Cytoskeleton (Lecture 7)

Question 1

What 2 forms are actin microfilaments found in?

Answer 1

monomers (G-actin) or long chains (F-actin)

Question 2

True or False:

Actin microfilaments are highly conserved among eukaryotes.

Answer 2

True, actin microfilaments are hight conserved among eukaryotes.

Question 3

What variety of actin microfilaments are found in muscle tissues?

Answer 3

alpha-actin

Question 4

Which two varieties of microfilaments are found in non-muscle?

Answer 4

beta and gamma actin

Question 5

What are the 2 drugs that affect actin polymerization?

Answer 5

Cytochalasin and Phalloidin

Question 6

How does cytochalasin affect actin polymerization?

Answer 6

Blocks the growth of actin filament by binding to the barbed ends, thus blocking elongation which can inhibit movements (e.g: cell division).

Question 7

How does phalloidin affect actin polymerization?

Answer 7

Phalloidin binds to actin filaments and prevents dissociation.

Question 8

Which drug that affects actin polymerization of microfilaments can be used in the laboratory to allow visualization of actin filaments with fluorescent dyes?

Answer 8

Phalloidin

Question 9

Name 3 of the 8 actin-binding proteins?

Answer 9

Spectrin
Dystrophin 
Villin and Fimbrin 
Calmodulin and Myosin I 
alpha-Actinin 
Filamin

Question 10

Which actin-binding protein is found in RBCs?

Answer 10

Spectrin

Question 11

Which 2 actin-binding proteins both bond cortical cytoskeleton to the plasma membrane?

Answer 11

Spectrin and Dystrophin

Question 12

Which 2 actin-binding proteins cross-link actin in microvilli?

Answer 12

Villin and Fimbrin

Question 13

Which 2 actin-binding proteins cross-links actin to plasma membrane in microvilli?

Answer 13

Calmodulin and Myosin I

Question 14

Which actin-binding protein cross-links stress fibers and connects actin to protein-plasma membrane complex complexes?

Answer 14

alpha-Actinin

Question 15

Which actin-binding protein cross-links actin at wide angles to form screen-like gels?

Answer 15

Filamin

Question 16

Name 4 of the 7 actin-binding proteins that control treadmilling?

Answer 16

Thymosin 
Profilin
Gelsolin 
Cofilin 
Arp2/3 
Phalloidin 
Latrunculins

Question 17

What does thymosin do?

Answer 17

Captures actin monomers and prevents actin monomers from being polymerized.

Question 18

What does Profilin do?

Answer 18

Binds to actin monomers and prevents monomers from being polymerized.

Facilitates exchange of bound ADP for ATP - which favors polymerization

NOTE: that only ATP-actin monomers can be assembled into F-actin

Question 19

What is the function of Gelsolin?

Answer 19

Destabilizes F-actin and caps actin filaments, preventing loss and addition of G-actin

In presence of Calcium ion, fragments actin filament and remains bound to the plus end

Question 20

What does Cofilin do?

Answer 20

Triggers depolymerization of ADP-bound actin at the minus end

Question 21

What does Arp2/3 do?

Answer 21

Initiates growth of F-actin from sides of existing filament - causes branching. Thus, changes the shape of the actin.

Question 22

What does phalloidin do?

Answer 22

It prevents depolymerization by binding to actin filaments.

Question 23

What does Latrunculins do?

Answer 23

It binds to G-actin and induces F-actin depolymerization.

Question 24

In what type of cells would you expect to find an abundance of intermediate filaments?

Answer 24

Cells subject to mechanical stress.

Question 25

Are intermediate filaments found as desmosomes and hemidesmosomes.

Answer 25

YES | **Intermediate filaments strengthen epithelial cells as desmosomes and hemidesmosomes.

Question 26

What three specific types of proteins make up the cytoskeleton?

Answer 26

1) Microfilaments (actin) (7 nm thick) 2) Intermediate Filaments (8-10 nm thick) 3) Microtubules (25 nm in diameter)

Question 27

Which two domains of intermediate filaments determine its specific function?

Answer 27

The head and tail domains.

Question 28

Label, in order, the general structure/assembly of an intermediate filament, starting at the inner must layer.

Answer 28

central rod of 2 polypeptides form a coiled dimer --> dimers associate in staggered antiparallel fashion to form tetramers --> tetramers assemble end to end to form protofilaments --> 8 protofilaments are wound together to form filaments.

Question 29

Why are intermediate filaments more stable than actin?

Answer 29

Intermediate filaments are more stable than actin and do not demonstrate dynamic behaviors such as treadmilling because dimers form a staggered antiparallel structure that are polymerized and do not have distinct ends, thus treadmilling is not possible.

Question 30

What are the 2 main functions of intermediate filaments?

Answer 30

1) form a cytoplasmic network in most cells | 2) associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell.

Question 31

How many types of intermediate filament types are there?

Answer 31

there are 6 types

Question 32

True or False: Protofilaments of microtubules have a fast-growing plus end and a slow-growing minus end.

Answer 32

True, thus they have polarity.

Question 33

True or False: The plus end grows more slowly than the minus end in the presence of low calcium ion concentration.

Answer 33

False, the plus end grow more rapidly than the minus end in the presence of low calcium ion concentration.

Question 34

What happens if the concentration of tubulin-GTP drops?

Answer 34

GTP at the plus end is hydrolyzed and dimers are lost.

Question 35

What happens if there are high concentrations of tubulin-GTP?

Answer 35

The dimers are added more rapidly than GTP is hydrolyzed, and the microtubule grows.

Question 36

What are the 4 factors that inhibit microtubule polymerization?

Answer 36

Colchicines (arrest mitotic cells at meta-phase) Colcemid Vincristine (anti-cancer drug) Vinblastin (anti-cancer drug -- used for Hodgkins Lymphoma)

Question 37

What factor stabilizes microtubules?

Answer 37

Taxol (used to treat breast caner, thus also an anti-caner drug)

Question 38

How many heads does myosin I have?

Answer 38

1 head

Question 39

Which myosin class has 2 heads?

Answer 39

myosin II

Question 40

Kinesin and Cytoplasmic dynein both have how many heads?

Answer 40

both have 2 heads each.

Question 41

What does the tail of myosin II bind to?

Answer 41

myosin II

Question 42

What does the tail of both cytoplasmic dynein and kinesin bind to?

Answer 42

Vesicles

Question 43

What does the tail of myosin I bind to ?

Answer 43

cell membrane

Question 44

What does the head of myosin I and myosin II bind to?

Answer 44

actin

Question 45

what does the head of kinesin and cytoplasmic dynein bind to?

Answer 45

microtubules

Question 46

The direction of the head motion of myosin I and myosin II is towards what?

Answer 46

barbed (plus) end

Question 47

Does kinesin move towards the plus end or minus end of microtubules? What about cytoplasmic dynein?

Answer 47

Kinesin towards the plus end | Cytoplasmic dynein towards the minus end