Ch.16-The Cytoskeleton

Question 1

1. What are the differences between microtubules, intermediate filaments and actin filaments?

Answer 1

A) Microtubules:
= Hollow tubes that can grow and shrink very rapidly; they are the largest.

B) Intermediate filaments:
= The strongest; composed of a variety of different proteins that are expressed in different types of cells

C) Actin:
= Linear polymers of globular actin subunits; only one type; smallest

Question 2

2A. What are microtubule filaments made of/composed?

Answer 2

= are made up of tubulin heterodimers&raquo_space; alpha and beta tubulin

Question 3

2B. What are Intermediate filaments made of/composed?

Answer 3

= are composed of a variety of proteins; made up of subunits that wrap around each other and then stack together; tetramers twisted in a ropelike filament

Question 4

2C. What are actin filaments made of/composed?

Answer 4

= are made up of actin subunits

Question 5

3A. What is the function of microtubule filaments?

Answer 5

= act as highways for transport of vesicles, organelles, and chromosomes

Question 6

3B. What is the function of Intermediate filaments?

Answer 6

= provide mechanical support for the plasma membrane where it comes into contact with other cells or with the extracellular matrix

Question 7

3C. What is the function of actin filaments?

Answer 7

= Provide structure and shape to a cell

Question 8

4A. Where in the cell would you find microtubule filaments?

Answer 8

Most, not all, originate from the centrosome

Question 9

4B. Where in the cell would you find Intermediate filaments?

Answer 9

Extensive network found in the cytoplasm

Question 10

4C. Where in the cell would you find actin filaments?

Answer 10

Cytoskeleton

Question 11

5A. Polymerization of Actin

Answer 11

• Actin filament binds to ATP, hydrolyzes it to ADP.
• ARP complex controls where actin filaments grow
• Some proteins (like Thymosin) control actin subunit availability.

Question 12

5B. Polymerization of Microtubules

Answer 12

• Gamma-tubulin ring starts growth of microtubules and is found in centrosomes;
• minus end of microtubule attaches to gamma-tubulin ring while the plus end grows and shrinks

Question 13

6. What is the difference between the plus and the minus end of actin and microtubules?

Answer 13

• Microtubules:
  = The “plus end” ends with a beta tubulin and the “minus end” ends with an alpha tubulin;
• Prefers to grow on the “plus end” while it grows and shrinks slowly on the “minus end.”
• Actin:
  = The ARP complex binds to the “minus end” and can start nucleation;
• The “plus end” is what grows and shrinks

Question 14

7. What are GTP caps? What types of filaments have these caps?

Answer 14

= is a specially altered nucleotide that controls growth and shrinkage on a microtubule.

• If the GTP cap is lost, the microtubule shrinks.
• If the GTP cap is regained, growth restarts

Question 15

8. Compare the mechanical properties of actin, microtubules and intermediate filaments.

Answer 15

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Question 16

9. Name an intermediate filament.

Answer 16

= Keratin

Question 17

10. How can a mutation in a keratin gene lead to blistering?

Answer 17

Mutation causes basal cells to rupture between the nucleus and the hemi-desmosomes&raquo_space; inflammation response&raquo_space; blistering of the skin

Question 18

11. How are microtubules nucleated? Name the protein involved.

Answer 18

They are nucleated by gamma-tubulin rings

Question 19

12A. Name the protein involved when actin filaments are nucleated.

Answer 19

ARP complex

Question 20

12B. How are actin filaments nucleated?

Answer 20

ARP complex binds to the minus end of actin filaments and start nucleation

Question 21

13. What is a centrosome?

Answer 21

= The organizing center of animal cells, abundant with gamma-tubulin rings

Question 22

14A. How does profilin affect actin polymerization rates?

Answer 22

• Many actin subunits bound to profilin and make it more likely to be added to actin filament;
• profilin promotes growth of actin filaments

Question 23

14B. How does thymosin affect actin polymerization rates

Answer 23

• Controls actin subunit availability by binding to actin subunits;
• actin subunits not available for growth

Question 24

15. How does stathmin affect the polymerization rate of microtubules?

Answer 24

• Sequesters free tubulin and makes it unavailable to microtubule;
• addition of subunits is stopped and microtubule shrinks

Question 25

16. How does capping of filaments affect their polymerization rates?

Answer 25

- Capping stabilizes filaments; | - actin subunits ready to be added onto filament but cannot because of the cap

Question 26

17A. How do cross-linking proteins organize different assemblies of actin filaments?

Answer 26

Cross-linking proteins organize actin filaments differently by linking them in different ways

Question 27

17B. What impact would spectrin have on the properties of actin bundles?

Answer 27

= forms pentagonal or hexagonal arrangements meant to maintain plasma membrane and structure; long filaments

Question 28

17C. What impact would fimbrin have on the properties of actin bundles?

Answer 28

= parallel bundles of actin filaments; short and tight packing

Question 29

18. Why are there fimbrin crosslinking proteins in microvilli?

Answer 29

= To prevent things that are not wanted from entering the cell.

Question 30

19. Describe the cytoskeleton rearrangements that occur in a blood platelet when it is activated.

Answer 30

1. Actin filaments capped by capping protein in inactivated blood plate 2. Signal mediated Ca++ influx activates gelsolin 3. Severed actin filaments capped by gelsolin 4. Signal mediated slow rise in PIP2 5. Gelsolin and capping protein removed and rapid actin filament growth from man short fragments

Question 31

20. What does gelsolin do?

Answer 31

= Severs (cuts) actin filaments

Question 32

21. Why sever actin filaments during platelet activation?

Answer 32

- Allows new actin filaments to grow >>(because cut segments won’t have caps on them)<< - This accelerates assembly of new actin filaments.

Question 33

22A. What are myosins?

Answer 33

= ATP-dependent motor proteins best known for their role in muscle contraction

Question 34

22B. What filament are they associated with?

Answer 34

= they move along actin filaments

Question 35

23. In what part of the myosin protein is the motor activity located?

Answer 35

= The motor head

Question 36

24. How are organelles moved around a cell?

Answer 36

Motor proteins pull organelles along microtubules

Question 37

25A. What is a skeletal muscle cell?

Answer 37

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Question 38

25B. How is a skeletal muscle cell formed?

Answer 38

= form by the fusion of many muscle cell precursors called myoblasts

Question 39

26A. What is a myofibril?

Answer 39

= Basic rod like unit of muscle cell, consists of thin and thick filaments; Repeated subunits called sarcomeres and Z-discs.

Question 40

26B. Describe the structure of myofibrils in detail.

Answer 40

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Question 41

27A. What is a Z-disc?

Answer 41

= are narrow plate-shaped regions that separate neighboring sarcomeres

Question 42

27B. What is a sarcomere?

Answer 42

= composed of long, fibrous proteins as filaments that slide past each other during muscle contraction. - basic unit of striated muscle tissue (appear as repeating dark and light bands)

Question 43

28. How do your muscles move?

Answer 43

- Sliding of thin and thick filaments, using myosin motor heads

Question 44

29. Describe how a muscle cell receives a signal for contraction in detail.

Answer 44

1. Action potential goes down the axon 2. Eventually, a voltage-gated Calcium channel opens up and calcium rushes into cell 3. Acetylcholine vesicle opens and acetylcholine is released into the synaptic cleft 4. Acetylcholine opens Ca++ ligand channel 5. Voltage sensitive protein changes shape 6. Ca++ goes out of sarcoplasmic reticulum 7. Ca++ relieves tropomyosin blockage and exposes myosin binding site

Question 45

30A. What are transverse tubules?

Answer 45

= They are deep invaginations of sarcolemma

Question 46

30B. Why are transverse tubules necessary?

Answer 46

= allow depolarization of membrane to quickly penetrate interior of the cell

Question 47

31. Describe in detail how Ca2+ causes myofibrils to contract.

Answer 47

It relieves tropomyosin blockage and exposes myosin binding site so actin can attach and the muscle can shorten

Question 48

32. What is tropomyosin?

Answer 48

- It can cover up myosin binding sites on actin | - Troponin complex can shift tropomyosin off myosin binding sites.