actin

Question 1

3 independent protein networks in cytoskeleton

Answer 1

microfilaments 7-9nm
microtubules 25nm
intermediate filaments 10nm

Question 2

define the cytoskeleton

Answer 2

a 3D filamentous protein network that provides s cell’s shape, internal organisation and functional polarity

Question 3

describe the structure of a G-actin monomer

Answer 3

globular protein with 4 domains and 2 lobes separated by a deep cleft
ATP binding structure at the base of the cleft- binds ATP and Mg 2+

Question 4

what induces the polymerisation of G actins

Answer 4

binding of cations induces polymerisation into F actin filaments

Question 5

how can polarity be demonstrated

Answer 5

electron microscopy in myosin decoration experiements, excess of myosin SI is mixed with actin filaments
pointed end is (-)
barbed end is (+)

Question 6

structure of F actin filament

Answer 6

2 strands of monomers wound around each other helically

each complete turn is 14 subunits

Question 7

3 stages of actin polymerisation

Answer 7

nucleation, elongation, steady state

Question 8

what causes the lag phase and when doesn’t it occur

Answer 8

lag is caused by g monomers forming a nucleus for elongation, if a small number of F actin nuclei are added to a solution the lag period can be skipped

Question 9

what is property of steady state phase

Answer 9

no change in filament length

Question 10

what happens during the elongation phase

Answer 10

the short oligomer rapidly increases in length due to addition of actin monomers to both ends

Question 11

what does rate of addition of subunits depend on

Answer 11

ATP-G actin concentration in the solution

Question 12

property of the rate of loss of subunits

Answer 12

it is not affected by ATP-G concentrations and is similar at the (+) and (-) ends

Question 13

what is the critical concentration

Answer 13

the concentration of free G-actin at which rate of subunit addition = rate of subunit loss AT THE SAME END

Question 14

how is critical concentration calculated

Answer 14

rate of disassembly/ rate of assembly

Question 15

concentrations of free G actin at steady state

Answer 15

Cc+ < concentration < Cc-

Question 16

what powers treadmilling and describe the mechanism

Answer 16

ATP hydrolysis provides power for treadmilling
when ATP-actin is added to (+) it is hydrolysed to ADP + pi
Pi is slowly released so there is ADP-actin subunits towards (-)
ADP- G protein dissociates from (-)

Question 17

what is the role of profilin and mechanism

Answer 17

it catalyses the exchange of ADP to ATP which provides (+) a supply of ATP actin from ADP actin

1. prolifin binds ADP-g actin opposite the nucleotide binding cleft
2. The cleft is opened which enhances ADP loss
3. This is replaced by ATP forming a ATP-profilin complex
4. New actin subunit binds to (+) and prolifin dissociates

Question 18

what is the role of cofilin

Answer 18

binds and severs ADP-actin into fragements
this generates more free (-) which enhances ADP-actin loss
the released ADP is recharged by prolifin

Question 19

3 examples of structures that actin can form

Answer 19

microvilli, contractile stress fibres, adherens belt

Question 20

function of actin binding proteins

Answer 20

regulate the shape of the actin cytoskeleton e.g. fimbrin in microvilli and alpha-actinin bundles actin together in contractile fibres

Question 21

function of thymosin-beta 4

Answer 21

binds to ATP G-actin to inhibit its addition to either end of the filament, acts as a reservoir
it is in equilibrium with free ATP-G actin and can release it if needed

Question 22

what does cofilin specifically bind to

Answer 22

old bits of F actin filaments that contain ADP