The vast majority of mitochondrial proteins are encoded as precursors by the nuclear genome. A major aspect of mitochondrial biogenesis is therefore the transfer of nuclear-encoded, cytosplasmically synthesized precursor proteins across and into the mitochondrial membranes. During the past years the use of simple model organisms such as the yeasts S. cerevisiae and N. crassa has helped considerably to identify and unravel the structure and function of a substantial number of components involved in targeting of nuclear-encoded preproteins to mitochondria. Several pathways and a number of components were characterized that are involved in guiding mitochondrial preproteins to their specific sites of function. In particular, import of nuclear-encoded precursor proteins into and across the mitochondrial inner membrane is mediated by two distinct translocases, the TIM23 complex and the TIM22 complex. Both TIM complexes cooperate with the general preprotein translocase of the outer membrane, TOM complex. The TIM complexes differ in the their substrate specificity. While the TIM23 complex mediates import of preproteins with a positively charged matrix targeting signal, the TIM22 complex facilitates the insertion of a class of hydrophobic proteins with internal targeting signals into the inner membrane. Most recently the rapid progress of research has allowed elucidation of a new mitochondrial disease on the molecular level. This rare X-linked progressive neurodegenerative disorder, named Mohr-Tranebjaerg (MT syndrome), is caused by mutations in the DDP1 gene and includes sensorineural deafness, blindness, mental retardation and a complex movement disorder. The analysis of the novel pathomechanism is based on the homology of the affected DDP1 protein to a family of conserved yeast components acting along the TIM22 pathway. This contribution briefly summarizes the current knowledge of the pathways of protein import and proposes a mechanism to explain how defective import leads to neurodegeneration.
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