14.1 Structure and Organization of Actin Filaments

Question 1

() in the most abundant cytoskeletal protein of most cells (5-10% of total proteins in eukaryotic cells)

Answer 1

actin

Question 2

actin polymerizes to form () → thin, flexible fibers

Answer 2

actin filaments (microfilaments)

Question 3

each actin monomer () has tight binding sites that mediate head-to-tail interactions with 2 other monomers → forms a trimer (nucleation)

Answer 3

globular [G] actin

Question 4

polymerization of G actin leads to formation of ()

Answer 4

filamentous (F) actin

Question 5

why are actin filaments polar (i.e. have plus and minus ends)?

Answer 5

all actin monomers are oriented in the same direction

Question 6

describe “treadmilling” of actin filaments

Answer 6

ATP-actin is added to plus end while ADP-actin is dissociated from the minius end of a microfilament

Question 7

describe the general dynamics of actin filaments

Answer 7

1. the plus end of actin filaments elongates by the addition of ATP-actin monomers
2. actin bound to ATP associates with plus ends, ATP is then hydrolyzed to ADP
3. actin-ATP binds more readily to actin filaments compared to actin-ADP
4. ADP-actin found at the - end is less tightly bound, and thus actin monomers are more readily disassociated from this end

Question 8

actin-binding proteins regulate the ff:

Answer 8

• assembly and disassembly of actin filaments
• cross-linking of actin filaments into bundles and networks
• association of actin filaments to cell structures (e.g. plasma membrane)

Question 9

what are the principal proteins that stimulate the initiation and elongation of actin filaments?

Answer 9

1. formin
2. Arp2/3 complex (actin-related protein)

Question 10

proteins involved in the nucleation, initiation, and growth of actin filaments → they move along growing filament and add new ATP-acting monomers at plus end

Answer 10

formins

Question 11

binds actin monomers and stimulates exhange of bound ADP for ATP, increasing local concentration of ATP-actin → promotes actin polymerization

Answer 11

profilin

Question 12

initiates growth of branched actin filaments → important in driving cell movement at the plasma membrane

Answer 12

Arp2/3 complex

Question 13

actin-binding proteins that stabilize actin filaments by binding lengthwise along filament groove

Answer 13

tropomyosins

Question 14

actin-binding proteins that stabilize actin by binding to the plus or minus ends

Answer 14

capping proteins

Question 15

actin-binding proteins that associate microfilaments into bundles or networks

Answer 15

cross-linking proteins

Question 16

() are filaments cross-linked into parallel arrays

Answer 16

actin bundles

Question 17

() are filaments cross-linked in arrays that form 3D meshworks (have properties of semisolid gels)

Answer 17

actin networks

Question 18

actin-binding protein that severs filaments → generates new ends which are then available for polymerization or depolymerization

Answer 18

cofilin

Question 19

a drug that affects actin polymerization by binding to plus ends and block elongation → inhibits cell movement and even cell division

Answer 19

cytochalasins

Question 20

a drug that affects actin polymerization by binding to actin filaments and prevents dissociation

Answer 20

phalloidin

Question 21

network of microfilaments and associated proteins under the plasma membrane

Answer 21

cell cortex

Question 22

member of calponin family and is the major protein that provides the structural basis for the cortical cytoskeleton; a tetramer of two polypeptides, ⍺ and β

Answer 22

spectrin

Question 23

spectrin ends associate with short actin filaments → forms the ()

Answer 23

spectrin-actin network

Question 24

links the spectrin-actin network and the plasma membrane by binding to spectrin and band 3 (transmembrane protein)

Answer 24

ankyrin

Question 25

ankyrin binds to spectrin and ()

Answer 25

band 3 transmembrane protein

Question 26

() is another link that binds spectrin-actin junctions and glycophorin (transmembrane protein)

Answer 26

protein 4.1

Question 27

protein 4.1 binds spectrin-actin junctions and ()

Answer 27

glycophorin

Question 28

links actin filaments to transmembrane proteins that link to the extracellular matrix → maintains cell stability during muscle contraction

Answer 28

dystrophin

Question 29

what is the cause of Duchenne’s muscular dystrophy?

Answer 29

absence (complete KO) of dystrophin

Question 30

what is the cause of Becker’s muscular dystrophy?

Answer 30

abnormal expression of dystrophin

Question 31

specialized plasma membrane regions of fibroblasts that form contacts with the adjacent cells’ extracellular matrix

Answer 31

focal adhesions

Question 32

in focal adhesions, fibroblast cells attach to extracellular matrix by binding to transmembrane proteins called ()

Answer 32

integrins

Question 33

for fibroblasts, focal adhesions are also the attachment sites for large actin bundles called ()

Answer 33

stress fibers

Question 34

stress fibers are contractile fibers, cross-linked by (1) and stabilized by (2)

Answer 34

1. alpha-actinin
2. tropomyosin

Question 35

what proteins are involved in stress fiber binding in focal adhesions

Answer 35

1. talin - interacts with vinculin and integrin
2. vinculin - interacts with talin and actin filament

Question 36

cell-cell contacts in epithelial cell sheets

Answer 36

aderens junctions

Question 37

the continuous belt of cell-cell contacts around each epithelial cell in epithelial cell sheets

Answer 37

adhesion belt

Question 38

what are the proteins that are involved cell-cell contact in adherens junctions

Answer 38

1. cadherins - transmembrane proteins that mediate cell-cell contact by binding to cytoplasmic catenins
2. catenins - interact with actin filament and cadherins, anchoring the actin filaments to the plasma membrane

Question 39

extensions of the cell surface are usually based on (), either in permanent or transient conformations; the latter usually form in response to environmental signals

Answer 39

actin filaments

Question 40

cell surface extensions of moderate width; responsible for phagocytosis and movement of amoebas

Answer 40

pseudopodia

Question 41

broad, sheet-like extensions at the leading edge of fibroblasts

Answer 41

lamellipodia

Question 42

projections of the plasma membrane supported by actin bundles

Answer 42

filopodia

Question 43

fingerlike extensions on plasma membranes that are abundant on cell surfaces involved in absorption (e.g. epithelial cells); serve to increase surface area of the cell

Answer 43

microvilli

Question 44

in the epithelial cells lining the intestine, microvilli form a () (about 1000 microvilli per cell) on apical surface of cell

Answer 44

brush border

Question 45

intestinal microvilli contain parallel bundles of 20-30 microfilaments cross-linked by (1) and (2)

Answer 45

1. fimbrin
2. villin

Question 46

actin bundles of intestinal microvilli are attached to the plasma membrane by the calcium-binding protein (1), in association with (2)

Answer 46

1. calmodulin
2. myosin I

Question 47

how are extensions of the plasma membrane driven by branching and polymerization of underlying actin filaments

Answer 47

• growing actin filaments push against plasma membranes and drive formation of protrusions
• inhibition of actin polymerization blocks the formation of cell surface protrusions

Question 48

cell movement across a surface proceeds in 3 stages

Answer 48

1. extension of leading edge
2. attachment of leading edge to substratum
3. retraction of trailing edge

Question 49

like other cell surface protrusions, extension of the leading edge during cell motility involves

Answer 49

branching and polymerization of actin filaments

Question 50

formation of cell surface protrusions in response to external stimuli is mediated by small (), which are activated by signals stimulating cell movement

Answer 50

GTP-binding Rho proteins

Question 51

Rho proteins promote actin polymerization by:

Answer 51

• stimulating Arp2/3 complex (initiates growth of branched actin filaments)
• activating formins (initiates growth of linear filaments)
• activating WASP

Question 52

what is the role of cofilin in cell motility

Answer 52

• by cleaving existing filaments, it creates new plus ends to support filament branching and growth

Question 53

as new actin filaments extend into the growing cell, they also provide pathways for vesicles containing ()

Answer 53

lipids and proteins needed for continued extensions

Question 54

for slow-moving cells, cell attachment to surface requires ()

Answer 54

rebuilding cell-substratum adhesions (e.g. focal adhesions)

Question 55

retraction of the trailing edge into the cell body involves the action of (1) and (2)

Answer 55

1. small GTP-binding Arf proteins
2. Rho proteins

Question 56

what is the role of Arf GTPases in the retraction of the trailing edge

Answer 56

control membrane endocytosis and regulate Rho proteins to modulate actin polymerization and dynamic reorganization

Question 57

what is the role of Rho GTPases in the retraction of the trailing edge

Answer 57

causing the formation of stress fibers and actomyosin contractility providing tension for the cell to retract its tail and move forward