Targeting Signals and Import Machinery of Plastids and Plant Mitochondria

: Genetic information in plants is localized in three compartments: the nucleus, pastids and mitochondria. Despite both plastids and mitochondria containing their own DNA, most of the proteins of these organelles are nuclear encoded. The biogenesis of a functional organelle requires coordinate expression of both nuclear and organellar genomes and specific intracellular protein trafficking, processing and assembly machinery. Most nuclear-encoded organellar proteins are synthesized in the cytosol as precursor proteins with N-terminal signal peptides, required for sorting and targeting the protein to the correct organelle. Both plastid and plant mitochondrial signal peptides show a remarkable similarity in amino acid composition. They are rich in hydrophobic, hydroxylated and positively charged amino acid residues and deficient in acidic amino acids. However, despite great similarities of signal peptides, the plastid and mitochondrial protein targeting is specific. Yet, there is a group of proteins that are dually targeted to both organelles. Molecular chaperones and other cytosolic factors are involved in the import process: maintaining precursors in an import-competent conformation, guiding them to the organelle and assisting in transport through the membrane. Precursor proteins are transported into plastids through the translocons of the outer and the inner envelope membrane, TOC and TIC complexes, and into mitochondria through the translocases of the outer and the inner mitochondrial membrane, TOM and TIM complexes. Mitochondrial molecular chaperone Hsp70 functions as a molecular motor pulling precursor proteins into the mitochondria. The signal peptides are proteolytically cleaved off inside the organelles by highly specific organellar processing peptidases, the mitochondrial processing peptidases, that in plant mitochondria are an integral part of the cytochrome bc1 complex of the respiratory chain, and the stromal processing peptidase, SPP, in plastids. The mature proteins, after intraorganellar sorting, are assembled by molecular chaperones into functional oligomeric protein complexes, whereas the cleaved targeting peptides, potentially destructive for biological membranes, are degraded inside the organelles by a newly identified signal peptide degrading zinc metalloprotease, both in plastids and in mitochondria.