Targeting Signals and Default Destinations:
The final localization of proteins is determined by targeting signals or the absence of such. Proteins are translated on free ribosomes in the cytosol or on endoplasmic reticulum (ER)-bound ribosomes. If the proteins synthesized by free ribosomes do not have any localization signals, the default localization after translation is the cytoplasm. For proteins that are already targeted to the ER, if there are no additional localization signals, the default pathway is secretion (i.e., outside the cells).
Answer and Explanation: 1
The correct answer includes statements (c) and (d).
a. Proteins translocated into either compartment have an equal chance of ultimately being localized to the outside of the cell.
This is incorrect. Proteins targeted to the endoplasmic reticulum (ER) might end up outside the cells. If apart from the signal peptide that guides the synthesis of these proteins by ER-bound ribosomes, there is no additional localization signal, the default final destination will be outside the cell, via exocytosis. This scenario requires the removal of the signaling peptide once the protein is synthesized. On the other hand, matrix mitochondrial proteins with targeting signals are synthesized in the cytosol, on free ribosomes. Post-translationally, such signals are recognized, and the proteins are unfolded and guided through two protein complexes (TOM and TIM) inside the mitochondrial matrix. There is no chance for such proteins to end up outside the cell.
b. Insertion of proteins into both compartments takes advantage of an energy source in the form of an electrochemical gradient.
This is incorrect. No electrochemical gradient is required for the ER translocation of a growing polypeptide chain synthesized by ER-bound ribosomes. However, when a mitochondrial matrix protein is transferred through the outer translocase complex (TOM) and interacts with the translocase inner membrane complex TIM, the movement of the signal(targeting) peptide at the amino terminus of the protein is powered by the electrochemical gradient of hydrogen ions across the inner membrane.
c. Proteins destined for either compartment require sequences of amino acids which serve as targeting signals.
This is correct. Matrix mitochondrial proteins typically have targeting signals at their amino termini. ER-targeted proteins also have signal peptides.
d. Proteins destined for either compartment are inserted into their respective compartments in an unfolded state.
This is correct. The mitochondrial proteins are synthesized (i.e., translated) by free ribosomes in the cytosol. Post-translationally, such proteins are unfolded by chaperons and guided for import into the mitochondria. Proteins targeted to the ER begin their translation on free ribosomes in the cytosol; however, once the targeting signal peptide is made, the translation complex is recruited to the rough ER membrane, where a translocon (i.e., a channel) opens and the translation proceeds as the protein is threaded through the translocon. Therefore, in this process of co-translational translocation, the newly synthesized protein is still unfolded. The folding takes place in the ER lumen and is also assisted by chaperons.
e. The same signal can be used to direct a protein to either compartment, as long as the appropriate receptor recognizes the signal.
This is incorrect. The signals targeting matrix mitochondrial proteins and ER proteins to their proper compartments are different. The ER signal peptide is approximately 15-25 amino acids long, with an amino-terminal basic region, a highly hydrophobic core region, and a carboxy-terminal polar region that ends with a cleavage site recognized by a signal peptidase inside the ER. The mitochondrial targeting signals are typically 15 to 70 amino acids, and they are rich in positively charged basic amino acid residues.
f. Proteins destined for either compartment are allowed to fold in the cytoplasm and then are unfolded prior to insertion into their respective compartments.
This is incorrect. The matrix mitochondrial proteins fold in the cytoplasm, where they are synthesized on free ribosomes, except for a few proteins that are encoded by the mitochondrial DNA and are made inside the mitochondria. After translation in the cytosol, the folded matrix mitochondrial proteins are unfolded by chaperons prior to insertion through the translocases of the outer and inner mitochondrial membranes. The ER-targeted proteins with a signal peptide are translocated into the ER lumen as the translation takes place. The folding is inside the ER lumen.
g. Proteins destined for either compartment are likely to retain their amino terminal targeting signals.
This is incorrect. Typically (but not always), the amino-terminal targeting signal of an ER protein is cleaved by a signal peptidase localized at the luminal face of the ER. Similarly, for most mitochondrial proteins, there is a mitochondrial matrix signal peptidase that cleaves the targeting signals.
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fromChapter 4 / Lesson 10
Understand the mitochondria structure. Know how many membranes mitochondria have and learn the function of these membranes. Discover the function of mitochondria.