Chloroplast biogenesis: diversity and regulation of the protein import apparatus.

The biogenesis of chloroplasts is dependent on the coordinate expression of genes encoded in both nuclear and plastid genomes. The chloroplast protein import machinery plays key roles in organelle biogenesis by mediating the import and assembly of thousands of nuclear-encoded proteins into the organelle. It is now apparent that multiple levels of control exist to integrate the activities of the protein import apparatus into the network of chloroplast-nuclear communication that is essential to maintain organelle homeostasis. The import apparatus has diversified into small, functionally specialized gene families to coordinate the import of distinct classes of differentially expressed proteins. Protein targeting to chloroplasts also has evolved regulatory mechanisms that respond to cellular developmental and physiological changes, including redox sensing, phosphorylation, and dual targeting. Recent studies also have revealed new components that could represent additional levels of control on the import process.