lec 11- the cytoskeleton cont.

Question 1

what do intermediate filaments (IF) do?

Answer 1

provide support and mechanical strength

Question 2

what are IF made of?

Answer 2

70 different genes

Question 3

how many types of IF are there?

Answer 3

5 based off of the cells where they are found in

Question 4

how do IF’s interconnect to other cytoskeletal filaments?

Answer 4

via cross bridges

Question 5

what are the cross bridges called?

Answer 5

plectin

Question 6

what connects at the ends of plectin?

Answer 6

on one end there is a binding site for intermediate filament, and a binding site for any other cytoskeletal filament

Question 7

how does labelled keratin act when in a cell with IF’s?

Answer 7

it gets scattered in the IF

Question 8

what is keratin?

Answer 8

an IF that provides strength for epithelial cells, most diverse IF

Question 9

what does actin filament do?

Answer 9

provides motility, shape, structural support, and does muscle contractions

Question 10

what is the most abundant types of actin in the cell?

Answer 10

G-actin = globular
F-actin = filamentous
With ATP, they form into a helical filament

Question 11

what do the minus and plus end of the actin filament do?

Answer 11

minus end: the end with an exposed binding cleft which binds to the plus end in a filament
plus end: the other end where the minus end of a G-actin binds too

Question 12

what regulates polymerization and depolymerization of actin filament?

Answer 12

actin binding protein

Question 13

what is myosin?

Answer 13

an actin based motor protein that has a “head or motor domain

Question 14

what are the groups of myosin?

Answer 14

conventional and unconventional

Question 15

what type of myosins are the best studied?

Answer 15

type II (conventional) myosins

Question 16

what is an example of an unconventional myosin?

Answer 16

a myosin that transports vesicles and organelles

Question 17

how many polypeptide chains does a myosin II molecule have?

Answer 17

one pair of heavy chains and two pairs of light chains

Question 18

what does myosin II consist of?

Answer 18

1. a pair of globular heads that contain the catalytic site of the molecule
2. a pair of necks, each consisting of a single, uninterrupted alpha-helix and two associated light chains
3. a single, long, rod-shaped tail formed by the intertwining of long a-helical sections of the two heavy chains

forms into a fiber with heads pointing out and tails pointing in

Question 19

what is a muscle?

Answer 19

bundles of parallel muscle fibres joined by tendons to the bones that the muscle must move

Question 20

what are muscle fibres?

Answer 20

each fiber is a multinucleate cell formed during embryogenesis and specialized for contraction

Question 21

what is myofibril?

Answer 21

thinner cylindrical strands that make up muscle fiber

Question 22

what is a sarcomere?

Answer 22

the contractile unit of myofibrils each which has a very specific organization

Question 23

what is a sarcomere made of?

Answer 23

thin filaments (actin)
thick filaments (myosin)
and Z lines at the ends

Question 24

what side of the actin is anchored to the Z lines?

Answer 24

the plus end

Question 25

what does CapZ do?

Answer 25

caps actin to the Z end

Question 26

what is nebulin?

Answer 26

repeating actin binding motifs that binds actin filament to Z line

Question 27

what is myomesin?

Answer 27

bundles the myosin filaments

Question 28

what is titin?

Answer 28

extends through the myosin filament and attaches to the Z line-helps to prevent tearing of muscle

Question 29

do multiple myosins work at the same time?

Answer 29

no, individually

Question 30

explain the contractile cycle of actin and myosin:

Answer 30

1. ATP binds to myosin head, causing release of myosin from actin 2. ATP is hydrolyzed to ADP and Pi, causing nech to straighten and head to pivot which causes a weak binding to a new actin, myosin head is cocked and ready for power stroke 3. Pi releasing causes neck to go back to original conformation, tighter binding to actin, and creating a power stroke which moves thin filament 4. ADP is released and new ATP can now bind to repeat cycle

Question 31

what does calcium, tropomyosin and troponin do?

Answer 31

-tropomyosin blocks myosin binding sites on actin -troponin forms a complex with tropomyosin -motor neuron excitation signal, signal transduction pathway leads to calcium release from the SR, calcium binds to troponin, the troponin-tropomyosin complex moves opening binding sites, when signal stops calcium goes back to SR and muscle relaxes

Question 32

what does cofilin do?

Answer 32

binds ADP actin and severs filaments promoting depolymerization at minus end

Question 33

what does profilin do?

Answer 33

functions as an adenine nucleotide exchange factor, binds ADP actin at plus end changing conformation and allowing binding of ATP, binding results in dissociation of profilin, ATP actin then either joins growing filament at plus end or is bound by thymosin

Question 34

what does thymosin do?

Answer 34

sequesters G-actin preventing polymerization, displacement of thymosin allows binding of G-actin to the plus end

Question 35

are both ends of F-actin capped?

Answer 35

yes, it stabilizes the sarcomere by preventing plus end further growth or minus end loss of subunits

Question 36

can actin be linked to other actin filaments and the cell membrane?

Answer 36

yes

Question 37

what is the network of actin filaments and accessory proteins that underlies the plasma membrane called?

Answer 37

cell cortex

Question 38

what promotes actin branching and growth?

Answer 38

Arp2/3 complex: nucleates new branches off the side of existing filaments

Question 39

what activates Arps?

Answer 39

WASPs

Question 40

what are the steps in cell motility?

Answer 40

1 and 2. extracellular signal recognized and signal transduction cascade initiated 3 and 4. WASP activates Arp complex and Arp nucleates new actin filaments 5. The result is branch formation at plus end 6. membrane is pushed forward 7. caps terminate elongation 8. oldest part of filament at the minus end 9. F-actin severed and depolymerized at minus end 10. profilin exchanges GDP to GTP

Question 41

what does GDI do?

Answer 41

prevents Rho-GTPase from putting its tail into the membrane and interacting with GEF

Question 42

what does GEF do?

Answer 42

turns the GDP on the Rho into GTP turning it "on"

Question 43

how does Rho with GTP turn back into GDP?

Answer 43

GAP removes the phosphate on GTP, making it into GDP "off"

Question 44

what does Rho-GTPase do?

Answer 44

regulates actin nucleators

Question 45

What is tropomodulin?

Answer 45

Caps negative end and regulates the length of actin filament