Chaperone proteins, mitochondria

After synthesis In the cytosol, the soluble precursors of mitochondrial proteins (Including hydrophobic Integral membrane proteins) interact directly with the mitochondrial membrane. In general, only unfolded proteins can be imported Into the mitochondrion. Chaperone proteins such as cytosolic Hsc70 keep nascent and newly made proteins in an unfolded state, so that they can be taken up by mitochondria. Import of an unfolded mitochondrial precursor is initiated by the binding of a mitochondrial targeting sequence to an import receptor in the outer mitochondrial membrane. These receptors were first identified by experiments in which antibodies to specific proteins of the outer mitochondrial membrane were shown to inhibit protein import into... 

They may enter the cytosol and fold quickly into a compact form. This may require only a few seconds, whereas the translation process in the ribosome may take many seconds. The folding will therefore be cotranslational.525 Depending upon the N-terminal signal peptide the protein may later unfold and pass through a membrane pore or translocon into the endoplasmic reticulum (ER), a mitochondrion, chloro-plast, or peroxisome. Wherever it is, it will be crowded together with thousands of other proteins. It will interact with many of these, and evolution will have enabled some of these to become chaperones (discussed in Chapter 10).526... 

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