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Molecular Biology - Unit 2 - Zach H. > Cytoskeleton II - Organization and Behavior > Flashcards

Cytoskeleton II - Organization and Behavior Flashcards

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1
Q

What is Listeria monocytogenes?

A

Pathogenic bacteria that invade your intestinal cells, divide into new cells, and exhibit an interesting behavior.

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2
Q

True of False:

Most cells in the human body are bacteria.

A

True

10X more than human cells

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3
Q

What does Listeria monocytogenes cause?

A

Food poisoning, especially if you are immunologically deficient or immunocompromised.

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4
Q

Where does Listeria monocytoenes attach to on enterocytes?

A

Listeria attaches to receptors on enterocytes - and then enters and replicates in your intestinal cells.

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5
Q

What makes up the comet tails that Listeria leaves behind?

A

Actin filaments make up the comet tails associated with the bacteria.

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6
Q

List the accessory proteins that control assembly and positioning of cytoskeletal filaments of actin subunits.

A
  • Formin
  • Thymosin
  • Profilin
  • ARP Complex
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7
Q

List the accessory proteins that control assembly and positioning of cytoskeletal filaments of actin filaments.

A
  • Cofilin
  • Gelsolin
  • Capping Protein
  • Tropomyosin
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8
Q

What does the accessory protein of actin subunits formin do?

A

Nucleates assembly + remains associated with growing plus end (remember - for new large actin filaments to form, subunits must assemble into initial aggregate or nucleus - nucleation).

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9
Q

What does the accessory protein of actin subunits thymosin do?

A

Binds subunits and prevents assembly.

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10
Q

What does the accessory protein of actin subunits profilin do?

A

Binds subunits and speeds elongation.

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11
Q

What does the accessory protein of actin subunits ARP Complex do?

A

ARP (actin related protein) nucleates assembly to form a web and remains associated with minus end.

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12
Q

What does the accessory protein of actin filaments cofilin do?

A

Binds ADP-actin filaments and accelerates disassembly.

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13
Q

What does the accessory protein of actin filaments gelsolin do?

A

Severs actin filaments and binds to plus end.

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14
Q

What does the accessory protein of actin filaments capping protein do?

A

Prevents assembly and disassembly at plus end.

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15
Q

What does the accessory protein of actin filaments tropomyosin do?

A

Stabilizes filament.

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16
Q

List the accessory proteins that are involved in filament bundling, cross linking, and attachment to membranes.

A
  • Fimbrin
  • alpha-actinin
  • filamin
  • spectrin (RBC cytoskeleton and HS)
  • ERM Family:
    > Ezrin
    > Radixin
    > Moesin
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17
Q

List the accessory proteins that are involved in tubulin dimers.

A
  • Staminin
  • gamma-TuRC (gamma-tubulin ring complex)
  • TIPS (plus end tracking proteins)
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18
Q

List the accessory proteins that are involved in microbubules.

A
  • Kinesin 13
  • Katanin
  • MAPS (microtubule associated protein)
  • XMAP215
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19
Q

List the accessory proteins that are involved in filament cross linking of microtubules.

A
  • Tau (a MAP protein); MAP2

- Plectin

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20
Q

What is the function of the accessory protein, plectin?

A

Cross-linking protein that links microtubules to intermediate filaments.

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21
Q

What is the function of the accessory protein, tau and MAP2?

A

Both cause bundling of microtubules.

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22
Q

What is the function of the accessory protein, XMAP215?

A

A microtubule associated protein that stabilizes plus ends and accelerates assembly.

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23
Q

What is the function of the accessory protein, MAPS?

A

MAPS (microtubule associated protein) - stabilizes tubules by binding along sides.

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24
Q

What is the function of the accessory protein, katanin?

A

(Japenese word for “sword”) - severs microtubules.

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25
Q

What is the function of the accessory protein, kinesin 13?

A

Enhances catastrophic disassembly at plus end.

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26
Q

What is the function of the accessory protein, TIPS (plus end tracking proteins)?

A

Remains associated with growing plus ends, and can link them to other structures (e.g., membrane).

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27
Q

What is the function of the accessory protein, gamma-TuRN (gamma-tubulin ring complex)?

A

Mucleates assembly and remains associated with minus end.

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28
Q

What is the function of the accessory protein, staminin?

A

Binds subunits and prevents assembly.

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29
Q

Alpha- and beta-tubulin are the regular subunits for microtubules, but what is gamma-tubulin involved with?

A

Involved in the nucleation of microtubule growth.

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30
Q

What is the specific location that microtubules are nucleated from?

A

microtubule-organizing center (MTOC)

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31
Q

Microtubules grow outward from which end of the MTOC?

A

Microtubules grow outward from the MTOC from the plus end.

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32
Q

What is gamma-tubulin ring complex (gamma-TuRN) accessory protein responsible for?

A

The nucleation of microtubule growth.

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33
Q

What does the ring caused by the gamma-TuRC serves as a template for?

A

Template that creates a microtubule with 13 proto-filaments.

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34
Q

What is the major MTOC of animal cells called?

A

centrosome

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35
Q

Where is the centrosome located?

A

In the cytoplasm next to the nucleus.

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36
Q

What does the centrosome consist of?

A

Consists of a fibrous centrosome matrix to which the gamma-TuRCs are attached - greater than 50 copies of gamma-TuRCs.

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37
Q

What is embedded in the centrosome?

A

A pair of cylindrical structures, called centrioles.

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38
Q

What is the function of the centrioles?

A

They organize the centrosome matrix, ensuring its duplication during each cell cycle as the centrioles themselves duplicate.

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39
Q

Where are microtubules nucleated at?

A

Microtubules ARE NUCLEATED AT THE CENTROSOME AT THEIR MINUS END, with plus ends pointing outward and grow toward the cell periphery.

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40
Q

True of False:

Microtubules emanate in a star-like, astral conformation from centrosome.

A

True

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41
Q

True or False:

Microtubules are nucleated at the centrosome at their minus end, with the plus ends pointing outward and grow toward the cell periphery.

A

True

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42
Q

Where does the nucleation of actin filaments occur?

A

At or near the plasma membrane, thus actin filaments mostly accumulate at the cell periphery.

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43
Q

Where do actin filaments mostly accumulate?

A

At the cell periphery.

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44
Q

What determines the shape and movement of the cell surface?

A

Actin filaments in cell cortex determine the SHAPE and MOVEMENT of the cell surface.

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45
Q

True of False:

Actin filaments can form many different cell surface projections that help move cells over solid substrate.

A

True

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46
Q

What two types of regulated factors catalyses nucleation of actin filaments?

A
  • ARP Complex

- Formins

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47
Q

Actin filaments can form many different cell surface projections that help move cells over solid substrate. What are these different forms?

A
  • Lamellipodia, flat protrusive veils.

- Filopodia or microvilli, spiky bundles.

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48
Q

What percentage of two ARP proteins, Arp2 and Arp3, are identical to actin?

A

are 45% identical to actin

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49
Q

What other accessory protein does the two ARP proteins, Arp2 and Arp3, function similar to?

A

gamma- TuRC

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50
Q

What does the ARP complex do?

A

Nucleates actin filaments growth from the minus end, allowing elongation at the plus end.

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51
Q

You know that the ARP complex nucleates actin filaments growth from the minus end, allowing elongation at the plus end, but what else does this require?

A

activating factor

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52
Q

What happens to Arp2 and Arp3 in the absence of activating factor?

A

Arp2 and Arp3 are masked by their accessory proteins to prevent them from nucleating a new actin filament accessory proteins to prevent them from nucleating a new actin filament.

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53
Q

What does the binding of Arp2 and Arp3 to their activating factor induce?

A

Conformation change that resembles the plus end of actin filament, allowing actin monomers to bind, BYPASSING THE RATE-LIMITING STEP OF FILAMENT NUCLEATION.

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54
Q

How do Listeria and the ARP (Arp2/3) complex interact?

A
  • Bacteria surface causes local nucleation of actin filaments.
  • Listeria presents a surface protein called ActA.
  • ActA activates Arp2/3 complex - which causes local nucleation of actin filaments which are cross linked.
  • Growing filaments are the driving force to push cell through cytoplasm.
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55
Q

Listeria presents a surface protein called ActA. What does this protein do?

A

Activates Arp 2/3 complex - which causes local nucleation of actin filaments which are cross linked.

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56
Q

What are growing filaments the driving force for in Listeria?

A

To push cell through cytoplasm.

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57
Q

When does the ARP complex work the most efficiently?

A

When it is bound to the side of a preexisting actin filament - filaments cross linked.

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58
Q

At what degree does the filament that is branched off of the original filament grow?

A

70 degrees

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59
Q

What does repeated rounds of branching nucleation by ARP complex result in?

A

A highly branched web of actin filaments.

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60
Q

What does the accessory protein formin do?

A

Nucleate the growth of straight and un-branched actin filaments.

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61
Q

What does each fromin subunit have a binding site for?

A

actin monomer

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62
Q

True or False:

Formins are a large family of dimeric proteins (mouse has 15 formin genes).

A

True

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63
Q

How do formin dimers nucleate actin filament polymerization?

A

By capturing two monomers at the plus end or the growing end of an actin filament.

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64
Q

What does formin protein do after it forms a dimeric complex?

A

Nucleate the formation of a new actin filament and remain associated with the rapidly growing plus end as it elongates.

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65
Q

How do formins and gamma-TuRCs differ?

A

Formin dimers nucleate actin filament polymerization by capturing two monomers at the plus end or the growing end of an actin filament.

Gamma-TuRCs binds to minus end not plus end of filament - in microtubules.

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66
Q

How is there a large pool of actin polymers always kept available with all the nucleation of actin filaments occurring in the cell?

A

Thymosin - keeps actin monomers soluble so they are readily available for generating filaments.

Actin monomers bound to thymosin are in locked state, where they cannot associate with actin filaments, this causes high concentration of soluble actin monomers in cells.

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67
Q

What does the accessory protein profilin do?

A

Recruits actin monomers to the actin filament for polymerization.

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68
Q

How does profilin recuit actin monomers to the actin filament for polymerization?

A

> Profilin binds to the actin monomer to expose the site of actin that binds to the plus end of the actin filament.

> The addition of an action monomer to the filament induces conformation change in the actin that reduces its affinity for profilin.

> So the profilin falls off, leaving actin filament one subunit longer.

> Profilin competes with thymosin in binding to individual actin monomers.

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69
Q

What two accessory proteins are involved in the regulation of availability of actin monomers for actin polymerization?

A
  • Thymosin

- Profilin

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70
Q

What do MAPs do in the regulation of stability of microtubule filaments?

A

MAPs stabilize microtubules against disassembly, contain at least two domains, one domain for binding to microtubule and another that project outward.

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71
Q

True or False:

Microtubule-associated proteins (MAPs) are the proteins that bind along the sides of microtubuels and alter the filament’s stability and mechanical properties.

A

True

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72
Q

Compare and contrast MAP2 vs. Tau in the regulation of stability of microtubule filaments.

A

> MAP2 - has a long projecting domain with a second microtubule-binding domain at the other end, thus forming bundles of stable microtubules that are kept widely spaced.

> Tau - binds to the microtubule at both its N- and C-termini, with a short projecting loop, forming bundles of more closely packed microtubules.

73
Q

Compare and contrast tropomyosin vs. cofilin in the regulation of stability of actin filaments.

A

Stabilization - Actin filaments are stabilized by the binding of tropomyosin (an elongated protein); binding of tropomyosin prevents the actin filament interacting with other proteins (tropomyosin is a key protein in erythrocye membrane skeleton).

Disassembly - Cofilin, also called actin depolymerizing factor, binds to both actin filament and free actin subunits.

74
Q

What does the binding of cofilin force the filament to do?

A

Twist a little more tightly, weakens the contacts between actin subunits, making the filament brittle and more easily severed.

75
Q

What does cofilin prefer to bind to?

A

ADP-containing actin filaments, thereby efficiently disassembling the older filaments.

76
Q

What accessory protein protects actin filaments from cofilin binding to them?

A

tropomyosin binding potects actin filaments from cofilin binding to them.

77
Q

What accessory proteins are considered catastrophe factors (change from growth to rapid shrinkage)?

A
  • Kinesin-13 protein family

- XMAP215

78
Q

What does the catastrophe factor, XMAP215, function to do?

A

Stabilizes free ends of microtubule and inhibits the switch from a growing to a shrinking state.

79
Q

What does the catastrophe factor, kinesin-13, function to do?

A
  • Increase the rate at which a microtubule switches from a growing to a shrinking state.
  • They bind to microtubule ends and pry proto-filaments apart by lowering the activation energy barrier that prevents a microtubule from springing apart into the curved proto-filament.
80
Q

How do kinesin-13 accessory proteins pry protofilaments apart?

A

By lowering the activation energy barrier that prevents a microtubule from springing apart into the curved protofilament.

81
Q

What are the two different types of actin filament organization?

A

> Bundle

> Web-like

82
Q

What accessory protein mediates the bundle organization of actin filaments?

A

formin

83
Q

What accessory protein mediates the web-like organization of actin filaments?

A

ARP complex

84
Q

What do actin filament cross-linking proteins help to do?

A

Stabilize and maintain the bundle and web-like structures of actin filaments.

85
Q

What are the two classes of actin filament cross-linking proteins?

A
  • Bundling protein -> cross links actin filaments into a parallel array, alpha-actinin, fibrin, and villin.
  • Gel-forming protein -> holds two actin filaments together at a large angle to each other to create a looser meshwork.
86
Q

List the three bundling proteins, which are actin filament cross-linking proteins.

A
  • alpha-actinin
  • fimbrin
  • villin
87
Q

Compare and contrast the functions of alpha-actinin and fimbrin.

A

alpha-actinin -> cross-links actin filaments into loose bundles, allowing myosin II to enter to make actin filaments contractile.

fimbrin -> cross-links actin filaments into tight bundles, excluding myosin II.

alpha-actinin and fimbrin tend to exclude one another because of different functions

88
Q

What is the function of the accessory protein, villin?

A

Villin, like fimbrin, helps to cross-link many tightly bundled actin filaments.

  • found in microvilli
89
Q

Where is villin found?

A

microvilli

90
Q

What are microvilli, where villin is found?

A

Microvilli are the finger-like extensions of the plasma membrane on the surface of many epithelial cells.

91
Q

What connects the sides of actin filament bundles to the plasma membrane?

A

lateral sidearms (myosin-I and calmodulin)

92
Q

What does the accessory protein, filamin promote?

A

The formation of a loose and highly viscous gel-like network by clamping together 2 actin filaments roughly at right angles.

93
Q

Why do cells require the actin gel formed by filamin?

A

In order to extend membrane projections which help cells to crawl across a solid surface.

94
Q

What can cells that lack filamin not do?

A

Cannot crawl properly, and they produce disorganized membrane blebs.

95
Q

What does the ERM (including ezrin, radixin, and moesin) family of proteins mediate?

A

Mediates the attachments between actin and plasma membrane.

96
Q

What is the activation of ERM proteins regulated by?

A

Both intracellular and extracellular signals - phosphorylation activates ERM.

97
Q

You know that the activation of ERM proteins is regulated by both intracellular and extracellular signals - phosphorylation activates ERM. Which two binding sites are exposed?

A

One to bind to an actin filament and one to bind to a transmembrane protein.

98
Q

What protein functions in the attachment of cortical actin cytoskeleton to plasma membrane?

A

ERM (including ezrin, radixin, and moesin)

99
Q

Review slide 63!

A

Summary of Accessory Proteins for Actin Filaments

100
Q

What accessory proteins function on actin subunits?

A

> profilin
ARP complex
formin
thymosin

101
Q

What accessory proteins function on actin filaments?

A 
> tropomyosin
> ERM
> filamin
> alpha-actinin
> fimbrin
> cofilin
102
Q

What accessory proteins function on microtubule filaments?

A 
> XMAP215
> MAPs
> MAP-2
> Tau
> Kinesin 13
103
Q

What accessory proteins function on alpha/beta tubulin dimer?

A

> gamma-TuRC

104
Q

What do cytoskeletal motor proteins do?

A

> Bind to a polarized cytoskeletal filaments and use the energy derived from repeated cycles of ATP hydrolysis to move steadily along it.

> Carry membrane-enclosed organelles (cargo) to their appropriate locations in the cell.

> Cause cytoskeletal filaments to exert tension, or to slide against each other, generating force that drives muscle contraction, or cell division.

> Motor proteins associate with their filament tracks through a “head” region, or motor domain, that binds and hydrolyzes ATP.

> The motor domain (head) determines the identity of the track and the direction of movement along it.

> The tail determines the identity of the cargo (represents the biological function of the motor protein).

105
Q

What does the motor domain (head) of cytoskeletal motor proteins determine?

A

Determines the identity of the track and the direction of movement along it.

106
Q

What does the tail of the cytoskeletal motor protein determine?

A

Determines the identity of the cargo (represents the biological function of the motor protein).

107
Q

How do motor proteins associate with their filament tracks?

A

Through a “head” region, or motor domain, that binds and hydrolyzes ATP.

108
Q

What was the first motor protein identified?

A

Skeletal muscle myosin: myosin II.

109
Q

What does the structure of myosin II, an actin-based motor protein, consist of?

A

Consists of 2 heavy chains (coiled coil of two alpha helices) and 4 light chains.

110
Q

What does each heavy chain of myosin II have that binds ATP?

A

Each heavy chain has a globular head (motor) domain at its N-terminus that binds ATP, followed by a very long alpha-helix that forms an extended coiled-coil that mediates heavy chain dimerization.

111
Q

Where does the 2 light chains of myosin II bind close to?

A

Close to the N-terminus head domain.

112
Q

What does the long coiled-coil tail bundle itself with?

A

The tails of other myosin molecules.

113
Q

What do the tail-tail interactions between myosin II molecules form?

A

Form thick filaments that have lots of myosin heads oriented in opposite directions and projected to the outside of filament.

114
Q

True of False:

Each myosin head binds and hydrolyzes ATP, using the energy of ATP hydrolysis to walk toward the plus end of an actin filament.

A

True

the ATP-driven sliding of actin filaments results in contraction

115
Q

What is does kinesin do?

A

Protein that uses ATP to walk along a microtubule track to move vesicles.

116
Q

What does kinesin consist of?

A

Consist of 2 heavy chains and 2 light chains per active motor, these form 2 globular headmotor domains and an elongated coiled-coil tail responsible for heavy chain dimerization.

117
Q

True or False:

The kinesin family is a large protein family in which the motor domain is the only common element.

A

True

118
Q

What do most kinesin protein families have?

A

Most of them have the motor domain at the N-terminus of the heavy chain and walk toward the plus end of microtubule.

119
Q

True of False:

Kinesin proteins carry a binding site in the tail for a membrane-enclosed organelle.

A

True

120
Q

What are the two major branches of the dynein motor protein?

A
  • cytoplasmic dyneins

- axonemal dynein

121
Q

What are the characteristics of cytoplasmic dynein?

A
  • heavy-chain homodimers, with 2 large motor domains as heads
  • Function: vesicle trafficking, localization of the Golgi apparatus near the center of the cell.
122
Q

What are the characteristics of axonemal dynein?

A
  • heterodimers and heterotrimers, with 2 or 3 motor-domain heads
  • Function: specialized for rapid and efficient sliding movements of microtubule that drive the beating of cilia and flagella.
123
Q

What is the largest known molecular motor protein and the fastest?

A

Dynein

124
Q

What are dynein motor proteins composed of?

A

2 or 3 heavy chains (include the motor domain) and a large and variable number of associated intermediate chains and light chains.

125
Q

Is dynein or kinesin a minus-end-directed microtubule motor?

A

Dynein

126
Q

What is the function of cytoplasmic dyneins?

A

Vesicle trafficking, localization of the Golgi apparatus near the center of the cell.

127
Q

What is the function of axonemal dyneins?

A

Specialized for rapid and efficient sliding movements of microtubule that drive the beating of cilia and flagella.

128
Q

What causes the power stroke, which moves the actin filament?

A

Phosphate release causes the power stroke, which moves the actin filament.

129
Q

What are the steps in how motor proteins work?

A
  • attached
  • released
  • cocked
  • force-generating
  • attached
130
Q

What stage of how motor proteins work is this?

At the start of the cycle, a myosin head lacking a bound nucleotide is locked tightly onto an actin filament in a rigor configuration. In an actively contracting muscle, this state is very short-lived, being rapidly terminated by the binding of a molecule of ATP.

A

Attached

131
Q

What stage of how motor proteins work is this?

A molecule of ATP binds to the large cleft on the “back” of the head and immediately causes a slight change in the conformation of the domains that make up the actin-binding site. This reduces the affinity of the head for actin and allows it to move along the filament.

A

Released

132
Q

What stage of how motor proteins work is this?

The cleft closes like a clam shell around the ATP molecule, triggering a large shape change that causes the head to be displaced along the filament by a distance of about 5 nm. Hydrolysis of ATPoccurs, but the ADP and inorganic phosphate produced remain tightly bound to the protein.

A

Cocked

133
Q

What stage of how motor proteins work is this?

A weak binding of the myosin head to a new site on the actin filament causes release of the inorganic phosphate produced by ATP hydrolysis, concomitantly with the tight binding of the head to actin. This release triggers the power stroke - the force-generating change in shape during which the head regains its original conformation. In the course of the power stroke, the head loses its bound ADP, thereby returning to the start of a new cycle.

A

Force-Generating

134
Q

How does motor proteins work (unidirectional movement)?

A
  • works by coupling ATP hydrolysis to conformational change.
  • each cycle propels the motor protein forward in a single direction along a filament to a new binding site on the filament.
  • each conformational change is coupled to change in the binding affinity.
135
Q

What does most cell behavior depend on?

A

Labile structures that appear at specific stage of the cell cycle or in response to external signals then disappear once they complete their jobs.

136
Q

What does most cell behavior require?

A

Requires coordinated processes, the dynamic assembly and disassembly of cytoskeletal filaments, the regulation and modification of their structure by accessory proteins, and the action of motor proteins moving along the filaments.

137
Q

What is cell migration or crawling, a highly integrated multi-step process, important in?

A

> embryonic morphogenesis
tissue repair and regeneration
immune surveillance

138
Q

Does cell crawling also have a role in many cancers?

A

Yes - when cells in a primary tumor invade neighboring tissues and crawl into blood vessels or lymph vessels and are thereby carried to other sites in the body to form metastases.

139
Q

In animals, almost all cell locomotion occurs by what method?

A

crawling

140
Q

What must cells produce?

A

polarity - front vs. rear

cell must move forward but also must drag rear - retraction of rear of cell

141
Q

Cell migration can be conceptualized as a cyclic process, involving three distinctive activities. What are these three activities?

A

> Polarization and Protrusion
Adhesion (attachment) and Traction
Re-traction

142
Q

In crawling, what is polarization and protrusion?

A

The molecular processes at the front and the back of a moving cell are different. Actin-rich structures are pushed out at the front of the cell.

143
Q

In crawling, what is adhesion and traction?

A

Adhere to extracellular matrix (ECM) or adjacent cells via transmembrane receptors linked to the actin cytoskeleton. Adhesions serve as traction sites for migration as the cell moves forward over them.

144
Q

In crawling, what is re-traction?

A

Adhesions are disassembled at the cell rear, allowing the bulk of the railing cytoplasm to be drawn forward.

145
Q

There are 3 different types of protrusive structures, all of which have a dense core of actin filaments and no membrane-enclosed organelles. What are the 3 types?

A

> Filopodia (microspikes)
Lamellipodia
Pseudopodia

146
Q

What are pseudopodia formed by?

A

By amoebae and neutrophils.

147
Q

What is a pseudopodia?

A

Three-dimensional projections filled with an actin-filament gel.

148
Q

What are lamellipodia formed by?

A

By epithelial cells, fibroblasts, and neurons.

149
Q

What are lamellipodia?

A

Two-dimensional, sheet-like structure.

150
Q

What do lamellipodia contain?

A

Contains cross-linked mesh of actin filaments, most lie in a plane parallel to the solid substratum.

151
Q

What are filopodia (microspikes) formed by?

A

migrating fibroblasts

152
Q

What are filopodia?

A

one-dimensional and contain a core of long, bundled actin filaments.

153
Q

What do filopodia contain?

A

Contain a core of long, bundled actin filaments.

154
Q

Actin filaments are polarized in lamellipodia of a keratocyte.

A

Remember - the ARP complex nucleates actin filaments growth from the minus end, allowing elongation at the plus end.

Attaches at 70 degree angle and forms actin web.

The ARP complex is highly concentrated near the front of the lamellipodia, here actin nucleation is most common.

155
Q

What is the role of coffilin in lamellipodia?

A

> Filament nucleation is at the leading edge.
Filament depolymerization occurs at sites well behind the leading edge.
Remember - cofilin binds ADP-actin filaments, accelerates disassembly.
The actin web unergoes treadmilling: assembling at the front and disassembling at the back.
This polarity is a driving force of membrane protrusion.

156
Q

Mechanism of protrusion of the actin meshwork at the leading edge.

A

> Nucleation is at the front and newly nucleated actin filaments are attached to the sides of pre-existing filaments (70 degree angle).
Actin polymerization is mediated by the ARP2/3 complex by binding to the sides or tip of pre-existing filaments.
FILAMENTS ELONGATE, PUSHING THE PLASMA MEMBRANE FORWARD.
After newly polymerized actin subunits hydrolyze their bound ATP in the filament lattice, the filaments become susceptible to depolymerization by cofillin.
This cycle causes a spatial separation and the delayed ATP hydrolysis set the basis to maintain an UNIDIRECTIONAL TREADMILLING PROCESS IN THE LAMELLIPODIUM.

157
Q

What is chemotaxis?

A

A cell movement in a direction controlled by a gradient of a diffusible chemical.

158
Q

Give an example of chemotaxis?

A

Neutrophils, toward a source of bacterial infection by detecting peptides that are derived from bacteria proteins.

159
Q

Regulation of cell migration by external signals.

A

External signals trigger a family of GTPase monomer proteins called the Rho family to set up cell polarity by influencing the organization of the apparatus required for cell migration.

160
Q

What is the cycle of Rho GTPases?

A

> A variety of cell-surface receptors trigger global structural rearrangements in response to external signals.
All the external signals converge inside the cell on monomeric GTPases: the Rho monomeric GTPase family - Cdc42, Rac, and Rho.
Rho proteins act as molecular switches to control cell processes by cycling between an active, GTP-bound form and inactive, GDP-bound form.

161
Q

What does Rac activation promote?

A

Promotes actin polymerization at the cell periphery leading to the formation of lamellipodia extensions.

162
Q

What does activation of Cdc42 trigger?

A

Triggers actin polymerization and bundling to form filopodia.

163
Q

What does Rho activation promote?

A

Promotes the bundling of actin filaments into stress fibers.

164
Q

What is the major targets for Rac, which mediates actin polymerization in protrusions?

A

Are the WASp (Wiskott-Aldrich Syndrome protein) family of ARP complex activators (nucleation of actin filament growth).

Rac also activates PAK, which activates filamin (web-forming cross-linker) and inactivates myosin light chain kinase (MLCK).

165
Q

What does decreased MLC phophorylation lead to?

A

Myosin II filament disassembly and a decrease in contractile activity.

166
Q

What does the activation of Rho lead to?

A

Nucleation of actin filaments by formins and increases contraction by myosin II, promoting the formation of contractile actin bundles - increaes in contractile activity - pulls cell from the rear.

167
Q

What does the activation of myosin II by Rho require?

A

ROCK (Rho kinase), by inhibiting MLC phosphatase or directly phosphorylating MLC.

168
Q

What does ROCK activate?

A

LIM kinase, which in turn contributes to the formation of stable contractile actin filament bundles by inhibiting cofilin.

169
Q

What is the molecular mechanism during chemotaxis?

A

> at the front, binding of the chemo-attractant ligand to its G-protein coupled receptor which generates PIP3 that in turn activates Rac; Rac then activates ARP complex and lamellipodial protrusion.

> the same receptor also stimulates G12/13-protein coupled receptor, that triggers the activation of Rho.

> the two pathways are mutually antagonistic, thus, Rac activation dominates in the front and Rho activation stays in the rear.

> This enables the cell to maintain its functional polarity with protrusion at the leading edge and contraction at the back.

170
Q

Where are microtubules primarily nucleated at and what is this mediated by?

A

At the centrosome, is mediated by gamma-tubulin

171
Q

Where are actin filaments nucleated near and what mediates this?

A

Near the plasma membrane and mediated by ARP complex and formin.

172
Q

True or False:

The kinetics of filament assembly and disassembly are controlled by different accessory proteins that bind to free subunits or the filaments themselves.

A

True

173
Q

True or False:

Some of actin accessory proteins assemble filaments into larger structure by cross-linking filaments.

A

True

174
Q

Where do motor proteins get their energy from to move along microtubule or actin filaments?

A

ATP hydrolysis

175
Q

What do motor proteins mediate?

A

The sliding of filaments and the transport of cargo along filament tracks.

176
Q

What are myosin superfamily members?

A

Motor proteins moving on actin filaments.

177
Q

What does cell crawling, important in embryonic development and wound healing, require?

A

The generation and maintainance of structure polarity, which is influence by external signals.

178
Q

During cell migration, protrusion is formed at the leading edge (by assembly of new actin filaments), newly protruded part adheres to the substratum, motor proteins (myosin II) generate contractile force to move cell body forward.

A

True

179
Q

For a cell to migrate, what must it generate and maintain?

A

Must generate and maintain an overall structure polarity, which is influenced by external signals.

Molecular Biology - Unit 2 - Zach H. (11 decks)

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