13–9 The KDEL receptor must shuttle back and forth between the ER and the Golgi apparatus to accomplish its task of ensuring that soluble ER proteins are retained in the ER lumen. In which compartment does the KDEL receptor bind its ligands more tightly? In which compartment does it bind its ligands more weakly? What is thought to be the basis for its different binding affinities in the two compartments? If you were designing the system, in which compartment would you have the highest concentration of KDEL receptor? Would you predict that the KDEL receptor, which is a transmembrane protein, would itself possess an ER retrieval signal?
13–10 How does the low pH of lysosomes protect the rest of the cell from lysosomal enzymes in case the lysosome breaks?
Figure Q13–2 SNARE requirements for vesicle fusion (problem 13–7).
(A) Scheme for measuring the fusion of vacuolar vesicles. (B) Results of fusions of vesicles with different combinations of v-SNAREs and t-SNAREs. The SNAREs present on the vesicles of the two strains are indicated as v (v-SNARE) and t (t-SNARE).
13–11 Melanosomes are specialized lysosomes that store pigments for eventual release by exocytosis. Various cells such as skin and hair cells then take up the pigment, which accounts for their characteristic pigmentation. Mouse mutants that have defective melanosomes often have pale or unusual coat colors. One such light-colored mouse, the Mocha mouse (Figure Q13–3), has a defect in the gene for one of the subunits of the adaptor protein complex AP3, which is associated with coated vesicles budding from the trans Golgi network. How might the loss of AP3 cause a defect in melanosomes?
Figure Q13–3 A normal mouse and the Mocha mouse (problem 13–11). In addition to its light coat color, the Mocha mouse has a poor sense of balance. (Courtesy of
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