Targeting, import, and processing of nuclear gene-encoded proteins into mitochondria and peroxisomes.

I have herein discussed two gene-enzyme families in maize whose protein products participate to purge toxic oxidants from cells, and are thus of importance to all aerobic organisms. We have demonstrated that plant mitochondria import precursor proteins (i.e., preSOD-3) in a manner analogous to other eukaryotic cells. The "transit peptide" (TP) of preSOD-3 is 31 amino acids long and has similar properties to other reported TPs for mitochondrial and chloroplastic proteins. Import to peroxisomes is uniquely different from that for mitochondria and chloroplasts in that no consensus presequence seems to be involved. Instead, targeting signals seem to be integral parts of peroxisomal proteins. These studies are important from a genetic engineering perspective in that it is not only important to express genes in specific tissues, but also within specific subcellular compartments. Expression of donor genes in appropriate cellular compartments of host cells will be critical for maintaining a stable, functional protein in the plant. Manipulation of metabolic pathways in plants requires that engineered proteins be efficiently and effectively expressed in the correct subcellular compartment and during the appropriate developmental time frame. The use of engineered proteins with altered topogenicity may not only prove useful in studying the role(s) of such proteins in metabolism, they may also serve to enhance our understanding of inter-organellar communication within the cell.