Answers To Review Questions

2. D. The KDEL motif does not activate a translocation process; rather it holds proteins bearing the sequence in the endoplasmic reticulum if they have already been translocated there.

3. A. GAP means GTPase activator protein, so the GTPase will hydrolyze GTP to GDP more rapidly once the GAP has been added. Concerning other answers: (B) A GEF, a guanine nucleotide exchange factor, would do this. (C) This won't happen because there is no GEF present to cause the GTPase to release its GDP and then bind a GTP. (D) This is nonsense—hydrolysis of GTP yields GDP, not the other way around.

4. D. Glycosylation occurs in the endoplasmic reticulum and in the Golgi apparatus.

5. E. The lysosomal sorting signal is mannose-6-phosphate. Concerning other answers: (B) It is true that the oligosaccharide that contains the mannose-6-phosphate signal is attached to an asparagine residue, but asparagine by itself is not a lysosomal targeting signal. (C) Ran is concerned with nuclear import/export. (D) Ribose is a pentose sugar so it does not have a number 6 carbon!

6. E. All of these are required for endocytotic vesicles to form. Receptors specific for a ligand that is present in the extracellular medium, adaptor proteins, and clathrin are required for the generation of a clathrin-coated invagination of the plasma membrane; dynamin is then required to pinch the invagination off to make an endocytotic vesicle.

7. A. All the other answers are more or less nonsensical. In particular, it is important to note that ribosomes are found at the endoplasmic reticulum by virtue of a signal sequence on the protein that they are synthesizing, not because the ribosome itself is any different from those that are free in the cytosol, as was implied by answers B through E.

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