MITOCHONDRIAL PROTEIN IMPORT IN PLANTS - SIGNALS, SORTING TARGETING, PROCESSING AND REGULATION

Mitochondrial biogenesis requires a coordinated expression of both the nuclear and the organellar genomes and specific intracellular protein trafficking, processing and assembly machinery. Most mitochondrial pr oteins are synthesised as precursor proteins containing an N-terminal extension which functions as a targeting signal, which is proteolytica lly cleaved off after import into mitochondria. We review our present knowledge on components and mechanisms involved in the mitochondrial p rotein import process in plants. This encompasses properties of target ing peptides, sorting of precursor proteins between mitochondria and c hloroplasts, signal recognition, mechanism of translocation across the mitochondrial membranes and the role of cytosolic and organellar mole cular chaperones in this process. The mitochondrial protein processing in plants is catalysed by the mitochondrial processing peptidase (MPP ), which in contrast to other sources, is integrated into the bc(1) co mplex of the respiratory chain. This is the most studied component of the plant import machinery characterised to date. What are the biochem ical consequences of the integration of the MPP into an oligomeric pro tein complex and how are several hundred presequences of precursor pro teins with no sequence similarities and no consensus for cleavage, spe cifically cleaved off by MPP? Finally we will address the emerging are a of the control of protein import into mitochondria.