Actin cytoskeleton

Question 1

What is the role of the cytoskeleton in cells?

Answer 1

Maintains and changes cell shape and organization.

Question 2

What gives cells polarity?

Answer 2

Microtubules and actin filaments and their organization.

Question 3

What are the three types of cytoskeletal filaments?

Answer 3

Actin filaments, microtubules, and intermediate filaments.

Question 4

What is G-actin?

Answer 4

Globular monomeric actin.

Question 5

How many actin isoforms exist in humans?

Answer 5

Six.

Question 6

What is F-actin?

Answer 6

Filamentous polymerized actin.

Question 7

What gives actin filaments polarity?

Answer 7

Uniform orientation of actin monomers.

Question 8

Which end of actin filament has the ATP-binding cleft exposed?

Answer 8

The minus end.

Question 9

What are the three stages of actin filament formation?

Answer 9

Nucleation, elongation, and steady state.

Question 10

Which end of the actin filament grows faster?

Answer 10

The plus end.

Question 11

What is treadmilling?

Answer 11

Addition of subunits at plus end and loss at minus end, causing subunit flux.

Question 12

What drives treadmilling in actin?

Answer 12

ATPase activity of actin.

Question 13

What do capping proteins do?

Answer 13

Bind to ends of filaments to regulate growth/shrinkage.

Question 14

What is the role of formins in actin nucleation?

Answer 14

Stabilize early dimers and promote elongation.

Question 15

What is the role of FH1 domain in formins?

Answer 15

Binds profilin-ATP-actin.

Question 16

What is the role of FH2 domain in formins?

Answer 16

Stabilizes actin dimer and promotes elongation.

Question 17

What does Arp2/3 complex do?

Answer 17

Promotes branching of actin filaments.

Question 18

What angle does Arp2/3 form with the new branch?

Answer 18

70°.

Question 19

What activates the Arp2/3 complex?

Answer 19

Nucleation-promoting factors (NPFs).

Question 20

How is formin activation controlled?

Answer 20

Binding of Rho-GTP relieves autoinhibition.

Question 21

How is Arp2/3 complex activated?

Answer 21

Binding of NPFs and Cdc42-GTP.

Question 22

What defines higher-order actin structures?

Answer 22

Crosslinking proteins and membrane adapters.

Question 23

What is myosin?

Answer 23

An actin-based motor protein.

Question 24

How many subunits are in myosin II?

Answer 24

Six (2 heavy chains, 4 light chains).

Question 25

What is the function of myosin head domain?

Answer 25

Binds actin and ATP for movement.

Question 26

What is the motor domain in myosin?

Answer 26

Head and neck regions plus light chains.

Question 27

What provides energy for myosin movement?

Answer 27

ATP hydrolysis.

Question 28

What is the function of myosin tail domain?

Answer 28

Cargo binding.

Question 29

What are bipolar filaments in myosin?

Answer 29

Myosin filaments with opposite orientations.

Question 30

What drives actin-myosin contraction?

Answer 30

ATP hydrolysis and neck conformational change.

Question 31

What is a sarcomere?

Answer 31

The contractile unit of skeletal muscle.

Question 32

Where are the plus ends of actin filaments in sarcomeres?

Answer 32

Toward the Z disc.

Question 33

What is titin?

Answer 33

Protein connecting Z disc to M band, centers myosin.

Question 34

What is nebulin?

Answer 34

Protein regulating actin filament length.

Question 35

How is skeletal muscle contraction triggered?

Answer 35

Action potential leads to Ca²⁺ release.

Question 36

What does Ca²⁺ bind during muscle contraction?

Answer 36

Troponin.

Question 37

How does Ca²⁺ affect tropomyosin?

Answer 37

Shifts it to expose myosin binding sites.

Question 38

What regulates contraction in smooth muscle?

Answer 38

Not troponin/tropomyosin; alternative mechanisms.

Question 39

What structures drive cell migration?

Answer 39

Lamellipodia and filopodia.

Question 40

What polymerizes at the leading edge in migration?

Answer 40

Actin filaments via Arp2/3.

Question 41

How is the actin cytoskeleton connected to ECM?

Answer 41

Via focal adhesions and integrins.

Question 42

What pushes the rear of the migrating cell forward?

Answer 42

Actin-myosin contraction.

Question 43

What proteins regulate actin cytoskeleton dynamics?

Answer 43

Rho, Rac, and Cdc42 GTPases.

Question 44

What activates small GTPases?

Answer 44

GEFs (guanine nucleotide exchange factors).

Question 45

How is GTPase activity regulated in cells?

Answer 45

Locally by external signals.