Exam 4 - Actin slides

Question 1

Fibers of actin filament are made of what?

Answer 1

Polymerized protein

Question 2

What is Phalloldin?

Answer 2

F-Actin stabilizer

Question 3

What is cytochalasins?

Answer 3

F-Actin destabilizer

Question 4

MTs had GTP nucleotide activity, F-Actin has what?

Answer 4

ATP activity

Question 5

Where is F-actin located?

Answer 5

Cytoplasm, Beta-actin in Nucleus

Question 6

Where are myosin motors located?

Answer 6

Muscle, cytoplasm, cell periphery

Question 7

What is the directionality of myosin’s?

Answer 7

mostly barbed end (+), except Myosin VI

Question 8

Barbed end of Microfilament is + or -?

Answer 8

Plus end

Question 9

Pointed end of F-Actin is + or -?

Answer 9

Minus end

Question 10

What is NOT a feature shared by both MTs and actin filaments?

Answer 10

Both depolymerize rapidly when their (+) end cap hydrolyzes a nucleotide.

Question 11

What are 3 features shared by both MTs and actin filaments?

Answer 11

1. Polar
2. Sensitive to the free subunit concentrations of their (+) ends
3. Plus end is the faster growing end.

Question 12

What is G-actin?

Answer 12

Globular actin

Question 13

How is F-actin formed?

Answer 13

G-actin polymerizes with nucleus to form F-actin

Question 14

What does the rate and direction of growth depend on in actin?

Answer 14

Free actin concentration

Question 15

How would you specifically block F-actin elongation from the pointed end?

Answer 15

Add a binding factor for the barbed end of G-actin

Question 16

What are the functions of actin?

Answer 16

1. Structural support
2. Contraction
3. Migration

Question 17

Small G protein activation does what?

Answer 17

Regulates actin organization

Question 18

Rac interacts with ____.

Answer 18

Lamellipodia

Question 19

Rho interacts with ____.

Answer 19

Stress fibers

Question 20

Cdc42 interacts with ____.

Answer 20

Filopodia

Question 21

What are actin stress fibers?

Answer 21

Contractile bundles of actin filaments and myosin that are anchored to focal adhesions at the plasma membrane and function in cell adhesion.

Question 22

What does ADF/Cofilin do?

Answer 22

Actin-associated protein: actin filament disassembly (ADF = actin depolymerizing factor)

Question 23

What does ARP2/3 do?

Answer 23

Actin-associated protein: Actin filament polymerizing/branching

Question 24

What does Gelsolin/Villin do?

Answer 24

Actin-associated protein: Actin filament severing

Question 25

What does the spectrin family do?

Answer 25

Actin-associated protein: Actin filament cross-linking

Question 26

What does the myosin family do?

Answer 26

Actin-associated protein: multiple motor functions in cytoplasm and muscles

Question 27

What does tropomyosin do?

Answer 27

Actin-associated protein: Regulates myosin-actin interactions in sarcomeres, controlled by troponin.

Question 28

What does Troponin do?

Answer 28

Actin-associated protein: Regulates myosin-actin interactions in sarcomeres via tropomyosin (Ca2++ dependent)

Question 29

Describe ADF/Cofilin.

Answer 29

1. Three highly conserved and highly (70-82%) identical genes: cofilin 1, cofilin 2, and destrin. 2. Ubiquitous actin-binding factor required for the reorganization of actin filaments. 3. Recycles "older" ADP-F-actin. 4. Helps cells to maintain ATP-G-actin pool for sustained motility. 5. Involved in ruffling membranes and at the leading edge of mobile cells. 6. Changes the average twist of filaments

Question 30

F-actin affinity is regulated by what?

Answer 30

pH, phosphorylation, and phosphoinositides.

Question 31

Where is cofilin 1 found?

Answer 31

Non-muscle (n-cofilin)

Question 32

Where is cofilin 2 found?

Answer 32

In muscle (m-cofilin)

Question 33

What is destrin known as?

Answer 33

ADF = actin depolymerizing factor

Question 34

Describe Profilin.

Answer 34

1. Binds to monomeric actin at the barbed end (+ end) (preference for ATP-G-actin) ** 2. Regulates the direction of filament growth. 3. Controls spatial and temporal growth of F-actin. 4. Sequesters actin from the pool of polymerizable actin monomers. ** 5. Involved in the dynamic turnover and restructuring of the actin cytoskeleton (essential for organ development, wound healing, and hunting down of infectious intruders). 6. Over 50 binding partners 7. Profilin is the major allergen present in birch, grass, and other pollen. **

Question 35

Describe Gelsolin/Villin.

Answer 35

1. Actin can be cross-linked into a gel by actin cross-linking proteins. Gelsolin can turn this gel into a sol (colloidal suspension) 2. Key regulator of actin filament assembly and disassembly (produces fresh uncapped barbed ends) 3. Highly efficient actin filament severing factor. ** 4. Can be found in the cytosol and mitochondria as well as in blood plasma. ** 5. Ca++ regulated, binds to the barbed end. ** 6. Key player in podosomes. ** 7. 82 kDa protein complex with six subunits.

Question 36

What is fimbrin?

Answer 36

Actin bundling protein

Question 37

Describe ARP2/3 complex.

Answer 37

1. Key factor for actin branching, e.g. in lamellipodia. ** 2. Simultaneously controls nucleation of actin polymerization and branching of filaments. 3. Typically found in cellular regions characterized by dynamic actin filament activity e.g. lamellipodia, motile cells ** 4. ARP2/3 stimulates actin polymerization by creating a new nucleation core (unlike creating a free barbed-end such as gelsolin or cofillin do) 5. Activated by WASp (Wiscott Aldrich Syndrome Protein. WAS = X-linked immune dysregulation) ** 6. Seven-subunit complex: Two of its subunits, the Actin-Related Proteins ARP2 and ARP3 closely resemble the structure of monomeric actin and serve as nucleation sites for new actin filaments. **

Question 38

Describe the Spectrin Family (e.g. alpha-Actinin).

Answer 38

1. Actin filament crosslinkers and linkers to the plasma membrane. ** 2. Spcetrin cleavage causes the membrane to form blebs and ultimately to be degraded, usually leading to the seat of the cell. 3. Extended spectrin cytoskeleton in Erythrocytes. 4. alpha-actinin (non-muscle): cross linking and binding actin filaments to the plasma membrane. ** 5. alpha-actinin (muscle): localized to the Z-disc, involved in anchoring actin filaments to the Z-disc. **

Question 39

Which of the following statements are wrong? A. The ARP2/3 complex is involved in actin filament branching. B. Branched actin filaments grow with the pointed end at their outer tips. C. The ARP2/3 complex inserts fully into the "mother" actin filament and triggers branching. D. The F-actin branching by ARP2/3 is triggered by two actin-like domains. E. B and C are both wrong.

Answer 39

E