Journal of Bioenergetics and Biomembranes, Vol. 22, No. 6, 1990 MINI-REVIEW Current Views on Chloroplast Protein Import and Hypotheses on the Origin of the Transport Mechanism ~ E. Kathleen Archer 2 and Kenneth Keegstra 3 Received March 21, 1990 Abstract 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 pep- tides, describe what is known about the import process, and present some hypotheses on the evolutionary origin of the import mechanism. Key Words: Chloroplast; import; protein; evolutionary origin; binding; translocation. Introduction 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 ~Abbreviations: DHFR, dihydrofolate reductase; EPSP synthase, 5-enolpyrovylshikimate-3- phosphate synthase; hsp heat-shock protein; LHCP II, light-harvesting chlorophyll a/b binding protein; OEE 16, 23, and 33, the 16-, 23-, and 33-kDa proteins of the oxygen-evolving complex; pr, precursor; rubisco, ribulose-l,5-bisphosphate carboxylase/oxygenase; SS, rubisco small subunit. 2Department of Biology, Trinity College, Hartford, Connecticut 06106. 3Department of Botany, Birge Hall, 430 Lincoln Drive, University of Wisconsin, Madison, Wisconsin 53703. 789 0145-479X/90/1200-0789506.00/0 © 1990 Plenum Publishing Corporation