Cytoskeleton

Question 1

Describe microfilaments (actin)

Answer 1

They are highly conserved in eukaryotes.

They bind to specific transmembrane proteins directly or indirectly.

Question 2

What are G-actin and F-actin?

Answer 2

G-actin is actin as a monomer and F-actin is actin polymerized.

Question 3

What is the first step of actin polymerization?

Answer 3

Nucleation- ATP binds tightly to G-actin, G-actin binds tightly with 2 other G-actin to form a trimer.

Question 4

Do all actin filaments display polarity and why or why not?

Answer 4

Yes they do because all actin monomers are oriented in the same direction.

Question 5

Which end of the chain can actin monomers be added?

Answer 5

Either side, but the barbed side is 5-10x faster than the pointed side.

Question 6

Once the G-actin + it’s bound ATP is added to the chain what happens to the ATP?

Answer 6

It is hydrolyzed to ADP

Question 7

What determines the rate of actin polymerization?

Answer 7

The concentration of actin monomers in the cytoplasm.

Question 8

Which one dissociates more readily from the filament? ADP actin or ATP actin?

Answer 8

ADP actin

Question 9

What does very low G-actin concentration encourage?

Answer 9

The disassembly of actin filaments.

Question 10

What does intermediate G-actin concentration encourage?

Answer 10

Treadmilling

Question 11

What is Treadmilling?

Answer 11

An equilibrium state where as one G-actin is removed, another is added resulting in zero net growth.

Question 12

What does high concentration of G-actin encourage?

Answer 12

The addition of G-actin at both ends, resulting in growth.

Question 13

Once actin microfilaments are polymerized, what do they form?

Answer 13

A double helical chain.

Question 14

What are cytochalasins?

Answer 14

They block the elongation of actin by binding to barbed ends which inhibits cell movement (mitosis)

Question 15

What is phalloidin?

Answer 15

It prevents dissociation by binding to actin filaments.

Can be labeled with fluorescent dyes so you can see actin filaments.

Question 16

Even though actin has the same structure in all cells…

Answer 16

It has many different functions and their functions are determined by binding proteins.

Question 17

Arrange the following in ascending order:

Microtubukes, microfilaments, intermediate filaments

Answer 17

Microfilaments, intermediate filaments, and microtubules

Question 18

What is the role of the actin binding protein spectrin?

Answer 18

Found in RBCs, binds cytoskeleton to cell membrane.

Question 19

What is the role of the actin binding protein dystrophin.

Answer 19

Binds the cytoskeleton to the plasma membrane.

Question 20

What do the actin binding proteins villin and fimbrin do?

Answer 20

Cross-link actin in microvilli

Question 21

What are the roles of the acting binding proteins calmodulin and myosin 1?

Answer 21

Cross-link actin to the plasma membrane in microvilli.

Question 22

What is the role of the actin binding protein alpha actinin?

Answer 22

Cross-links stress fibers and connects actin to protein-plasma membrane complexes.

Question 23

What is the role of the actin binding protein filamin?

Answer 23

Cross-links actin at wide angles to form screen like gels.

Question 24

What is the role of thymosin is controlling treadmilling?

Answer 24

Captures actin monomers and prevents them from polymerizing.

Question 25

How does profilin control treadmilling?

Answer 25

Binds to actin monomers preventing them from being polymerized. They also facilitate the exchange of ADP for ATP so that the monomer can be added to the chain.

Question 26

How does gelsolin control treadmilling?

Answer 26

Destabilizes F-actin and caps actin filaments preventing loss and addition of G-actin. In presence of calcium ion, fragments actin filaments and remains bound to the plus end.

Question 27

How does cofilin control treadmilling?

Answer 27

Triggers depolymerization of ADP bound actin at the minus end.

Question 28

How does Arp 2/3 control treadmilling?

Answer 28

Initiates growth of F-actin from sides of existing filament causing branching.

Question 29

How does latrunculins control treadmilling?

Answer 29

Binds to G-actin and induces F-actin depolymerization.

Question 30

What is the role of intermediate filaments?

Answer 30

The cell undergoes mechanical stress and they provide strength to the cells. (Nucleus, neurons, and macula)

Question 31

What is the structure of intermediate filaments?

Answer 31

They have a common monomer..a helical rod with heads and tails. The heads and tails determine the function.

Question 32

How do intermediate filaments form a coiled dimer?

Answer 32

The central rod of 2 polypeptides come together aligned head to head and tail to tail.

Question 33

How do dimers associate and why is this important?

Answer 33

They associate into a staggered antiparallel fashion to form tetramers. This doesn't allow them to have distinct ends and makes them more stable than actin-no treadmilling.

Question 34

What do tetramers assemble to form?

Answer 34

Protofilaments | 8 protofilaments form a filament.

Question 35

How do intermediate filaments work in the cytoplasm?

Answer 35

They connect with other cytoskeleton elements for form structure within the cell.

Question 36

What are the types of intermediate filaments and what is each associated with?

Answer 36

``` Type I- acidic keratin Type II- neutral and basic keratin Type III- vimentin,desmin,glial fibrillary acidic protein, peripherin. TypeIV- neurofilaments Type V- nuclear lamins Type-VI- nestin ```

Question 37

What are microtubules composed of?

Answer 37

Tubulin dimers alpha and beta unit. Protofilaments are longitudinal rows of Tubulin dimers. 13 protofilaments arranged parallel to form a cylinder with a hollow core = microtubule

Question 38

Protofilaments have a fast growing plus end and a slow growing minus end? True or False?

Answer 38

True

Question 39

Explain treadmilling in microtubules

Answer 39

GTP bound Tubulin dimers bind to the plus (growing end) end. * the plus end grows much faster in low Ca conc After polymerization, the GTP is hydrolyzed to GDP. Tubulin becomes less stable and dimers dissociate at the minus end.

Question 40

What happens when there are high levels of GTP-tubulin? And what happens when there are low levels?

Answer 40

The dimers are added more rapidly than the GTP is being hydrolyzed and dimers are dissociating. The reverse happens at low levels.

Question 41

What factors inhibit microtubule polymerization?

Answer 41

Colchicines Colcemid Vincristine - anti cancer drug Vinblastin- used for Hodgkin's lymphoma

Question 42

What factors stabilize microtubules?

Answer 42

Taxol- used for breast cancer