1-32 Cytoskeleton I Microfilaments

Question 1

describe the mechanism that regulates actin assembly into microfilamens

Answer 1

when an actin monomer is bound to ATP, it assembles into a filament (Adding to + end)

when the actin subunit has its ATP hydrolyzed to ADP, it leaves the filament (at minus end)

These processes occuring simultaneously leads to the process of treadmilling

Question 2

What regulates the form and function of microfilaments?

what can they do?

Answer 2

Various actin binding proteins

they can use nucleate assembly (branching), block assembly (end caps), cross links, generate force, and regulate motility

Question 3

Mysin does what? (general)

Answer 3

Generates Force

Question 4

Myosin I

associations?

allows for?

Answer 4

standalone head-and-tail structured motor protein

Head associates with actin fibers

tail associates with membranes

allows for vesicular transport in the cell

Question 5

Myosin II

Answer 5

two headed molecule, each head contains ATPase motor domains

they cause contraction of actin filament bipolar arrays (heads move towards the plus ends of the opposite actin filaments)

Called thick filament in muscle cells or “nonfilaments” in muscle cells

Question 6

troponin

Answer 6

regulates motility

Question 7

filamin

Answer 7

regulates crosslinking

Question 8

alpha/beta capping protein

Answer 8

caps filaments

Question 9

ARP complex

Answer 9

actin-related-protein complex

promotes assembly of new filament branches which push out membrane at the leading edge.

Allows for formation of microvilli, contractile rings, stress fibers, lamellipodia, and filopodia

Question 10

phases of contraction

Answer 10

Attatched - myosin head attatched to thin filament in rigor conformation

released - binding of ATP causes myosin head to detach from the thin filament

cocked - when cleft region of myosin head clamps around ATP, myosin cocks back. ATP hydrolyzed, but ADP and Pi stay tightly bound to myosin

Force generating - myosin binds to the actin, causing Pi to be released. Triggering the power stroke causing the myosin to lose its ADP, returning it to the attatched phase.

Question 11

describe the mechanism of muscle contraction regulation

Answer 11

1. SR opens calcium channels to release Ca+2 ions into cytosol
2. Ca+2 binds to troponin C
3. conformational change causes movement of Troponin I and troponin T
4. troponin T moves tropomyosin away from multiple mysoin binding sites along the actin
5. during muscle relaxation, SR uses Ca-ATPase to pump Ca+2 back into SR lumen

Question 12

alpha actinin and fimbrin

Answer 12

bundle

Question 13

talin

Answer 13

anchors filaments to other structures such as the PM

Question 14

ARP

Answer 14

actin related protein complex - promotes assembly of new filament branches, which push the membranes out at a leading edge.

a

Question 15

ARP allow for

Answer 15

the formation of microvilli, contractile rings, stress fibers, lamellipodia, and filopodia

Question 16

dissocation of the inorganic phosphate..

Answer 16

increases affinity of the myosin head for the actin filament

Question 17

ATP binding to the myosin head

Answer 17

decreases the myosin heads affinity for actin

Question 18

dissociation of ADP is stimulated by

Answer 18

translocation of the myosin head back to its original conformation

Question 19

dystrophin

Answer 19

anchors the sarcoglycan (SG) and dystroglycan (DG) complexes to the plasma membrane.

this protein is defective in muscular dystrophy

Question 20

where do branched filaments occur and what is the role of the ARP complex?

Answer 20

branched filaments are found in lamellapoida, and each branch point is formed by an ARP complex, which stimulates assembly at the branch.

Question 21

why is actin filament structural polarity important?

Answer 21

polarity is important for directed assembly, which can generate directional changes in cell shape when the actin is associated with the cell membrane

polarity also important for directional movement of myosin

Question 22

why does Myosin II form bipolar thick filaments but not myosin I

Answer 22

tails of myosin II molecules form coiled-coil rods that self associate.

myosin I lacks this tail domain, and instead has a tail that binds to membrane

Question 23

what effect does the binding of calcium to the troponin/tropomyosin complex have on the interaction between Myosin II and actin in skeletal muscle?

Answer 23

when calcium binds, the complex moves out of the way and allows myosin to bind to actin

Question 24

what is the difference between the relaxed state of muscle that is not contracting, and the rigor state of muscle post mortem

Answer 24

in relaxed muscle, ATP concentrations are high, calcium concentrations are low, myosin is detatched from actin

in rigor, ATP is depleted, calcium leaks out of SR and myosin binds to actin

Question 25

how is ATP hydrolysis linked with actin filament assembly dynamics?

Answer 25

assembled actin hydrolyzes its bound ATP to ADP, and ADP actin dissociates easily from free minus ends of filaments.

exchange of ADP for a fresh ATP allows actin monomers to bind with high affinilty to filament plus ends, leading to treadmilling

Question 26

The neurofilament class of intermediate filaments provides tracks for retrograde transport of neurotropic viruses.

Answer 26

False

Question 27

The activation of contraction in non-muscle cells, smooth muscle cells, and striated (skeletal) muscle cells all involve regulation of myosin activity by ____________.

Answer 27

regulation of intracelluar calcium ion concentrations

Question 28

Which of the following cellular motility events only depends on the regulated assembly of actin filaments?

A. Cytokinesis

B. Lamellipodium formation

C. Mitosis

D. Fibroblast migration

E. Transport of vesicles from the Golgi to lysosomes

Answer 28

B

Question 29

A major difference between myosins that cause contraction of actin bundles in both muscle and non-muscle cells, and myosins that move vesicles along actin filaments, is that the contractile myosins can interact with _____.

Answer 29

themselves to form bipolar arrays

Question 30

The normal distribution of endoplasmic reticulum and Golgi membrane systems in a typical mammalian cell is dependent upon ___________.

Answer 30

microtubules and microtubule associated motor proteins

Question 31

Microvilli are protrusions of the plasma membrane whose shape is maintained by _____.

Answer 31

microfilaments

Question 32

Motility supported by molecular motor proteins depends on motors attaching to and carrying specific cargoes along linear cytoskeletal elements. Eukaryotic cilia and flagella are powered by the motor protein dynein. What are the ‘cargoes’ carried by flagellar dyneins?

Answer 32

doublet microtubules

Question 33

place these events in order

Which sequence of numbers lists the events that follow activation of skeletal muscle contraction, labeled 1-5 below, in their correct order?

1. ATP binds to the myosin head
2. Calcium binds to troponin C.
3. Tropomyosin moves and exposes the myosin binding site on actin.
4. Calcium is released from the sarcoplasmic reticulum.
5. Myosin binds to actin and undergoes a conformational change.

Answer 33

4,2,3,5,1