Contractile Proteins

Question 1

Give some examples of structures that utilize actin?

Answer 1

Microvilli, filopodia, contractile ring of cytokinesis

Question 2

Accesorry proteins assemble actin filaments into actin bundles and actin networks, what are the differences?

Answer 2

Actin bundles are cross-linked into closely packed parallel arrays

Actin networks are loosely cross-linked in orthogonal arrays that form a meshwork and are flexible

Question 3

How is cross-linking acheived?

Answer 3

by peptides and polypeptides have at least two actin-binding sites

Question 4

How are the size and shape determined for the filaments?

Answer 4

the crosslinking proteins determine how close/far they will be from eachother EX: contractile are further away for myosin, parallel bundles are closer together because dont have myosin

Question 5

Fimbrin binds to actin filaments as a monomer and holds two parallel filaments close together… what is an example of this?

Answer 5

Microvilli which increase cell absorption

Question 6

Alpha-actinin binds to actin filaments as a dimer, which is a larger protein so there is more space between each actin filament called contractile bundles, what does this allow room for?

Answer 6

allows for motor protein (myosin) to interact during contraction, loosely bundled due to size of alpha actinin protein

Question 7

Filamin is a large actin binding protein which binds as a dimer, creating a 3D network, crossing actin filaments. Where are the actin binding domains located?

Answer 7

They are located on the opposite ends of the dimer (dimerization domain)

Question 8

What are abundant beneath the plasma membrane and what do they do? Example?

Answer 8

3D networks are abudant and they determine the shape of the cell, aid in movement, stability and flexibility of the membrane.
EX: spectrin in erythrocytes

Question 9

What occurs when there is a mutation in the cytoskeleton proteins in erythrocytes (spectrin/ankyrin/4.1)?

Answer 9

Hereditary spherocytosis, decrease RBC, jaundice, become very round

Question 10

What are pseudopodias?

Answer 10

based on actin filaments cross-linked into a 3D network responsible for phagocytosis

Question 11

What are lamellipodia?

Answer 11

sheet like extensions on the edge of the cell, containing a network of actin filaments

Question 12

What are filopodia?

Answer 12

thing projections of the plasma membrane supported by actin bundles, based on assembly (+ towards outside) and disassembly (- towards inside) of actin filaments

Question 13

How does myosin move along actin?

What is the most common in skeletal muscle?

What are the three domains of myosin?

Answer 13

1. move along via ATP hydrolysis
2. Myosin II is most common
3. Head neck and tail

Question 14

What does the head domain of myosin consist of?

Answer 14

Actin binding and ATP binding sites, ATPase activity

Question 15

What does the neck domain of myosin consist of?

Answer 15

Flexible region, binds myosin light chain peptides (common see more proteins here)

Question 16

What does the tail domain of myosin consist of?

Answer 16

intertwined to bring myosin regions in close proximity, bind membrane/organelles

Question 17

What are the steps for how myosin moves along F-actin? (4)

Answer 17

1. ATP binds myosin, causing a conformational change and releases head from actin
2. Hydrolysis of ATP to ADP + Pi, Myosin in cocked state until binding to actin
3. Binds actin releases Pi (energy) to drive power stroke
   (conformational change in neck, moves actin filament)
4. ATP binds to myosin, releasing ADP, head released from actin

Question 18

The amount of velocity and powerstoke of the myosin depends on what?

Answer 18

If there is a longer neck domain on the myosin, the velocity and quickness of the reaction will be increased.(ex: myosin V will have faster velocity than myosin II due to longer neck)

Question 19

In the sarcromere, there is a positive end at the Z disk and negative charge at the M band/ end of the actin, which way does myosin move towards?

Answer 19

Move towards the positive charge on z disk to contract the muscle

Question 20

For smooth muscle, how is folded myosin activated to contract with actin, making the cell become globular?

Answer 20

When there is an increase in Calcium due to an extracellular signal, the myosin light chain becomes phosphorylated, unfolded, and active

Question 21

What is an example of a non-muscle cell contraction, and how does it occur?

Answer 21

An example is cytokinesis, bundles of f-actin and myosin II form the contractile ring, which contracts creating a cleavage furrow

Question 22

How is vesicular transport done regarding actin-myosin?

Answer 22

Myosin V moves towards positive end of actin, myosin VI moves towards minus end of actin, activated when cargo is present, attached by tail

Question 23

How is cell migration done by actin-myosin contractions?

Answer 23

Actin and myosin contractile bundles pull focal adhesions off the plasma membrane, allowing it to migrate.

Actin stess fibers are in middle and actin is plentiful at the leading edge of a cell

Question 24

What is duchene muscular dystrophy (DMD)?

Answer 24

X-linked recessive disorder, muscle wasting, wheel chair by 12 y/o, due to mutations in dystrophin gene

Question 25

What mutation is worse, in frame or out of frame mutations?

Answer 25

out of frame (DMD) in frame will still have partial activity of gene, called Becker muscular dystrophy

Question 26

What does dystrophin do in muscle contractions?

Answer 26

Relieves stress of the contraction and releases the energy laterally and longitudinally, without will see bursting/rupture of muscle cell and cell death, scar tissue grows.