BIOL 4374/BCHS 4313 Cell Biology Exam #2 March 22, 2001

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1 BIOL 4374/BCHS 4313 Cell Biology Exam #2 March 22, 2001 SS# Name This exam is worth a total of 100 points. The number of points each question is worth is shown in parentheses. Good luck! 1. (2) In the absence of any signal or targeting sequence, the expected subcellular location for any protein is the cytoplasm. 2. (2) The energy required for translocating mitochondrial proteins from the cytoplasm to the mitochondrial matrix is provided by proton-motive force. 3. (2) A common feature of mitochondrial, chloroplast, peroxisome and nuclear proteins is: a) they all have N-terminal signal sequences b) they all translocate to their target tissue as folded proteins answer - c c) they all are translated completely in the cytoplasm d) their signal sequences are removed e) all of the above 4. (2) A characteristic of nuclear localization sequences is: answer - b a) hydrophobic amino acid residues. b) basic amino acid residues c) acidic amino acid residues d) N-terminal location e) cleavage after movement into the nucleus 5. (3) Name three functions of SRP. 1. Binds to the signal sequence 2. Stops translation 3. Binds ribosome 4. Brings ribosome to the SRP receptor 6. (4) Signal peptidase is found in the ER lumen and functions to cleave off signal sequences. 1

2 7. (3) A protein consists of the following topogenic features: a signal peptide, stop-transfer sequence, signal-anchor sequence and a stop-anchor sequence arranged sequentially, N to C, and separated by various hydrophilic amino acids. Draw how this protein would orient in the membrane making sure to define the cytosolic and luminal sides of the membrane and where the N and C terminus of the protein will be. Cytoplasm Sig. anchor Stop transf. C ER lumen N stop anch. 8. (2) Which enzyme catalyzes disulfide bond formation and where does this occur in the cell? 1. Protein disulfide isomerase 2. found in ER lumen 9. (4) An excess of unfolded proteins builds up in the ER lumen, which triggers a response that results in synthesis of _(hsc70)(calnexin)(hac1) and (calreticulin) (PDI) (peptide prolyl isomerase) to increase protein folding capacity. 10. (2) What amino acid sequence acts as a signal to return a protein to the ER from the Golgi? KDEL 11. (3) Oligosaccharide-protein transferase is found in the _ER lumen, and transfers an oligosaccharide comprised of 9 mannose, 2 actyl-glucosamine and 3 glucose residues from _dolichol to _asparagine (ASN). 12. (2) O-linked glycosylation occurs on which amino acids? Answer - e a) serine b) asparagine c) tyrosine d) threonine e) two of the above 2

3 13. (4) The targeting signal for localization of acid hydrolases to the lysosome is mannose-6-phosphate, which is released from its receptor in late endosomes due to low ph. 14. (4) Proteins destined for regulated secretion form aggregates with _chromogranin B_ and secretogranin II. 15. (4) Proteolytic processing occurs in secretory vesicles, where endopeptidases cleave proproteins at dibasic amino acid residues to generate mature proteins. 16. (3) LDL particles are brought into the cell via the process of endocytosis, then LDL is released from its receptor in late endosomes because of low ph, and once the LDL receptor is delivered to lysosomes, mature (i.e. non-esterified) cholesterol is released. 17. (2) The formation of vesicles that transport cargo from the ER to the cis Golgi is controlled by: a) Clathrin answer - C b) COPI c) COPII d) none of the above e) all of the above 18. (2) The formation of vesicles that transport cargo from the cis Golgi to the ER is controlled by: a) Clathrin answer - B b) COPI c) COPII d) none of the above e) all of the above 19. (2) Once clathrin coated vesicles are formed, they are disassembled by: a) ARF b) SARIanswer - C c) Hsc70 d) Rab proteins e) AP1 3

4 20. (3) V-SNARES are located on the surface of _vesicles, and bind to specific T-SNARES on target membranes during the process of vesicular targeting/fusion. 21. (2) Decoration of actin filaments with myosin is commonly used to: answer - C a) attach actin filaments to cell membranes b) disassemble actin filaments c) reveal the polarity of actin filaments d) reveal the polarity of myosin filaments e) none of the above 22. (2) Treadmilling occurs when the G-actin concentration is > C c + and < C c (2) Actin filaments are destabilized by: answer - D a) phalloidin b) actinomysin D c) colcemid d) cytochalasin e) taxol 24. (4) Inside the cell, the actin binding protein profilin promotes actin assembly and Thymosinβ4 (Tβ4) inhibits actin assembly. 25. (2) When ATP binds to the myosin head it promotes: answer - C a) binding of myosin to a new actin subunit b) pivoting of the myosin head and generation of movement c) release of actin d) none of the above 26. (3) Show where actin filaments and myosin head and tail domains are in the diagram of a a sarcomere from skeletal muscle shown below. Actin actin Myosin heads myosin tails myosin heads Z disc A1 Zone A1 Zone Z disc A band 4

5 27. (2) acts to keep myosin-thick filaments in the center of the sarcomere. a) Nebulin b) CapZ answer - C c) Titin d) profilin 28. (3) During skeletal muscle contraction, Ca +2 ions bind to troponin C, which alters the conformation of tropomyosin, thereby allowing myosin to bind to actin filaments. 29. (2) A doublet microtubule has protofilaments. Answer - C a) 13 b) 15 c) 23 d) 33 e) (4) Centrioles and γ-tubulin are components of _(MTOCs)(centrosomes), and promote the assembly of microtubules. 31. (4) Microtubules are polar structures, where α-tubulin is exposed on the - end and β-tubulin is exposed on the + end. 32. (2) Growing microtubule ends are typically associated with: answer - A a) a GTP cap b) a GDP cap c) γ-tubulin d) phalloidin 33. (2) The process by which microtubules grow and shrink is called Dynamic Inastability. 34. (4) Kinesin mediates _(anterograde) (+-end directed)_ movement along microtubules and dynein mediates (retrograde) ( - end directed) movement along microtubules. 35. (2) In flagella, ATP is required for dynein to walk along. a) the central doublet b) radial spokes answer - E c) the A tubule d) basal bodies e) the B tubule 5

6 36. (4) During anaphase A, chromosome segregation is caused by the disassembly of kinetochore microtubules, followed by separation of the spindle poles by forces on polar and astral microtubules during anaphase B. 37. (3) Name three differences that distinguish intermediate filaments from either microfilaments or microtubules. 1. Don t bind to NTPs 2. Once formed are very stable 3. no role in motility 4. no polarity 5. many different protein constituents 6