The Import of Nuclear-Encoded Proteins into Mitochondria

The Import of Nuclear-Encoded Proteins into Mitochondria 

Less than a dozen or so polypeptides are synthesized on mitochondrial ribosomes, accounting for 5-1.5% of the mitochondrial protein. Three of the polypeptides have been identified as components of cytochrome oxi-dase, four are associated with oligomycin-sensitive ATPase, and one is a component of cytochrome fc. It is not known at this time whether these mitochondrial translation products are structural or catalytic components of these inner-membrane complexes. 

Although mitochondria possess a protein-synthesizing system active both in vivo and in vitro, it is clear that the vast majority of mitochondrial proteins are synthesized on cytoplasmic ribosomes and are coded for by nuclear DNA. This conclusion is based, in part, on the observations that 85-95% of mitochondrial protein can be labeled with radioactive amino acids in the presence of chloramphenicol, but not cyclohexmide, and that only small qualitative differences can be detected in mitochondrial proteins between respiratory-competent yeast cells and cytoplasmic petites.  

These facts bring us to one of the major outstanding problems in mitochondrial biogenesis what is the mechanism whereby the vast majority of mitochondrial proteins, coded for by nuclear DNA and synthesized on cytoplasmic ribosomes, traverse the mitochondrial membrane barriers and enter the closed organelle Since the mitochondrion is impermeable to proteins of even relatively low molecular weight, a number of models have been proposed to account for the transport of products of cytoplasmic protein synthesis into mitochondria, but convincing experimental support for these is not yet available. 

One mechanism for the transfer of particular translation products across membrane barriers which has received extensive experimental support is the vectorial release of secretory proteins into the cisternal space of the endoplasmic reticulum. In this system, secretory proteins are synthesized selectively on polysomes attached to the rough endoplasmic reticulum and are subsequently released from the membrane-bound ribosomes through the adjacent membrane into the cisternum of the reticulum for later transport from the cell.  

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