Lecture 13: Cytoskeleton

Question 1

Define cytoskeleton and its two functions

Answer 1

• the cytoskeleton is a network of protein filaments extending throughout the cytoplasm in all eukaryotic cells.
• functions:
  1) provides structural framework, determining cell shape and organisation of the cytoplasm.
  2) responsible for the movement of cell and internal transport of organelles and other structures (mitotic chromosomes).

Question 2

State 3 principal types of protein filaments

Answer 2

1) intermediate filaments (10 nm)
2) microtubules (25 nm)
3) actin filaments (7 nm)

Question 3

These 3 types of protein filaments are held together and ______ to subcellular organelles and the ________ by a variety of accessory proteins.

Answer 3

• linked

- plasma membrane

Question 4

What are the main functions of intermediate filaments?

Answer 4

• enables cells to withstand mechanical stress when plasma membrane stretches (growth or movement), but are not directly involved in movement.
• form network throughout pm, surround the nucleus and extend to cell periphery where they are anchored to the pm and are indirectly connected to neighboring cells through desmosomes (cell junctions).

Question 5

Intermediate filaments form what kind of lamina and its function?

Answer 5

form nuclear lamina on the inner face of the nuclear envelope which a) supports nuclear membrane, b) provides attachment sites for chromatin.

Question 6

What is the difference between intermediate filaments and other protein filaments?

Answer 6

They’re composed of more than 50 intermediate filament proteins expressed in different types of cells whereas actin and microtubules are polymers of a single type of proteins (actin and tubulin).

Question 7

Name the classes of intermediate filament protein and their cell

Answer 7

cytosolic
a) keratins: epithelial cell
b) vitamins: muscle cell
c) neurofilaments: nerve cell
nuclear
lamins: all nucleated cells

Question 8

What are the main functions of microtubules?

Answer 8

Separating chromosomes during mitosis, intracellular transport of membrane-bound vesicles and organelles, cell movement (disassembly and assembly).

Question 9

Define the structure of microtubules

Answer 9

• polar structures composed of tubulin dimers (alpha and beta tubulin) which polymerize to form microtubules, consisting of 13 linear protofilaments assembled around a hollow core.
• protofilaments are composed of head-to-tail dimers arranged in parallel with two distinct ends; a fast growing (+), slow growing (-).
• polarity determines direction of movement along microtubules, (-) to (+).

Question 10

Do both alpha and beta tubulin bind GTP? Whats the difference between them

Answer 10

Yes both beta (tail), alpha (head) bind GTP.

• beta bound to GTP is unstable, hydrolyzed shortly after polymerization, weakening the binding affinity of tubulin for adjacent molecules. This favors depolymerization and resulting in dynamic instability of microtubules.
• alpha bound to GTP is stable.

Question 11

Explain the assembly and disassembly of microtubules

Answer 11

Assembly:
assembly continues as long as there’s high conc of tubulin bound to GTP, which are added faster than GTP is hydrolyzed thus retaining GTP cap at the growing end.

Disassembly:
Caused by low conc of tubulin bound to GTP, the rate of polymerization slows down, thus GTP cap is not retained at the growing end, resulting in shortening of microtubule.the

Question 12

Major microtubule- organizing center is

Answer 12

the centrosome where microtubules are anchored from (-) ends and extend outward from it.

Question 13

Structural components of a centrosome

Answer 13

a) pair of centrioles perpendicular to each other, b) gamma tubulin complexes forming ring structures with diameters similar to microtubules serve as nucleation sites for the assembly of microtubules bound to (-) end.

Question 14

Steps on reorganization of microtubules during mitosis (cell division)

Answer 14

interphase: centrioles and centrosomes duplicate, and remain together on one side of the nucleus.

prophase: separate and move on opposite sides of the nucleus
forming two poles of the mitotic spindle, and chromosomes condense.

metaphase: nuclear envelope disassembles, microtubules reorganize to form the mitotic spindle, condensed chromosomes align at the centre of the spindle.
anaphase: chromosomes move toward the spindle poles along kinetochore microtubules driven by disassembly and shortening of kinetochore microtubules.

Question 15

Name three types of microtubules in metaphase cells

Answer 15

1- kinetochore: attach condensed chromosomes at the centromere, play a role in separating mitotic chromosomes.

2- polar: are not attached to chromosomes, stabilized by overlapping with each other in center of cell.

3- astral: extend outward from the centrosomes to cell periphery and have exposed (+) ends.

Question 16

Role of motor proteins

Answer 16

Move membrane bound vesicles and organelles (cargo) along microtubules and use ATP hydrolysis to travel and carry cargo along microtubules in one direction.

Question 17

Name and define two families of motor proteins in microtubules

Answer 17

(1) kinesins: move toward (+) end, away from centrosome.

(2) dyneins: move toward (-) end, toward the centrosome.

Question 18

What are the main functions of actin filaments?

Answer 18

provides mechanical support, determines cell shape, enables cells to migrate.

Question 19

Actin filaments are highly concentrated at the _____ of the cell, where they form __________.

Answer 19

• periphery.

- 3D network underlying the plasma membrane.

Question 20

What are actin filaments made out of and appear as?

Answer 20

• made out of protein actin which exist as a globular monomer called G-actin, or a filamentous polymer called F-actin where G-actins polymerize to form F-actins.
• appear as a double stranded helix.

Question 21

Do actin filaments have a distinct polarity?

Answer 21

Yes, and because of this their (+),tail, and (-), head, ends are distinguishable as head-to-tail polymerization.

Question 22

What is the first step in actin polymerization?

Answer 22

The first step is nucleation, the formation of three G-actin monomers (G-actin trimers) are able to grow by the reversible addition of monomers to both ends, but the (+) end elongates 10x faster than he (-) end.

Question 23

Two forms of G-actin in the cytosol

Answer 23

1- ATP-bound (predominant state), ATP G-actin monomers polymerize much faster and dissociate much slower than ADP actin monomers.
2- ADP-bound.

Question 24

Describe the assembly/disassembly process of actin filaments

Answer 24

(1) ATP-actin monomers associate with the fast growing (+) end.
(2) ATP bound to actin is hydrolyzed shortly after polymerization.
(3) hydrolysis of ATP to ADP reduces strength of binding between monomers, decreasing stability of the polymer.
(4) ADP-actin dissociates from the slow-growing (-) end.
(5) release of ADP-actin stimulates the exchange of ADP for ATP, resulting in the formation of ATP-actin monomers that can be re-polymerized into filaments.

Question 25

Name three actin binding proteins and their role in the assembly and disassembly of actin filaments

Answer 25

1- Cofilin: mainly responsible for actin filament disassembly, increases the rate of dissociation of ADP-actin monomers from the (-) end and remains bound, preventing their reassembly into filaments.

2- Profilin: can reverse the effect of cofilin by stimulating assembly of actin monomers (ADP to ATP-actin monomers), where ATP-actin monomers dissociate from cofilin and are available for re assembly.

3- Arp2/3: profilin bound to ATP-actin monomers can be re-polymerized into filaments, including new filaments nucleated by Arp2/3, which serve as nucleation sites to initiate assembly of new filaments.

Question 26

Cofilin, profiling, and Arp2/3 proteins act together to promote

Answer 26

Rapid turnover of actin filaments and remodeling of the actin cytoskeleton which is require for cell movement and cell shape.

Question 27

The activity of cofilin, profiling, and Arp2/3 are controlled by cell signalling mechanisms (T/F)

Answer 27

True, actin polymerization is regulated in response to environmental stimuli.

Question 28

Name and define family of motor proteins in actin filaments

Answer 28

Myosin family, in which myosin bind and hydrolyze ATP providing the energy to move along actin filaments from the (-) end toward the (+) end.

Question 29

What are the two subfamilies of myosin family of motor proteins?

Answer 29

a) myosin I: found in all cell types.
- single molecule with one globular head and tail that attaches to another molecule or organelle (cargo), moved along the actin filament by the motor activity of its head.
- head interacts with filaments and has ATP-hydrolyzing motor activity enabling movement, the tail varies between different types of myosin I and determines what’s moved.
- moves from trans Golgi carrying secretory vesicle toward pm, or carries pm and moves it relative to cortical actin filaments, thus deforming/pulling the membrane into another shape.
b) myosin II: most abundant in muscle cells.