Mitochondria contain about 1500 different proteins, of which only 1% are synthesized on ribosomes found within the mitochondrial matrix. The majority of proteins found in this organelle are encoded by nuclear genes which are translated by cytosolic ribosomes, and subsequently imported and assembled into existing mitochondria.
Recent findings have also linked the processes of mitochondrial protein import and assembly to that of mitochondrial morphology and dynamics. Additionally, this system may play a key role during the mediation of mitochondrially-mediated apoptosis. The mitochondrial protein import system is adaptable, and can be altered by physiological perbutations in energy status and hormones. Also, errors in the molecular mechanisms of protein import can lead to several diseases.
All nuclear-encoded proteins enter mitochondria via the translocase machinery of the outer membrane (TOM complex), and are then sorted to one of the four subcompartments of mitochondria by the interaction of the targeting information encoded within the imported protein and the translocation machinery. The components of the import machinery actively interact with each other during import, processing and assembly.
Mitochondrial protein unfolding and trafficking
Before being assimilated into existing organelles, mitochondrial preproteins synthesized in the cytosol are termed precursor proteins. All precursor proteins contain targeting and sorting signals which direct them to different sorting machineries of the import apparatus. Furthermore, precursor proteins cannot pass the translocase machinery while they are folded, and require cytosolic chaperones which help unfold them into import competent states, and even guide them toward import receptors.
Approximately 40% of mitochondrial precursor proteins possess a positively charged a-helix at the N-terminus which is cleaved after import. This portion of the precursor includes crucial mitochondrial targeting signals (MTS), whereas the remainder of precursor proteins that lack this a-helix have their targeting information within the mature part. The MTS helps guide precursor proteins towards the organelle and contains information that will sort them into the outer membrane (OM), inner membrane (IM), intermembrane space (IMS) or the matrix.
Cleavable presequences are typically 15-100 amino acids long and were the first class of presequences to be discovered. All precursor proteins destined to the matrix contain this signal. Several IM and IMS proteins also contain presequences, but they are not completely cleaved off. The targeting and sorting signals of precursor proteins may be further differentiated if they possess two (bipartite) or more signals, which require further processing. The nature of these additional second sorting signal may also determine the sorting of the precursor protein. For instance, proteins destined for the IM and IMS contain almost identical MTS and are sorted very similarly, except for a hydrophobic sorting signal which differentiates the two subsets. Finally, mitochondrial IMS proteins contain a special variety of internal signals rich in cysteine residues, which are particularly important as these proteins undergo several redox steps during import.
Signal anchor proteins such as the OM receptors, Tom20 and Tom70, contain a non-cleavable MTS at the C-terminus. Unlike the precursor variants of the IM and IMS mentioned earlier, it is possible to have a presequence-like internal signal present after the hydrophobic region.