A correctly addressed protein may fail to be targeted to an organelle if it folds too soon into its final three-dimensional shape. For example, movement of proteins into the mitochondrial matrix requires that a protein must move through a channel through the outer and inner membranes. This channel is just wide enough to allow an unfolded polypeptide to pass through. Our cells have proteins called chaperones, which as the name indicates, "look after" proteins. Chaperones use energy derived from the hydrolysis of ATP to keep newly synthesized proteins destined for the mitochondrial matrix in an unfolded state. As soon as the protein moves through the channels and into the matrix, the matrix targeting sequence is cleaved. The protein now folds into its correct shape. Some small proteins can fold without help. Larger proteins are helped to fold in the mitochondrial matrix by a chaperone protein called chaperonin, which provides a surface on which another protein can fold. Chaperonin itself does not change shape when helping another protein to fold.
Certain stresses that cells can experience, such as excessive heat, can cause proteins to denature (page 206). The cell responds by making proteins called heat-shock proteins in large amounts. The heat-shock proteins bind to misfolded proteins, usually to a hydrophobic region exposed by denaturation, and help the protein to refold. Like chaperone proteins, the heat-shock proteins are not themselves changed, but instead form a platform on which the denatured protein can refold itself. Heat-shock proteins are found in all cell compartments and also in bacteria.
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