The Proteomes of Chloroplasts and other Plastids

The post-genomic era of biology has seen a significant shift in focus, from the genes themselves to the proteins they encode. Recent large-scale studies on the proteomes of chloroplasts and others types of plastid have provided significant new insights into the biogenesis, evolution, and functions of these organelles, and have raised some interesting questions. Many of the proteins that define several important sub-organellar compartments (including the envelope and thylakoid membrane systems, the stroma and plastoglobules) have been identified, and this information has been used to make in silico predictions about the entire complement of proteins in each case. Proteomics has revealed that a relatively large number of proteins inside chloroplasts do not possess canonical targeting information (such proteins lack transit peptides for engagement of the general import machinery), and this has led to the elucidation of novel and unusual pathways of chloroplast protein traffic. For example, it is now clear that some Arabidopsis proteins pass through the endoplasmic reticulum and Golgi en route to the chloroplast, and that these proteins may become glycosylated along the way. Comparative studies have been used to characterise organellar proteome changes in response to various environmental cues or genetic perturbations, whilst other approaches have shed light on the oligomerisation and covalent modification of plastidic proteins.