Current views on chloroplast protein import and hypotheses on the origin of the transport mechanism

Most chloroplastic proteins are synthesized as precursors in the cytosol prior to their transport into chloroplasts. These precursors are generally synthesized in a form that is larger than the mature form found inside chloroplasts. The extra amino acids, called transit peptides, are present at the amino terminus. The transit peptide is necessary and sufficient to recognize the chloroplast and induce movement of the attached protein across the envelope membranes. In this review, we discuss the primary and secondary structure of transit peptides, describe what is known about the import process, and present some hypotheses on the evolutionary origin of the import mechanism.Abbreviations DHFR dihydrofolate reductase - EPSP synthase 5-enolpyrovylshikimate-3-phosphate synthase; hsp heat-shock protein - LHCP II light-harvesting chlorophylla/b binding protein - OEE 16, 23, and 33 the 16-, 23-, and 33-kDa proteins of the oxygen-evolving complex - pr precursor - rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - SS rubisco small subunit