficient to direct proteins across the chloroplast envelope and into the stroma; the transit sequence of the small subunit of Rubisco (abbreviated SSU) has been shown to direct import into chloroplasts of cytosolic, mitochondrial and bacterial proteins when placed at their N-terminus (3 ). Conversely, SSU devoid of its transit sequence cannot be imported into isolated chloroplasts (4 ).