A. The problem is that vesicles having two cytoskeleton serves as a retention signal and may different kinds of V-SNAREs in their membrane could dock on either of two proteins. Hence, depending on cell type, at least different membranes three different mechanisms for api-cal-basolateral B. The answer to this puzzle is presently not orting are known, but we can predict that ells must have ways of turning the docking 4 (55) Answer4. Normally, cells divide ability of SNAREs on and off. when they have grown to a certain size. This This may through other proteins on trol is clearly defective in the two mutant that are, for example strains In the case of gee cells, cell size increases copackaged in the ER with SNAREs into without ever triggering cell division, suggesting transport vesicles and facilitate that the mutant cell the inter actions of the correct v-SNARE with its the t-sNARE in the cis golgi example, now permanently inhibit MPF In wee network ells. on the other hand the mutant control protein triggers cell div ision prematurely, before 3(10 5 h Answer3. Membrane proteins are cells have grown to the appropriate size.This orted to either the apical or baso domains could be a control protein, for example, that no by several different mechanisms longer inh ibits MPF, so that mPf becomes active ng proteins to the appropriate dor prematurely. In fact, there is a yeast cell-cycle ma membrane involves sorting in control prote in called Weel, which is a kinase trans-Golgi network. Except for the GPI anchor, that phosphorylates MPF on a uses its acts as an apical or basolat inactivation, yeast cells with a no unique sequences have been identified wee-I gene have a short cell cycle and are small that target proteins to the apical or basolateral one bepatocy tes where all newly made apical and basolateral proteins are first delivered together from the trans -Golgi network to the basolateral nembrane. From there, both ap ical and vesicles. With in endosomes, there is then sorting appropriate domain. The attachment of integral membrane proteins to the6 A. The problem is that vesicles having two different kinds of v-SNAREs in their membrane could dock on either of two different membranes. B. The answer to this puzzle is presently not known, but we can predict that cells must have ways of turning the docking ability of SNAREs on and off. This may be achieved through other proteins that are, for example, copackaged in the ER with SNAREs into transport vesicles and facilitate the interactions of the correct v-SNARE with the t-SNARE in the cis Golgi network. 3 （10 分）。 Answer3. Membrane proteins are sorted to either the apical or basolateral domains by several different mechanisms. One mechanism for targeting proteins to the appropriate domain of the plasma membrane involves sorting in the trans-Golgi network. Except for the GPI anchor, which acts as an apical or basolateral targeting signal, no unique sequences have been identified that target proteins to the apical or basolateral domain. Another mechanism operates in bepatocytes where all newly made apical and basolateral proteins are first delivered together from the trans-Golgi network to the basolateral membrane. From there, both apical and basolateral proteins are endocyto-sed to the same vesicles. Within endosomes, there is then sorting and transport to the appropriate domain. The attachment of integral membrane proteins to the cytoskeleton serves as a retention signal and may assist in the apicai-basolateral sorting of some proteins. Hence, depending on cell type, at least three different mechanisms for api-cal-basolateral sorting are possible. 4.(5 分) Answer4. Normally, cells divide only when they have grown to a certain size. This size control is clearly defective in the two mutant strains. In the case of gee cells, cell size increases without ever triggering cell division, suggesting that the mutant cell-cycle control protein has lost its ability to monitor cell size. It might, for example, now permanently inhibit MPF. In wee cells, on the other hand, the mutant control protein triggers cell division prematurely, before cells have grown to the appropriate size. This could be a control protein, for example, that no longer inhibits MPF, so that MPF becomes active prematurely. In fact, there is a yeast cell-cycle control protein called Wee1, which is a kinase that phosphorylates MPF on a site that causes its inactivation; yeast cells with a mutation in the wee-1 gene have a short cell cycle and are small