cytoskeleton II: actin & cell motility

Question 1

microfilaments are

Answer 1

polarized with a plus and minus end

Question 2

there are two steps in the filament formation:

Answer 2

1. nucleation

2. extension

Question 3

once nucleation is achieved,

Answer 3

extension proceeds rapidly

Question 4

nucleation occurs from ___ end

Answer 4

minus

Question 5

why does nucleation not occur spontaneously?

Answer 5

would require very high actin concentration

Question 6

nucleation is catalyzed by

Answer 6

different proteins that determine the structure of polymerized actin

Question 7

actin related protein (ARP2/3) nucleation leads to

Answer 7

branched filaments

Question 8

formin nucleation leads to

Answer 8

parallel bundles

Question 9

microfilament polymerizes from the

Answer 9

plus end following nucleation

Question 10

_____ are involved in regulating polymerization and depolymerization

Answer 10

over 60 accessory proteins

Question 11

profilin binds to

Answer 11

actin and prevents polymerization

Question 12

capping proteins:

Answer 12

cap the plus or minus end to inhibit polymerization or depolymerization

Question 13

cap plus end

Answer 13

inhibit polymerization

Question 14

cap minus end

Answer 14

inhibit depolymerization

Question 15

cofilin severs

Answer 15

actin filaments and induces depolymerization

Question 16

Actin in epithelial junctions:

Answer 16

Actin anchors proteins involved in tight junctions and adherens junctions that hold epithelial cells together.

Question 17

The proteins anchored by actin in adherens junctions are

Answer 17

cadherins and catenins.

Question 18

Decreased association of actin with the adherens junction proteins leads to

Answer 18

loss of cell-to-cell adhesion.

Question 19

Loss of cell adhesion increases

Answer 19

cell motility resulting in epithelial-to-mesenchymal transitions, which can cause cancer.

Question 20

Actin in microvilli on the apical surface of epithelial cells:

Answer 20

Microvilli consist of actin bundles, held together by cross-linking proteins (villin and fimbrin) and attached to the plasma membrane by myosin-I and calmodulin.

Question 21

Microvilli function to:

Answer 21

increase the surface area of epithelial cells for housing cell transporters

Question 22

Loss of microvilli in the small intestine leads to

Answer 22

microvilli inclusion disease, which causes intractable diarrhea and dehydration in infants born with the disease.

Question 23

Molecular motors are proteins that

Answer 23

can transform energy from ATP hydrolysis into motion.

Question 24

Myosins are

Answer 24

1. actin-associated motor proteins.

Question 25

Myosins are made up of

Answer 25

coiled-coil dimers that polymerize into filaments.

Question 26

Myosin have two heads that bind to

Answer 26

actin and ATP and a tail that binds to cargo. These molecular motors “walk” along actin towards the plus end with the hydrolysis of bound ATP.

Question 27

There are three main classes of myosins:

Answer 27

1. those involved in cargo transport
2. those responsible for muscle contraction
3. unconventional myosins.

Question 28

Myosin-II is the molecular motor protein responsible

Answer 28

for muscle contraction.

Question 29

Myosin-II is made of a .

Answer 29

coiled-coil complex of myosin filaments

Question 30

This coiled complex of myosin II forms the

Answer 30

thick filament that “walks” along actin thin filaments during muscle contraction.

Question 31

Thick filaments have many heads for

Answer 31

binding actin, thus only a small percentage of heads are actin-bound at one time.

Question 32

transport myosins, which have a ____duty ratio

Answer 32

50:50

(half of the heads are bound at one time).

Question 33

Myosins I and V are the unconventional myosins that are associated with:

Answer 33

membranes and bind to organelles through their tails

Question 34

Myosin I and V are responsible for

Answer 34

moving these organelles around the cell along F- actin microfilaments.

Question 35

Cells commonly translocate by

Answer 35

amoeboid movement.

Question 36

Amoeboid movement is governed by

Answer 36

changes in the actin cytoskeleton, induced by extracellular signals.

Question 37

Mechanism of amoeboid locomotion:

Answer 37

1. protrusion
2. attachment
3. traction
4. detachment

Question 38

protrusion:

Answer 38

signaling from RAC and WASp causes F actin to polymerize and form flat projections called lamellipodia

Question 39

lamellipodia result from

Answer 39

polymerization catalyxes by actin related proteins Arp2/3

Question 40

filopodia form in

Answer 40

protrusion

Question 41

filopodia are the result of

Answer 41

polymerization catalyzed by forming and have attached receptors for “sampling” the environment in front of the cell during movement

Question 42

attachment:

Answer 42

the lamellipodia protrusions attach with the substratum ahead

Question 43

traction

Answer 43

the cell is pulled forward by tension created by anchored actin protrusions and myosin molecular motors, which slide actin filaments along each other within the cell

Question 44

detachment

Answer 44

adhesions behind the new site of the anchorage are released to allow for translocation of the cell.
This release is acheived through the depolymerization of action of the protein cofilin

Question 45

Cell motility is required during

Answer 45

development as undifferentiated cells migrate to their proper location.

Question 46

Neural crest cells originating from the neural tube must migrate

Answer 46

throughout the body to give rise to pigment cells and cells of the peripheral nervous system.

Question 47

The actomyosin ring is an

Answer 47

actin ring structure with associated mysoins that forms around a dividing cell.

Question 48

the actomyosin ring contracts through the

Answer 48

action of the myosin heads and ATP hydrolysis during cytokinesis to separate the daughter cells.

Question 49

The actomyosin ring is highly

Answer 49

highly regulated, so that the timing of its formation and contraction can give rise to daughter cells of varying symmetry.

Question 50

The site and timing of contraction of the actomyosin ring regulates the _______
This is important in allowing for _______

Answer 50

symmetry of cell division.

asymmetric cell divisions.

Question 51

examples of asymmetric cell divisions

Answer 51

1. cells giving rise to RBCs divide so that the nucleus is separated from the remainder of the cell
2. In platelet formation, a cell duplicates its genetic material before dividing and later divides asymmetrically many times to give rise to many cell fragments
3. cells can be left connected by canals by the persistence of the actomyosin ring following division
4. The division of epithelial cells through a symmetric vertical axis ensures that cells maintain a monolayer distribution

Question 52

G actin

Answer 52

globular actin

assembles for form two stranded helical filaments (F-actin)

Question 53

F actin is made up of

Answer 53

G actin

Question 54

Microfilaments (MF) are critical for cell

Answer 54

shape
movement
polarity

Question 55

phalloidin,

Answer 55

extracted from the highly toxic fungus Amanita phalloides (“death cap”), which binds to and stabilizes F-actin (causing a net increase in actin polymerization).

Question 56

amoeboid movement is important in

Answer 56

1. development
2. host defense
3. cancer

Question 57

Wiskott-Aldrich syndrome (WAS): inheritance

Answer 57

WAS is a rare, X-linked immunodeficiency disease.

Question 58

Lissencephaly:

is a

Answer 58

severe defect of brain development resulting in a smooth cortical surface, i.e., the absence gyri

Question 59

WAS mutation

Answer 59

results from WASp mutations.

Question 60

WAS clinical symptoms

Answer 60

thrombocytopenia (reduced platelet number and size) and recurrent infections.

Question 61

WAS macrophages and neutrophil leukocytes have been shown to be

Answer 61

migration- and chemotaxis-deficient.

Question 62

Lissencephaly : caused by

Answer 62

Loss-of-function of n-cofilin, an actin filament depolymerizing factor, results in lissencephaly and the associated severe mental retardation.