Answer Bank Rab hydrolyzes its bound GTP into GDP, which conceals its hydrophobic domain. The cytosolic domains of v-SNARES and t-SNAREs coil around each other to form a four-membered coiled coil. v-SNAREs bind to t-SNAREs. The inner layer of each bilayer fuse together. The outer layer of each bilayer fuse together. Rab-GTP binds to a tethering protein in the target membrane.
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Which of the following about the function of Rab proteins is INCORRECT? Rab proteins participate in fusion by binding directly to the SNARE proteins and facilitate the disassembly of the trans-SNARE complex following vesicle fusion. Rab7 is a specific Rab protein that allows tethering of vesicles to the late endosome. Rab proteins are GTPases that need to be in the GTP-bound state to be able to bind effector proteins on the target compartment. Rab proteins function in tethering a vesicle to a donor compartment. There are a wide range of Rab proteins, each of which is specialized in the traffic of vesicles between specific membrane compartments in a cell.
Madhur L.
Which of the following statements about vesicular membrane fusion is FALSE? A) When a vesicle fuses with a target membrane, the cytosolic portion of the transmembrane protein inserted into the vesicle will remain facing the cytosol. B) Water molecules must be displaced when a vesicle and its target membrane fuse. C) The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion. D) The interactions of the v-SNAREs and the t-SNAREs pull the vesicle membrane and the target organelle membrane together so that their lipids can intermix. E) The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell's exterior.
Shaiju T.
SNAREs exist as complementary partners that carry out membrane fusions between appropriate vesicles and their target membranes. In this way, a vesicle with a particular variety of v-SNARE will fuse only with a membrane that carries the complementary t-SNARE. In some instances, however, fusions of identical membranes (homotypic fusions) are known to occur. For example, when a yeast cell forms a bud, vesicles derived from the mother cell's vacuole move into the bud where they fuse with one another to form a new vacuole. These vesicles carry both v-SNAREs and t-SNAREs. Are both types of SNAREs essential for this homotypic fusion event? To test this point, you have developed an ingenious assay for fusion of vacuolar vesicles. You prepare vesicles from two different mutant strains of yeast: strain B has a defective gene for vacuolar alkaline phosphatase (Pase); strain A is defective for the protease that converts the precursor of alkaline phosphatase (pro-Pase) into its active form (Pase) (Figure $Q 13-2 A$ ). Neither strain has active alkaline phosphatase, but when extracts of the strains are mixed, vesicle fusion generates active alkaline phosphatase, which can be easily measured (Figure $\mathrm{Q} 13-2$ ). Now you delete the genes for the vacuolar V-SNARE, t-SNARE, or both in each of the two yeast strains. You prepare vacuolar vesicles from each and test them for their ability to fuse, as measured by the alkaline phosphatase assay (Figure $\mathrm{Q} 13-2 \mathrm{B})$ What do these data say about the requirements for V-SNAREs and t-SNAREs in the fusion of vacuolar vesicles? Does it matter which kind of SNARE is on which vesicle?
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