A putative mitochondrial signal peptide was attached to the N-terminus of the DHFR protein and expressed in mammalian cells. Mitochondria were isolated, and a fraction was osmotically shocked briefly. Both osmotically shocked and untreated pools of mitochondria were treated with protease. It was observed that the DHFR was intact in both cases.

Which of the following statements best describes the function of the signal peptide?

Correct option is D

EXPLANATION-

If DHFR is protected from protease even after osmotic shock, this implies that it is inside the inner membrane, specifically in the mitochondrial matrix.
Osmotic shock disrupts the outer membrane, exposing intermembrane space proteins to protease, but not those in the matrix.
Thus, DHFR must be located in the mitochondrial matrix, where it remains protected from protease digestion.
Hence, the signal peptide must be directing DHFR all the way into the mitochondrial matrix, confirming that Option (d) is correct.