Schematic representation of the main mitochondrial preprotein import pathway. Majority of mitochondrial proteins are synthetized in the cytosol as preproteins with the cleavable N-terminal targeting signal (so-called presequence). Subsequently, preproteins pass the barrier of the mitochondrial outer membrane through the TOM complex and are transported to the final destination with the assistance of the inner mitochondrial membrane translocase, TIM23 complex. Interestingly, the import of preproteins into mitochondria can be posttranslationaly regulated, for instance by reversible phosphorylation of TOM components or proteolytic degradation of crucial subunits of TIM23 translocase. These mechanisms will be discussed during the lecture.