on. Cycl in is degraded by ubiq- uitin-dependent microtubules, although only dynein occurs on the destruction in proteosomes, and the activation of microtubules of cilia and flagella. Kinesin is a e mitotic Cak most likely causes the plus-end directed micro tubular motor, and dynein ubiquitination of the cyclin, but with a substantial among its other roles, is a minus-end directed delay. As discussed in Chapter 5, ubiquitination microtubu lar motor. In spite of their similarities tags proteins for degradation in proteasomes in function, they are not homologous proteins, assume quite different Answer: The actual explanation is that the three-dimensional shapes. They are not members single amino acid change causes the protein to ora proteinfamily 后期A与后期B ented by aperone proteins in the er from ex iting this danelle. t therefore accumulates in the ER七、综合问答题任选一题,20分) 1.答:翻译后转运与共翻译转运 men and is eventually degraded. Alternative 跨膜运输 terpretations might have been: (1)the mutation affects the stability of the protein in the 小泡运输 核孔运输 bloodstream so that it is degraded much faster in the blood than the normal protein, or(2)the 2. mutation inactivates the ER signal sequence and 同:有关卡,有周期蛋白与周期蛋白激酶 异:CDC2CDC28哺乳动物不同的激酶与多 prevents the protein from entering the ER. ( 3) nother explanation could have been that the种周期蛋白。 retention hich would have 八、附加题(每题5分,共15分) 1. Answer: The extracts appear to R. One could d isting structures that are functionally equivalent to centrosomes (as evidenced by between using the presence of pericentrin) antibodies against the protein nucleate microtubule growth. to follow its transport in the cells(see Panel 5-3, 2. Answer: The functions of sRP were pp.158-159) demonstrated in a series of experiments cell-free protein-synthesizing system and mRNA 六、比较题(每题5分,共10分) encoding pre-prolactin, a typical 1. Answe secretory protein. When the mRNA was incubated in the cell-free translational Both dynein and kinesin are large motor system in the absence of SRP and microsomes proteins that convert the chemical energy of ATp the complete prot tein with its signa sequence was produced. The addition of SRP into movement. Both are found affiliated with to the incubation mixtures caused protein3 on. Cyclin is degraded by ubiq-uitin-dependent destruction in proteosomes, and the activation of the mitotic Cdk most likely causes the ubiquitination of the cyclin, but with a substantial delay. As discussed in Chapter 5, ubiquitination tags proteins for degradation in proteasomes. 3. Answer：The actual explanation is that the single amino acid change causes the protein to misfold slightly so that, although it is still active as a pro-tease inhibitor, it is prevented by chaperone proteins in the ER from exiting this organelle. It therefore accumulates in the ER lumen and is eventually degraded. Alternative interpretations might have been: (1) the mutation affects the stability of the protein in the bloodstream so that it is degraded much faster in the blood than the normal protein, or (2) the mutation inactivates the ER signal sequence and prevents the protein from entering the ER. (3) Another explanation could have been that the mutation altered the sequence to create an ER retention signal, which would have retained the mutant protein in the ER. One could distinguish between these possibilities by using fluorescent-tagged antibodies against the protein to follow its transport in the cells (see Panel 5-3, pp. 158-159). 六、比较题（每题 5 分，共 10 分） 1. Answer: Both dynein and kinesin are large motor proteins that convert the chemical energy of ATP into movement. Both are found affiliated with microtubules, although only dynein occurs on the microtubules of cilia and flagella. Kinesin is a plus-end directed microtubular motor, and dynein, among its other roles, is a minus-end directed microtubular motor. In spite of their similarities in function, they are not homologous proteins, and they assume quite different three-dimensional shapes. They are not members ora proteinfamily. 2. 后期 A 与后期 B 七、综合问答题(任选一题，20 分) 1. 答：翻译后转运与共翻译转运 跨膜运输 小泡运输 核孔运输 2. 答： 同：有关卡，有周期蛋白与周期蛋白激酶； 异：CDC2,CDC28,哺乳动物不同的激酶与多 种周期蛋白。 八、附加题（每题 5 分，共 15 分） 1. Answer: The extracts appear to contain structures that are functionally equivalent to centrosomes (as evidenced by the presence of pericentrin), which nucleate microtubule growth. 2. Answer:The functions of SRP were demonstrated in a series of experiments utilizing a cell-free protein-synthesizing system and mRNA encoding pre-prolactin, a typical secretory protein. When the mRNA was incubated in the cell-free translational system in the absence of SRP and microsomes, the complete protein with its signal sequence was produced. The addition of SRP to the incubation mixtures caused protein