ABSTRACT
Mitochondria are ubiquitous organelles housing enzyme complexes that make a decisive contribution to energy production and conservation in all eukaryotic cells using oxygen. Key enzymes of major metabolic routes also reside inside mitochondria. Only few mitochondrial proteins are produced inside the organelle; most are synthesized on cytoplasmic ribosomes and have to be imported into mitochondria. Additionally, the maintenance of the small mitochondrial genome and expression of its genes requires the synthesis and import of an amazingly large amount of mitochondrial proteins encoded by nuclear genes (Attardi and Schatz, 1988; Grivell, 1995). As a result, a large part of the genome of a eukaryotic cell encodes proteins that end up inside mitochondria. For the recently completely sequenced genome of the yeast Saccharomyces cerevisiae, that can live without carrying out oxidative phosphorylation, it can be calculated that almost 20% of its ~6000 genes will encode mitochondrial proteins (Johnston, 1996). All these proteins have to be recognized as being mitochondrial, imported into the organelle and have to end up in one of the four possible mitochondrial locations: the outer membrane, the intermembrane space, the inner membrane, or the mitochondrial matrix. This requires both promiscuity and specificity of the mitochondrial import apparatus.
