Barnase is a 110-amino-acid bacterial ribonuclease that is often used as a model

Question

Barnase is a 110-amino-acid bacterial ribonuclease that is often used as a model for studies of protein folding and unfolding. It forms a compact folded structure that has a high energy of activation for unfolding (about 85 kJ/mole). It is resistant to proteolysis by most proteases. Can such a protein be imported into mitochondria?

Considering the information about Barnase described in Question above...

To the N-terminus of barnase, you add 35, 65, or 95 amino acids (aa) from the N-terminus of pre-cytochrome b2, all of which include the cytochrome’s mitochondrial import presequence signal. N35-barnase is not imported, N65-barnase is imported at a low rate, and N95-barnase is imported very efficiently into isolated mitochondria. None of these N-terminal extensions have any measurable effect on the stability of the barnase domain.

What is the reason for the different rates of import observed for barnase with the tags considering that all of the three tags have the mitochondrial presequence signal?

Explanation / Answer

Precursor proteins are unfolded and imported into mitochondria. The ribonuclease barnase is imported into mitochondria only after unfolding. Mitochondria unfold this protein by unravelling it only from its N terminus. Barnase forms a compact folded structure that has a high energy of activation for unfolding which is about 85 kJ/mole. This energy for unfolding is provided by N95-barnase. N65-barnase falls little short of providing this energy and N35-barnase cannot provide this energy. So, N95-barnase is imported very efficiently into isolated mitochondria, N-65 barnase is imported at a low rate and N35-barnase is not imported.

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