Mechanism of Protein Transport across the Chloroplast Envelope.

Photosynthetic eukaryotes have evolved an elaborate system of nuclear-chloroplast interdependence to ensure the coordinate regulation of photosynthesis and other cellular processes. One of the most dramatic consequences of the evolution of the chloroplast from original prokaryotic endosymbiont to a cellular organelle has been the transfer of the vast majority of genes encoding chloroplast polypeptides to nuclear chromosomes, thereby centralizing the control of many essential developmental and metabolic processes in the nucleus. Although the chloroplast has conserved its own genome and is able to synthesize a number of polypeptides, the vast majority of the chloroplast proteins (approximately 90%) are encoded in the nucleus and are translated on free cytoplasmic polysomes. Following the completion of translation, these polypeptides are imported into the chloroplast, directed to one of six suborganellar subcompartments, and assembled into functional proteins. The central role of protein import in chloroplast biogenesis was recognized when the firstnuclear gene encoding a chloroplast polypeptide was identified and sequenced (Dobberstein et al., 1977). A general overview of the import process during the past two decades is now available from the research of numerous laboratories. More recently, attention in the field has turned to identifying components of the import apparatus in hopes of defining the molecular mechanism of protein import. The results of these investigations have revealed a pathway of import that parallels the targeting and membrane translocation systems of other organelles, especially the mitochondrion. However, investigations of the mechanism of chloroplast protein import have revealed fundamental differences among the structure and functions of the import machinery of chloroplasts and other organelles. These differences provide important insights into the distinct evolutionary origins and mechanistic variations of different protein translocation systems. In this review we will provide a summary of our current