Question

Indicate true (T) and false (F) statements below regarding the mitochondrial protein import system. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTF.

( ) Mitochondrial proteins should fold natively twice: once in the cytosol and once inside the organelle.
( ) β-barrel proteins that are abundant in the mitochondrial outer membrane are imported from the cytosol independently of the TOM complex.
( ) 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.
( ) At least two signal sequences are required to direct proteins to the mitochondrial matrix.

Answer 1

The correct answers indicating which statements are true and false in relation to the mitochondrial protein import system are 'FTTF'. Proteins import into mitochondria requires the TOM complex, and typically only one signal sequence is necessary for directing a protein to the mitochondrial matrix.

Step-by-step explanation:

The correct sequence of true (T) and false (F) statements is FTTF.

Proteins destined for the mitochondria are synthesized in the cytosol and have to be imported into the mitochondrion. They do not fold natively twice; instead, they fold in the cytosol and are then unfolded during transport across the mitochondrial membranes. They are not imported independently of the Translocase of the Outer Mitochondrial membrane (TOM) complex; in fact, protein import into mitochondria requires this complex.

The targeting signal sequences that direct proteins to the mitochondrial matrix indeed form an alpha-helix with positively charged residues near the N-terminus and hydrophobic or uncharged residues predominantly on the side that interacts with the membrane. However, only a single signal sequence is typically required to direct a protein to the mitochondrial matrix, although some proteins may have additional internal or C-terminal sequences that affect their final localization.