11 Jun Cell Communication -The endogenous GTPase activity
Question
Chapter 16 Cell Communication
1) The endogenous GTPase activity of G-proteins serves to:
(a) stimulate the activity of enzymes by producing energy.
(b) synthesize cGMP as a second messenger.
(c) synthesize GTP as an energy source
(d) hydrolyze GTP returning the G protein to a pre-stimulated level of activity 2) G protein-coupled receptors activate G proteins by reducing the strength of GDP binding, allowing GDP to
dissociate and GTP, which is present at much higher concentrations, to bind. How do you suppose the
activity of a G protein would be affected by a mutation that causes its affinity for GDP to be reduced without
significantly changing its affinity for GTP?
3) Should RGS (regulator of G protein signaling) proteins be classified as GEFs (guanine nucleotide exchange
factors) or GAPs (GTPase activating proteins). Explain what role the activity (of RGS proteins) plays in
modulating G-protein-mediated responses in animals.
4) Antibodies are Y shaped molecules that carry two identical binding sites (at the variable region). Imagine
that you obtained an antibody that is specific for the extracellular domain of a receptor tyrosine kinase. If
cells were exposed to the antibody, would you expect the receptor tyrosine kinase to be activated,
inactivated, or unaffected? Explain…
5) Genes encoding mutant forms of a receptor tyrosine kinase can be introduced into cells that also express
the normal receptor from their own genes. If the mutants genes are expressed at considerably higher levels
than the normal genes, what will be the consequences for receptor-mediated signaling of introducing genes
for the following mutant receptors?
A. A mutant receptor tyrosine kinase that lacks its extracellular domain? B. A mutant receptor tyrosine kinase that lacks its intracellular domain? Chapter 17 Cytoskeleton
1) In general terms, what are the cellular functions of intermediate filaments, microtubules, and actin
filaments?
2) The ?-tubulin subunit of an ??-tubulin dimer retains its bound GTP for a short time after it is added to a
microtubule, yielding a GTP cap whose size depends on the relative rates of polymerization and GTP
hydrolysis. A simple idea about microtubule growth dynamics is that the ends with GTP caps grow,
whereas ends without GTP caps shrink. To test this idea, you allow microtubules to form under conditions
where you can watch individual microtubules. You then sever (or cut) one microtubule in the middle using a
laser beam.
A. Would you expect the newly exposed plus and minus ends to grow or shrink? Explain.
B. What do you expect would happen to the newly exposed plus ends if you were to grow the
microtubules in the presence of an analog of GTP that cannot be hydrolyzed, and you then severed the
microtubules in the middle with a laser beam? 3) Phosphorylation of nuclear lamins regulates their assembly and disassembly during mitosis. You add a
drug to cells undergoing mitosis that inhibits the activity of an enzyme that dephosphorylates nuclear
lamins. What do you predict will happen to these cells? Why?
4) The graph in the figure below shows the time course of the polymerization of pure tubulin in vitro. You
can assume that the starting concentration of free tubulin is much higher than it is in cells. percentage of tubulin molecules
in microtubules C time at 37oC A.
B.
5) Explain the reason for the initial lag in the rate of microtubule formation.
Why does the curve level out after point C? Match the following labels to the numbered lines on the figure below.
1
2
3
+ 4
3
2
1 A.
B.
C.
D. minus end of microtubule
tail of motor protein
cargo of motor protein
head of motor protein Which of the two motors is most probably a kinesin? Explain your answer.
6) Some lower vertebrates such as fish and amphibians can control their color by regulating specialized
pigment cells called melanophores. These cells contain small, pigmented organelles, termed
melanosomes, that can be dispersed throughout the cell, making the cell darker, or aggregated in the
center of the cell to make the cell lighter. You purify the melanosomes from melanophores that have
either aggregated or dispersed melanosomes and find that:
1.
2. aggregated melanosomes co-purify with dynein;
dispersed melanosomes co-purify with kinesin. Given this set of data, propose a mechanism for how the aggregation and dispersal of melanosomes
occur. 7) In the cell, the concentration of actin monomer is higher than the concentration required for purified
actin monomers to polymerize in vitro. Thymosin is a protein that can bind actin monomers. If you were
to add a drug that inhibits the ability of thymosin to bind actin monomers, what effect would this have on
actin polymerization? Explain your answer. 8) You are examining a cell line in which activation of the Rho family member Rac promotes lamellipodia
formation. Which of the following statements is most likely to be true?
(a)
Cells carrying a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize
more unbranched actin filaments than normal cells.
(b)
Cells carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize
more unbranched actin filaments than normal cells.
(c)
Cells carrying a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize
more branched actin filaments than normal cells.
(d)
Cells carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize
more branched actin filaments than normal cells. 9) You isolate some muscle fibers to examine what regulates muscle contraction. When you bathe the
muscle fibers in a solution containing ATP and Ca2+, you see muscle contraction (experiment 3 in the
table below). Ca2+ is necessary, as solutions containing ATP alone or nothing do not stimulate
contraction and thus the muscle remains in a relaxed state (experiments 1 and 2 in Table Q17-49).
From what you know about the mechanism of muscle contraction, fill in your predictions of whether the
muscle will be contracted or relaxed for experiments 4, 5, and 6. Explain your answers.
Experiment
number added to muscle fibers muscle state 1 nothing relaxed 2 ATP relaxed
2+ 3 ATP and Ca 4 ATP, Ca2+, and a drug that inhibits troponin from binding Ca2+ 5 ATP and a drug that inhibits binding of tropomyosin to actin 6 a nonhydrolyzable analog of ATP contracted In what state would the muscle be if you added Ca2+ but no ATP?
Chapter 20 Cell Communities, Tissues
1) What are the main structures providing tensile strength in the following?
A.
animal connective tissue
B.
animal epidermis
C.
plant cell walls 2) Label the five different types of cell–cell junction shown in the Figure, and identify the apical and basal
surfaces of the epithelium.
What cytoskeletal elements are associated with each junction?
What is the function of each junction? 3) A stem cell divides into two daughter cells. One of the daughter cells goes on to become a terminally
differentiated cell. What is the typical fate of the other daughter cell? 4) Your friend is a pioneer in ES cell research. In her research, she uses an ES cell line that originated
from an inbred strain of laboratory mice called FG426. She has just figured out methods that allow her
to grow an entire liver from an ES cell and has successfully grown 10 livers. She demonstrates that the
newly grown livers are functional by successfully transplanting one of the new livers into a FG426
laboratory mouse.
You are particularly excited about this, because you have a sick pet mouse, Squeaky. You are very
attached to Squeaky, as you found him when you were out camping in New Hampshire. Unfortunately,
Squeaky has developed liver disease and will not live much longer without a liver transplant. After you
see your friend on TV talking about her new method for growing mouse livers, you immediately grab
your cell phone to ask her whether Squeaky could have one of the newly grown livers. Just as you are
about to dial your friend, you remember something you learned in cell biology and realize that instead,
you should ask your friend about possibly using therapeutic cloning for Squeaky’s benefit.
A. Why do you think that one of the newly grown livers may not work in Squeaky? B. Explain why therapeutic cloning would solve this problem. 5) Cancer is a disease of enhanced proliferation and cell survival. DNA repair mechanisms are normally
important for cell survival. When a cell senses DNA damage, the cell cycle is inhibited until the damage
is fixed. Given the importance of DNA repair mechanisms, how can their failure can lead to the
production of cancer cells with a competitive advantage over normal cells? 6) Ras is a GTP-binding protein that is often defective in cancer cells. A signal from a growth factor
through a receptor tyrosine kinase often stimulates normal cells to divide. When the receptor tyrosine
kinase binds the growth factor, Ras is stimulated to bind GTP. Ras in turn activates proteins that
promote cell proliferation. A common mutation in cancerous cells causes Ras to behave as though it
were bound to GTP all the time. A.
B. Why is this mutation advantageous to cancerous cells?
Your friend decides that the signaling pathway involving the Ras protein is a good target for
drug design, because the Ras protein is often defective in cancer cells. Your friend designs a
drug that will turn off the receptor tyrosine kinase by preventing it from dimerizing. Do you think
that this drug will affect cells that have a defective Ras protein that acts as if it were always
bound to GTP? Why or why not? 7) What do integrin receptors bind to extracellularly? intracellularly? 8) What are the features of stem cells? 9) What is therapeutic cloning? What is reproductive cloning? What is the difference between them? 10) What are iPS cells and how are they made? 11) Why are malignant tumors more dangerous than benign tumors? 12) What kind of mutations can convert a proto-oncogene into an oncogene? 13) What is a tumor suppressor? What are the differences between a tumor suppressor and an oncogene?
Essential Cell Biology, 4th Edition textbook Alberts et al., Garland Science, publisher.
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