Question

The signal sequence found in proteins that are sorted to the matrix of the mitochondrion is?

• A. an N-terminal amphiphilic α-helix that is cleaved from the protein by a signal peptidase in the matrix of the mitochondrion
• B. an N-terminal amphiphilic α-helix that is not removed from the protein
• C. an N-terminal sequence of at least eight hydrophobic residues that is not removed from the protein
• D. five adjacent (+)-charged amino acids found anywhere within the protein sequence; this signal sequence is not removed from the protein
• E. five adjacent (+)-charged amino acids found anywhere within the protein sequence; this signal sequence is cleaved from the protein by a signal peptidase in the matrix of the mitochondrion

Which of the following events occurs in the nucleus?

• A. the nuclear import receptor hydrolyzes a molecule of ATP in order to drive the active transport of a nuclear protein
• B. Ran-GDP binds the nuclear import receptor, causing the receptor to release its cargo
• C. the Ran protein releases a nuclear import receptor, causing the receptor to release its cargo
• D. Ran-GEF triggers nucleotide exchange on Ran, so that Ran releases GDP and binds GTP
• E. Ran-GAP activates the GTPase activity of Ran

Answer 1

Ans: The answer to the first question which asks about the pre-sequence of the peptide translocated into the mitochondrial matrix should be A.
The reason would be that the mitochondrial targeting signal sequence is of 2 types: One of them is the classical N-terminal cleavable matrix targeting sequences (presequences) and the other is the non-cleavable internal targeting sequences.
Presequences are rich in positively charged amino acids and hydroxyl-group containing amino acids but lack negatively charged acidic residues.
So, the options with amphiphilic N-terminal and these presequences are indeed cleaved by a Mitochondrial Processing Peptidase (MPP).

The answer to the second question about the events that occur in the nucleus especially during transport would be D.

It may be confusing, but you need to select events specific to the inside of the nucleus and not those that happen in the cytosol. Since the process of nuclear transport is an active process that requires energy in the form of GTP and this process is brought about by a protein called Ran (GTPase). Ran-GTP is present in the nucleus so it is involved in export and Ran-GDP is present in the cytosol so it is involved in import. There are also other things like GEF (Guanine Nucleotide Exchange Factor) and GAP (GTPase activating protein) present in the nucleus and cytoplasm respectively.

I hope you understand the process & if you still have doubts about anything feel free to refer to the comment section & I will get back to you.