Question

Which of these statements is TRUE regarding mitochondrial protein import?

A. Mitochondrial proteins should fold natively twice: once in the cytosol and once inside the organelle.
B. Signal sequences that target precursor proteins to the mitochondrial matrix form an α- helix in which positively charged residues cluster near its N-terminus, while uncharged or hydrophobic residues cluster near the other side.
C. β-barrel proteins that are abundant in the mitochondrial outer membrane are imported from the cytosol independently of the TOM complex.
D. At least two signal sequences are required to direct proteins to the mitochondrial matrix.
E. None of these statements are true about mitochondrial protein import.

Answer 1

The true statement about mitochondrial protein import is that signal sequences for mitochondrial targeting form an α-helix. Other statements are incorrect; mitochondrial proteins do not fold natively twice, β-barrel proteins need the TOM complex to be imported, and generally, only a single signal sequence is needed for directing proteins to the mitochondrial matrix.

Step-by-step explanation:

The correct statement regarding mitochondrial protein import is option B: Signal sequences that target precursor proteins to the mitochondrial matrix form an α-helix in which positively charged residues cluster near its N-terminus, while uncharged or hydrophobic residues cluster near the other side. This is due to the nature of the mitochondrial import machinery where proteins synthesized in the cytosol with a mitochondrial targeting sequence are directed to the mitochondria. The signal sequences are indeed characteristic of an α-helix, facilitating their recognition by specific receptors at the mitochondrial outer membrane.

The process of mitochondrial protein import involves proteins that are synthesized in the cytoplasm and are post-translationally transferred into the mitochondrion. Initially, the proteins are kept in an unfolded state by chaperone proteins like HSP70, allowing them to pass through the mitochondrial membranes. Upon entry into the mitochondria, the signal peptide is removed and the protein is refolded into its functional conformation with the aid of mitochondrial chaperones.