2. The Cytoskeleton

Question 1

what is the basic component of microfilaments

Answer 1

actin

Question 2

what comprises actin

Answer 2

G-actin assembles into F-actin (reversible process)

Question 3

where does ATP bind on actin

Answer 3

at the (-) end

Question 4

true or false:

actin is polarised

Answer 4

true

Question 5

ATP binds to actin extending the chain, what’s the formula:

Answer 5

G-actin + ATP -> ADP-f-actin

Question 6

where does growth occur on the actin chain

Answer 6

at the (+) end

Question 7

define critical concentration

Answer 7

the concentration of free ATP-G-actin at which assembly/disassembly are equal at one of the ends

Question 8

what happens if:
above critical concentration
below critical concentration

Answer 8

• the end will grow

- the end will shrink

Question 9

what are the units for critical concentration

Answer 9

micromolar

Question 10

what is + end CC

Answer 10

0.12 micromolar

Question 11

what is - end CC

Answer 11

0.6 micromolar

Question 12

what is treadmilling

Answer 12

rate of disassembly at one end is equal to rate of assembly at the other end

Question 13

what regulates assembly and disassembly

Answer 13

actin-binding proteins

Question 14

name 3 actin-binding proteins

Answer 14

profilin , Cofilin, thymosin beta4

Question 15

what does profilin do?

Answer 15

enhances exchange of ADP for ATP on G-actin

Question 16

what does Cofilin do?

Answer 16

enhances loss of ADP-actin from the -end

= facilitates disassembly

Question 17

what does thymosin beta4 do?

Answer 17

binds G-actin, to provide a reserve for actin when its needed for assembly

Question 18

what do capping proteins do

Answer 18

bind to filament ends - preventing assembly and disassembly

Question 19

name 2 capping proteins

Answer 19

CapZ and tropomodulin

Question 20

what do CapZ and tropomodulin do

Answer 20

CapZ = binds to and stabilises the + end

tropomodulin = binds to and stabilises the - end

Question 21

what does phalloidin toxin do

Answer 21

binds to f-actin preventing disassembly, can be used to stain actin (immunolabelling).
found in death cap mushroom

Question 22

name 2 actin cross-linking proteins

Answer 22

fimbrin (supports microvilli)

dystrophin (muscle cell cortex)

Question 23

what is dystrophin, and what happens in Duchenne muscular dystrophy

Answer 23

dystrophin is an adaptor protein that binds actin to cell-adhesion molecule dystroglycan

DMD = mutation impairs muscle function and shortens lifespan.

Question 24

what is the function of myosin

Answer 24

myosin uses ATP to generate movement to transport cellular components along actin filaments

Question 25

how many different types of myosin in the human body

Answer 25

40

Question 26

what does mutation to myosin II cause

Answer 26

familial hypertrophic cardiomyopathy

Question 27

how can you differentiate different types of myosin

Answer 27

all have similar head regions, but different tail regions, allowing them to move different distances

Question 28

what are the main structural differences between myosin V and myosin II

Answer 28

myosin V has longer necks, myosin V has globular cargo binding domains at its tail. myosin II proteins can assemble into bipolar filaments that have contractile functions

Question 29

what does the neck domain do in myosin V

Answer 29

acts as a lever arm for the conformational change used in the power stroke

Question 30

where is myosin II found

Answer 30

muscles

Question 31

what is the role of titin

Answer 31

holds myosin in position

Question 32

what is the role of nebulin

Answer 32

holds actin filaments in position

Question 33

what does tropomyosin do

Answer 33

blocks the myosin head from interacting with actin

Question 34

what happens when calcium ions are released from the SR

Answer 34

calcium ions interact with troponin to induce a conformation change to tropomyosin, removing it from the myosin binding site = contraction

Question 35

describe the steps involved in the myosin power stroke

Answer 35

1. ATP binds to myosin head, releasing it from actin 2. hydrolysis of ATP > ADP + Pi = myosin head rotates into cocked state 3. myosin head binds actin filaments 4. release of P and elastic energy straightens the myosin molecule = moving the actin filament left 5. ADP released, ATP binds = myosin head released

Question 36

what proportion of total protein in a typical animal is actin

Answer 36

5%

Question 37

what other process is reliant on actin, give an example

Answer 37

cell crawling = cell pushes forward via actin polymerisation | e.g. neutrophils migrating towards bacteria