Exam I 2009 key - Name Your GSI MCDB 428 Cell Biology Exam...

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Name ____________________________________ Your GSI_________________ MCDB 428 Cell Biology January 28, 2009 Exam I The exam has 12 questions from lectures 1-8 (the whole exam is 100 pts) Remember to pace yourself; the exam time is 2 hours For the short answer questions, you can continue on the back of the page if necessary Please write your name on the top right hand corner of each page The exam proctors will not answer questions during the test unless they concern typos, or unless essential information is missing Points will be deducted if what you write is incorrect whether or not it is relevant Good Luck! 1
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Name ____________________________________ 1. (8 pts) Like trans-membrane alpha helixes, beta-sheets sometimes form beta-barrel structures that allow a protein to integrate stably within a lipid bilayer. While formation of the beta-barrel is not dependent on the presence or absence of specific amino acids, the stability of this structure within the lipid bilayer requires that amino acid residues exposed on the outside of the barrel be hydrophobic. a) Ala-Leu-Ser-Cys-Asp-Val-Glu-Thr-Tyr-Trp-Leu-Ile A-L-S-C-D-V-E-T-Y-W-L-I b) Asp-Lys-Leu-Val-Thr-Ser-Ile-Ala-Arg-Glu-Phe-Met D-K-L-V-T-S-I-A-R-E-F-M c) Asp-Ser-Glu-Thr-Lys-Asn-Ala-Val-Phe-Leu-Ile-Leu D-S-E-T-K-N-A-V-F-L-I-L d) Thr-Leu-Asn-Ile-Ser-Phe-Gln-Met-Glu-Leu-Asp-Val T-L-N-I-S-F-Q-M-E-L-D-V e) Val-Leu-Glu-Phe-Met-Asp-Ile-Ala-Ser-Val-Leu-Asp V-L-E-F-M-D-I-A-S-V-L-D HINT: to distinguish between hydrophobic and hydrophilic amino acids the mnemonic “FAMILY VW” is helpful. (For your convenience, each sequence is also listed in both single letter and three letter code) a) Which, if any, of the following amino acid sequences would most likely be able to form a strand in a beta-barrel structure in a trans-membrane protein? The answer is D (4 pts) b) Explain the rationale by which you came to this conclusion. Because the side chains of the amino acids alternately project above and below the sheet, a sequence that could form a strand in an amphiphilic beta sheet should have alternating hydrophobic and hydrophilic amino acids. (4 pts) 2
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Name ____________________________________ 2. (5 pts) Ran GTPases act as a molecular switch to control nuclear transport. RanGEF controls the conformational state of Ran by inducing exchange of a GDP nucleotide for a GTP. RanGEF should be at a low concentration in the cytoplasm and at a high concentration in the nucleoplasm if nuclear import of NLS-bearing proteins is to be efficient and unidirectional. Because nuclear division involves breakdown of the nuclear envelope separating cytosolic and nucleoplasmic domains, explain how RanGEF is maintained at high concentration within the newly formed nuclei of the daughter cells. The RanGEF is RCC1, which associates with chromosomes during cytokinesis. As a result, once the nuclear envelope re-assembles in daughter cells the RanGEF will be maintained in the nucleoplasm.
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