Some Mitochondrial Proteins Are Transported after Translation

Some Mitochondrial Proteins Are Transported after Translation 

Some proteins, especially those destined for the eukaryotic mitochondria and chloroplasts, are transported after their synthesis on free polysomes is complete. Such transport is known as posttranslational transport. In the case of posttranslational transport it is believed that the polypeptide to be transported must be unfolded from its native folded configuration by a system of polypeptide-chain-binding proteins (PCBs) before it can pass through the membrane. Posttranslational transport into the mitochondrion requires both ATP and a proton gradient. Presumably the energy from one or both of these sources is used to unfold the protein or separate it from the PCB system so that it can pass through the membrane. 

The transport of proteins from the cytoplasm to the mitochondrion is further complicated by the fact that mito- 

Processing of insulin. Insulin is synthesized by membrane-bound polysomes in the /3 cells of the pancreas. The primary translation product is preproinsulin, which contains a 24-residue signal peptide preceding the 81-residue proinsulin molecule. The signal peptide is removed by signal peptidase, cutting between Ala (—1) and Phe (+1), as the nascent chain is transported into the lumen of the endoplasmic reticulum. Proinsulin folds and two disulfide bonds crosslink the ends of the molecule as shown. Before secretion, a trypsinlike enzyme cleaves after a pair of basic residues 31, 32 and 59, 60 then a carboxypeptidase B-like enzyme removes these basic residues to generate the mature form of insulin. 

As in the case of mitochondrial proteins, some of the proteins of chloroplasts are synthesized directly in the organelle, whereas others are synthesized in the cytosol and must be transported. The mechanisms for transport are very similar to those observed for transported mitochondrial proteins. 

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