Cytoskeleton 2

Question 1

Lamellipodium:

Answer 1

• a protrusion at the leading edge of a migrating cell.
  
  • caused by actin polymerization

Question 2

Two parts of cell migration via actin polymerization:

Answer 2

• formation of lamellipodium at the leading edge due to actin polymerization.
• collapse of the cell tail by actin depolymerization due to myosin II.

Question 3

Contractile Ring:

Answer 3

• the ring formed during cytokinesis whe two cells are dividing.
• formed by actin and myosin II.

Question 4

Adhesion belts:

Answer 4

• contractile bundles of actin and myosin filaments near the apical surface of epithelial cells.
• important for morphogenesis:
  
  • myosin II contraction drives tissue invagination and formation of the neural tube.

Question 5

The three classes of cytoskeletal motors:

Answer 5

• dynein
• kinesin
• myosin

all are ATPases and have multiple isoforms, encoded by multiple genes

Question 6

Kinesins:

Answer 6

• carry cargo from the centrosome (minus end) to the periphery (plus end) on microtubules

Question 7

Dyneins:

Answer 7

• carry cargo from the periphery (plus end) to the centrosome (minus end) on microtubules.
• much larger than kinesins

Question 8

Myosins:

Answer 8

• move cargo on actin
• most move from the minus end to the plus end

Question 9

Dynein, kinesin, and myosin are all:

Answer 9

ATPases

“mechanochemical enzymes”

Question 10

What two motors are related structurally and mechanistically?

Answer 10

kinesin and myosin

however, have different functions

Question 11

Particular isoforms of cytoskeletal motors …

Answer 11

• only move in one direction on an actin filament or microtubule.

Question 12

Molecular/cytoskeletal motors are the downstream targets of:

Answer 12

signaling cascades

Question 13

How do molecular motors function/move on a filament?

Answer 13

• They are enzymes (ATPases)
  
  • Chemical energy from binding/hydrolysis of ATPase leads to:
  • → intramolecular conformational change
  • → mechanical work

Question 14

What is the work done by molecular motors?

Answer 14

• generation of tension or movement of an object along an actin filament or microtubule

Question 15

Do intermediate filaments have motors?

Answer 15

NO

Question 16

Myosin structure:

Answer 16

tail → neck/hinge → light chains → motor/head

• Globular “head” is the motor domain; contains ATPase.
• Variable “Tail” domain contains coiled coil for dimerization, and/or binds to membrane or target vesicle.

Question 17

Classification of myosins is based on:

Answer 17

• motor domain (“head”) homology

Question 18

Kinesin head domains are structurally related to:

Answer 18

myosin and G-proteins

Question 19

Kinesin Structure:

Answer 19

tail → stalk → neck → motor/head

Question 20

What portion of the kinesin determines its polarity?

Answer 20

the neck region next to the motor/head domain

Question 21

What part of the kinesin contains the light chains?

Answer 21

the tail region

Question 22

Kinesin N-terminal motors have what polarity:

Answer 22

plus-end directed

Question 23

Kinesin C-terminal motors have what polarity:

Answer 23

minus-end directed

Question 24

What two molecular motors have large gene families?

Answer 24

myosin and kinesin

Question 25

Dynein motors:

Answer 25

* ATPase * minus-end directed * multi-subunit protein

Question 26

Dynein is what kind of protein:

Answer 26

* AAA protein * "ATPase Associated with diverse cellular Activities" * Typically 6 domains * ATPase domain connected to a microtubule via a stalk ## Footnote **ATPase NOT DIRECTLY ATTACHED TO MICROTUBULE**

Question 27

Primary ciliary dyskinesia (PCD) AKA "Kartagener syndrome"

Answer 27

* Dynein associated disease * respiratory tract infections; male infertility * dynein is mislocalized in the cell and does not make it to the cilia

Question 28

Cause of Primary ciliary dyskinesia (PCD) AKA "Kartagener syndrome"

Answer 28

* mutation in the outer arm ciliary dynein heavy chain leads to: * dynein outer arms in cilia are missing * cilia are immotile * chronic infection of the respiratory tract * males sterile

Question 29

What is the rate-limiting step in cytoskeletal motor function?

Answer 29

* product release following hydrolysis * a phosphate for myosin and kinesin * Binding to actin or microtubules accelerates the rate-limiting step.

Question 30

How do cytoskeletal motors work?

Answer 30

* All convert the energy of ATP hydrolysis into mechanical work and heat. * Binding and/or hydrolysis of ATP causes conformational change in the motor.

Question 31

What can lead to rigor mortis in muscle?

Answer 31

* depletion of ATP, which causes myosin to remain attached to muscle

Question 32

What determines the speed of myosin moving on an actin filament?

Answer 32

* rate of myosin ATPase and P_i release

Question 33

Steps in myosin II moving down an actin filament:

Answer 33

1. ATP binds and causes release of myosin motor from actin 2. recovery stroke occurs 3. ATP hydrolyzed 4. myosin re-binds to actin 5. phosphate from ATP hydrolysis released 6. power stroke 7. **_repeat_**

Question 34

Where/when can you regulate myosin II function?

Answer 34

* at the kinetic step when ATP is hydrolyzed * at the step when a phosphate from ATP hydrolysis is released

Question 35

What causes strong binding of myosin II to actin, which ultimately leads to the power stroke?

Answer 35

* release of the phosphate residue from ATP hydrolysis

Question 36

How does the kinesin motor function?

Answer 36

Same as myosin: 1. Binding of ATP weakens affinity of kinesin for microtubule. 2. After ATP hydrolysis, kinesin binds with higher affinity to microtubule. 3. Binding to microtubule accelerates product release and conversion from weak to strong binding. 4. Power stroke occurs.

Question 37

Does kinesin bind to alpha or beta tubulin?

Answer 37

beta tubulin in the microtubules

Question 38

Myosin substrate, filament, and direction of movement:

Answer 38

* ATP * actin * plus-end (mostly)

Question 39

Kinesin substrate, filament, and direction of movement:

Answer 39

* ATP * microtubules * plus-end (mostly)

Question 40

Dynein substrate, filament, and direction of movement:

Answer 40

* ATP * microtubules * minus end

Question 41

What determines the localization of a cytoskeletal motor?

Answer 41

the tail region of the motor

Question 42

Melanin transport:

Answer 42

* melanin produced by melanocytes * melanin placed in vesicles called melanosomes. * melanosomes transported on microtubules and actin filaments to the periphery of the cell where they are taken up by keratinocytes. ## Footnote **Myosin V is required for transport to the periphery.**

Question 43

What myosin transports melanin from the melanocytes to the keratinocytes?

Answer 43

Myosin V

Question 44

Besides myosin V, what else is required to move melanin from the nucleus of a cell to the periphery?

Answer 44

* myosin V tail binds melanophilin, which is bound to Rab27aGTP/melanin

Question 45

Griscelli type 1 and Elejalde syndromes:

Answer 45

* due to mutations in myosin Va, melanophilin or Rab27a * patients have silvery hair, light skin in child of dark-skinned parents * severe neurological effects * melanin in skin is clumped in basal layer * melanin in hair is clumped in center