1.1 Actin and Actin Motors

Question 1

What are the functions of the cytoskeleton (4)?

Answer 1

• provides structural support, shape to the cell
• helps provide internal organization
• enables cell movement
• enables intracellular transport

Question 2

What are the three filament types?

Answer 2

1. actin filaments
2. microtubules
3. intermediate filaments

Question 3

What is the function of actin filaments?

Answer 3

determine the shape of the cell surface, and needed for whole-cell locomotion

Question 4

What is the function of microtubules?

Answer 4

determine the position of organelles, and direct intracellular transport

Question 5

What is the function of intermediate filaments?

Answer 5

provide mechanical strength

Question 6

To complete their respective tasks, each filaments interact with [ ]. These proteins will [ ]. These accessory proteins include [ ].

Answer 6

1. hundreds of different accessory proteins
2. link filaments to each other, or to other cellular components
3. motor proteins

Question 7

Motor proteins utilizes ATP hydrolysis to [ ].

Answer 7

move various structures along the filaments, or to move the filaments themselves

Question 8

The cytoskeleton can remain [ ] but is quite [ ].

Answer 8

1. stable
2. dynamic

Question 9

Two examples of cell types with stable and differentiated morphology:

Answer 9

1. neurons
2. intestinal epithelium

Question 10

Stable cytoskeletal structures like [ ] must be maintained throughout the life of the cell.

Answer 10

microvilli

Question 11

[ ] cells turnover every few days.

Answer 11

intestinal epithelial

Question 12

What are stereocilia?

Answer 12

cells that last the life of the organism, unless damaged. If damaged, they are lost forever. This can result in a loss of hearing

Question 13

Cytoskeletal filaments are [ ].

Answer 13

protein polymers

Question 14

[ ] is specific to the type of filament.

Answer 14

protein subunit

Question 15

Each protein is [ ] in length, but form filaments that can span [ ].

Answer 15

1. only a few nanometers in length
2. hundreds of micrometers (1000x)

Question 16

Subunits of actin filaments and microtubules are also enzymes that [ ].

Answer 16

hydrolyze ATP and GTP, respectively

Question 17

What are the subunits for actin filaments?

Answer 17

globular actin or G-actin

Question 18

What are the three isoforms of actin?

Answer 18

1. alpha-actin (only in muscle cells)
2. beta-actin
3. gamma-actin (beta and gamma found together in almost all non-muscle cells)

Question 19

G-actin is [ ].

Answer 19

asymmetrical

Question 20

When G-actin subunits assemble, they arrange themselves [ ].

Answer 20

head-to-tail, forming a polar filament (F-actin)

Question 21

Actin filaments form a tight [ ].

Answer 21

right-hand helix

Question 22

What is the growth rate in F-actin (2)?

Answer 22

1. faster-growing “plus end”
2. slower-growing “minus end”

Question 23

Individual filaments are very [ ].

Answer 23

flexible

Question 24

What is filament nucleation?

Answer 24

several subunit-to-subunit interactions are required to acheive stability

Question 25

Nucleation phase is also called [ ].

Answer 25

"Lag phase"

Question 26

Instability of oligmers makes nucleation inefficient. However, [ ] gradually yield more stable nuclei.

Answer 26

formation of oligomers

Question 27

The cell can bypass the lag phase by using [ ] of preexisting filament fragments.

Answer 27

performed seeds

Question 28

What are the three phases of filament formation?

Answer 28

1. nucleation 2. elongation 3. steady state

Question 29

What is the concentration of subunits at the steady state?

Answer 29

at this state, the rate of subunit addition equals that of subunit disassociation

Question 30

What is the critical concentration?

Answer 30

the concentration of free subunits at this steady state (Cc = koff/kon)

Question 31

What is the different in critical concentration in vitro and in vivo?

Answer 31

the concentration is higher in vivo than in vitro

Question 32

The plus and minus ends have the [ ] affinity for actin subunits that are bound to an [ ].

Answer 32

1. same 2. ATP or ADP

Question 33

The koff and kon rates at the plus end are [ ] than at the minus end

Answer 33

much greater

Question 34

Rate of hydrolysis increases when [ ].

Answer 34

actin is incorporated into filaments. The ADP remains bound to actin subunit

Question 35

What are the two types of filament structures?

Answer 35

1. ATP-bound T form 2. ADP-bound D form

Question 36

ATP hydrolysis results in [ ], but the energy is not lost. The energy is stored in [ ].

Answer 36

1. free energy release 2. filament polymers

Question 37

Free energy change of subunit dissociation of a D form filament becomes more [ ] than the T form. The D form therefore has a [ ] koff/kon ratio than the T form.

Answer 37

1. negative 2. greater

Question 38

The Cc (D form) is [ ] than Cc (T form). Consequently, at certain concentrations, the D form will [ ] while the T form will [ ].

Answer 38

1. greater 2. shrink 3. grow

Question 39

In living cells, most actin subunits are in the [ ]. Concentration of free [ ] is about 10x higher than [ ].

Answer 39

1. T form 2. ATP 3. ADP

Question 40

The [ ] actin subunits remains in the [ ], more likely actin is to [ ].

Answer 40

1. longer 2. filament 3. hydrolyze the ATP

Question 41

When subunit concentration < Cc, hydrolysis of ATP may occur [ ] another subunit is added. The end would be converted from [ ] to [ ] form, and would dissociate, causing filament to [ ].

Answer 41

1. before 2. T 3. D 4. shrink

Question 42

When subunit concentration > Cc, addition would [ ] hydrolysis on both ends. Subunit addition at [ ] > than [ ]. Minus end is more likely to convert to [ ], while plus end remains in [ ].

Answer 42

1. outpace 2. plus 3. minus 4. D form 5. T form

Question 43

What is treadmilling?

Answer 43

growth at plus end equals shrinking at minus end

Question 44

What gives actin cytoskeleton its contractile feature?

Answer 44

myosin, which generates the force for muscle contraction

Question 45

What is the structure of myosin?

Answer 45

formed from two heavy chains and two sets of two light chains

Question 46

What is the structure of the heavy chain (2)?

Answer 46

1. globular head at the N-terminus where force is generated and ATP is hydrolyzed by the head group 2. Long alpha-helical chain forms coiled-coil with another heavy chain to facilitate dimerization

Question 47

Two light chains bind near the [ ] on each heavy chain.

Answer 47

N-terminal head domain

Question 48

What are tail-tail interactions?

Answer 48

each coiled-coil bundles with the tails of other myosin proteins

Question 49

What are thick filaments?

Answer 49

formed from tail-tail interactions. These have hundreds of myosin heads in each filament, oriented in opposite directions. but same on the same bare zone

Question 50

What are the actin and myosin interactions in skeletal muscle (3)?

Answer 50

1. actin surrounds myosin 2. ATP hydrolysis causes myosin head group to "walk" towrads the plus end of actin 3. opposing orientation of head groups causes the shortening of muscle (contraction)

Question 51

During development, many separate cells fuse to form [ ].

Answer 51

large single muscle cells (separate nuclei are retained in the single cell)

Question 52

What are myofibrils?

Answer 52

the basic contractile unit of muscle cell. Each is typically the length of the cell. Contains repeated units called sarcomeres

Question 53

What are sarcomeres?

Answer 53

ordered arrangement of thick and thin filaments (myosin and actin)

Question 54

Actin are attached by their [ ] to the [ ].

Answer 54

1. plus end 2. Z disc

Question 55

Actin-myosin assemblies can be used in non-muscle cells. This could allow for (3):

Answer 55

1. cell morphology 2. aid in cell division 3. facilitate cell migrations

Question 56

In non-muscle cells, the formation is [ ].

Answer 56

transient