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Molecular Cell Biology > (3) Chapter 13: The Cytoskeleton and Cell Movement > Flashcards

(3) Chapter 13: The Cytoskeleton and Cell Movement Flashcards

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1
Q

Describe the structure of the cytoskeleton

A
  • a network of protein filaments than extends throughout the cytoplasm of eukaryotic cells
  • not rigid
  • dynamic structure that is continually reorganized as cells move & change shape
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2
Q

What are functions of the cytoskeleton?

A

a) provides structural framework that determines cell shape & positions of organelles
b) responsible for general organization of the cytoplasm
c) movement of entire cells
d) internal transport of organelles and other structures

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3
Q

What are the three main protein filaments of the cytoskeleton?

A
  1. Actin filaments (microfilaments)
  2. microtubules
  3. intermediate filaments
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4
Q

What do Actin-binding proteins regulate?

A
  • Assembly & Disassembly of actin filaments
  • cross-linking into bundles & networks
  • associations with other cell structures
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5
Q

How many actin genes do mammals have? Where are they expressed?

A
  • 6 actin genes
  • 4 in muscle cells
  • 2 in nonmuscle cells
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6
Q

Describe the assembly and structure of actin filaments

A
  • each actin monomer (globular [G] actin has tight binding sites that mediate head to tail interactions with 2 other actin monomers to form filaments(filimentous [F] actin)
  • has 2 ends: pointed(ATP binds here) and barbed
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7
Q

How are actin monomers oriented? Why is this important to assembly?

A
  • actin monomers are oriented in same direction, so they have polarity
  • important in establishing direction of myosin movement relative to actin
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8
Q

Describe process of Actin polymerization

A
  • Nucleation Step: a trimer (of G actin) is formed and monomers are then added to either end
  • Process is reversible, filaments can be broken down if necessary
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9
Q

What is treadmilling?

A

-regulates actin filaments of the cell
-barbed end of filament grows 5-10 X faster then pointed end.
ADP

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10
Q

How is ATP hydrolyzed to ADP?

A

-actin bound to ATP associates with barbed ends of actin

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11
Q

ADP-actin

A

-less tightly bound than ATP-actin and dissociates at the pointed end

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12
Q

What are Formins

A
  • Actin binding proteins

- bind ATP-actin and nucleate initial polymerization of long unbranched actin filaments

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13
Q

What is profilin

A
  • actin binding protein

- binds actin monomers and stimulates exchange of bound ADP for ATP-actin

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14
Q

Arp 2/3 comp

A
  • actin-related proteins-

- initiate growth of branched actin filaments, important in driving cell movement at the plasma membrane

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15
Q

Tropomyosins

A

stabilize actin filaments by binding lengthwise along the groove of the filament

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16
Q

How do capping proteins stabilize actin?

A

-bind to the barbed or pointed ends

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17
Q

Cofilin

A

-severs filaments and generated new ends which are then available for polymerization of depolymerization

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18
Q

How do actin account for necessary cell movements and shape changes.

A

-actin binding proteins act together to promote rapid turnover of filaments and remodeling of the cytoskeleton which is needed for cell movements and shape change. -Their activities are controlled by signaling mechanisms in response to environmental signals

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19
Q

Actin bundles

A

filaments are cross-linked into closely packed parallel arrays

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20
Q

Actin networks

A

Filaments are cross-linked in arrays that form 3-D meshworks with properties of semisolid gels

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21
Q

Actin-bundling proteins

A

-small, rigid proteins that force filaments to align closely

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22
Q

Actin bundle types

A
  1. Parallel bundles

2. Contractile bundles

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23
Q

Parallel bundles

A
  • closely spaced filaments that are aligned in parallel, with the same polarity.
  • barbed ends are adjacent to the plasma membrane
  • uses fimbrin
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24
Q

Fimbrin

A

a bundling protein in parallel bundles

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25
Q

Contractile Bundles

A

-more widely-spaced filaments, cross-linked by alpha-actinin

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26
Q

What does increased spacing between filaments allow for?

A

-increased spacing between filaments allows motor protein myosin to interact with actin filaments

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27
Q

Filamin

A
  • forms flexible cross-links
  • “filamin dimer”
  • a flexible v-shaped molecule with actin-binding domains at the end of each arm
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28
Q

Cell Cortex

A

-network and associated proteins determines cell shape and is involved in activities such as movement

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29
Q

Why are RBC’s useful for studies of the cortical cytoskeleton?

A
  • have no nucleus or organelles, so plasma membranes and associated proteins are easily isolated
  • lack cytoskeletal components, so cortical cytoskelton is principal determinant of cell shape
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30
Q

Spectrin

A
  • a member of the calponin family of actin-binding proteins.
  • a tetramer of 2 polypeptide chains (alpha & beta).
  • ends of spectrin tetramers associate with short actin filaments, resulting in spectrin-actin network
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31
Q

Ankyrin

A

-links spectrin-actin network and the plasma membrane by binding to spectrin and a transmembrane protein

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32
Q

Give 2 examples of Ankyrin

A
  1. band 3

2. ex. protein 4.1 binds spectrin-actin juntion on transmembrane protein glycophorin

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33
Q

Stress fibers

A

contractile bundles

  • cross linked by alpha-actinin
  • stabilized by tropomyosin
  • assisted with the proteins talin and vinculin
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34
Q

Integrins

A

transmembrane proteins,

How fibroblasts attach to matrix

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35
Q

Focal adhesions

A

-site of attachment for large actin bundles called stress fibers

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36
Q

Adherens junctions

A
  • formed in sheets of epithelial cells

- cell to cell contacts that form an adhesion belt around each cell

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37
Q

Cadherins

A

-bind to cytoplasmic catenins, which anchor actin filaments to the plasma membrane

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38
Q

Microvilli

A
  • fingerlike extensions; abundant on cells involved in absorption
  • increase surface area for absorption by ten to twentyfold
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39
Q

Describe organization of microvilli

A
  • intestinal microvilli contain closely packed parallel bundles of 20 to 30 actin filaments
  • filaments are cross linked by fimbrin and villin
  • actin bundles attached to PM by Ca-binding protein calmodulin in association with myosin 1
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40
Q

Psudopodia

A

extensions of moderate width, responsible for phagocytosis and the movement of amoebas

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41
Q

Lamellipodia

A

-broad, sheetlike extensions at the leading edge of fibroblasts

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42
Q

Filopodia

A

thin projections of the plasma membrane supported by actin bundles

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43
Q

What are microtubules

A
  • rigid hollow tubes
    1. dynamic structures that undergo continual assembly and disassembly
    2. 13 protofilaments around a hollow core
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44
Q

What is the function of microtubules?

A

function in cell movement and determining cell shape

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45
Q

Describe the microtubule structure.

A
  • made of globular protein: Tubulin

- have polarity (plus and minus ends to determine direction of movement.

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46
Q

What is a protofilament?

A

head to tail arrays of tubulin dimers, arranged parallely

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47
Q

Tubulin dimer structure

A
  • have alpha tubulin and beta-tubulin.
  • each are encoded by related genes
  • has gamma-tubulin in centrosome to help initiate microtubule assembly
  • dimers polymerize and form microtubules
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48
Q

What happens to GTP bound to beta-tubulin shortly after polymerization? What does this process induce?

A

GTP is hydrolyzed to GDP; at the plus end.
-Process weakens binding affinity of tubulin dimers for each other and causes rapid depolymerization and loss of tubulin that was bound to GDP from the minus end.

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49
Q

What is dynamic instability?

A

Alternation between cycles of growth and shrinkage due to rapid GTP hydrolysis at the microtubule (-) end.

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50
Q

GTP cap function

A

-as long as GTP cap remains at the (+) end of the microtubule, growth continues

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51
Q

What happens if GTP is hydrolyzed more rapidly than new subunits are added to the microtubule?

A

GDP-bound tubulin at the (+) end of the microtubule leads to disassembly and shrinkage

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52
Q

What does the rapid turnover of microtubles allow for?

A

remodeling of cytoskeleton which occurs during mitosis.

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53
Q

What 2 drugs affect microtubule assembly?

A
  1. colchicine
  2. colcemid:microtubules disassemble
    - used as experimental tools and cancer treatments
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54
Q

What two drugs inhibit microtubule polymerization?

A
  1. Vincristine
  2. Vinblastine
    - both used in cancer chemotherapy bc they affect rapidly dividing cells
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55
Q

What drug stabilizes microtubules and blocks cell division?

A

Taxol

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56
Q

What are MAPs?

A

a) Microtubule associated proteins.
b) regulate growth and shrinkage of plus ends
c) Stabilize minus end of microtubules by proteins that prevent depolymerization

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57
Q

Centrosome

A
  • where most microtubules in animal cells are connected to
  • microtubules extend outward from centrosome
  • initiation sites for microtubule assembly
  • microtubles grow outward from minus ends anchored to centrosome
58
Q

What is the function of centrosome in mitosis?

A
  • microtubules extend outward from duplicated centrosomes to form the mitotic spindle
  • spindle helps seperate and distribute chromosomes to daughter cells
59
Q

g-tubulin ring complex

A
  • gamma tubulin in centrosome is associated with other proteins in a ring shaped structure
  • theory: complex bypasses rate-limiting nucleation step
60
Q

Centrioles

A

-cylindrical shape, with 9 triplets of microtubules
-also form basal bodies of filia and flagella
-a pair of centrioles is found in most animal cells
-

61
Q

pericentriolar material

A

surrounds centrioles that are oriented perpendiculat to each other

62
Q

Where does microtubule assembly found in plant cells, unicellular eukaryotes, and most meiotic animal cells?

A

-microtubule assembly is initiated by pericentriolar material

63
Q

How is microtubule stability regulated?

A
  • post-translational modification of tubulin by phosphorylation, and acetylation
  • modifications affect microtubule behavior by providing sites for binding of specific MAPs
  • MAPs allow cells to stabilize in specific locations and help determine cell shape and polarity
64
Q

MAP

A

microtubule assembly proteins

65
Q

Tau protein

A
  • MAP

- characteristic of lesions found in brains of Alsheimer patients

66
Q

What processes in nerve cells are supported by microtubules?

A

Axons

Dendrites

67
Q

How are microtubules arranged in axons?

A

-microtubules have plus ends towards tips that are associated with tau

68
Q

How are microtubules arranged in dendrites?

A

-microtubules are oriented in both directions and associated with MAP2

69
Q

What two families of motor proteins are responsible for powering movements in which microtubules participate?

A
  • Kinesins
  • Dyneins
  • thought to transport cargo in opposite direction
70
Q

Kinesin

A
  • helps to power movements

- move along microtubules toward the plus end

71
Q

Dyneins

A
  • help power movements

- Move along microtubules toward minus end

72
Q

What are roles of microtubules?

GIve an example

A
  • (main role) transport & position vesicles and organelles in the cytoplasm
  • ER extends to periphery of cell in association with microtubules (involves kinesin 1), drugs that depolymerize microtubules cause the ER retract toward the cell center
73
Q

Cilia

A
  • microtubule based
  • beat in a coordinated back-&-forth motion.
  • Either moves in the cell through fluid or moves fluid over the surface of the cell
74
Q

Flagella

A
  • microtubule based
  • longer than cilia
  • beat in a wavelike form
75
Q

What are cilia and flagella responsible for?

A

movement in eukaryotic cells

(some bacteria have flagella but they are protein filaments_

76
Q

Cilia & Flagella structural components (5)

A
  • axoneme
  • Nexin
  • Dynein
  • Basal body
  • 9+2 structure
77
Q

Axoneme

A

-conisists of microtubules in a 9+2 pattern, a central pair is surrounded by 9 outer doublets

78
Q

Explain structure of axoneme of cilia and flagella.

A
  • each of the 9 outer doublets in the 9+2 pattern arrangement is a complete A tubule fused to an incomplete B tubule.
  • Nexin links the tubules
  • 2 arms of dynein are attached to each A tubule
79
Q

Basal body

A
  • microtubule minus ends are anchored in a basal body
  • function: initiate growth of axonemal microtubules and anchor cilia and flagella to the surface of the cell
  • has 9 triplets of microtubules
80
Q

Axonemal Dyeneins

A

power motor activity of cilia and flagella

-movement occurs by sliding of outer microtubule doublets relative to one another via dynein binding and sliding

81
Q

How to dynein bind to tubules and move microtubules?

A
  • Dynein bases bind to A tubules, while head groups bind to B tubules of adjacent doublets
  • dyenein head groups move toward minus end of B tubule and microtubules bend
82
Q

Mitotic SPindle

A
  • the reorganization of microtubules during mitosis

- during mitosis, duplicated centrosomes migrate to from two poles of mitotic spindle

83
Q

What happens to microtubules during mitosis

A
  • rate of microtubule disassembly increases
  • microtubules shrink
  • # of microtubules emanating from 2 centrosomes increases
84
Q

What 4 types of microtubules make up the mitotic spindle?

A
  1. kinetochore
  2. chromosomal
  3. polar microtubules
  4. astral microtubules
85
Q

kinetochore microtubules

A

attach to the condensed chromosomes at the centromeres to stabalize them

86
Q

Chromosomal microtubules

A

connect to chromosome ends by chromokinesin

87
Q

Polar microtubules

A

not attached to chromosomes but are stabalized by overlapping with each other in the center of the cell

88
Q

Astral microtubules

A

-extend outward from the centrosomes with the plus ends enchored in the cell cortex.

89
Q

What happens in Anaphase A?

A
  • Chromosomes move toward spindle poles along kinetochore microtubules
  • kenesins depolyymerize and shorten tubules
  • kinetochore is shortened and disassembled
90
Q

What is a kinesin?

A

microtubule-depolymerizing enzymes that disassemble and shorten kinetechore & chromosomal microtubules

91
Q

What happens during Anaphase B?

A
  • Spindle poles seperate while polar microtubules elongate
  • overlapping polar microtubules slide against one another & push spindle poles apart.
  • spindle poles are pulled apart by astral microtubules
  • cytoplasmic dynein anchored to the cell cortex moves along astral microtubules in the minus-end direction
  • Simultaneous depolymerization of astral microtubules by middle motor kinesins leads to separation of the spindle pores
92
Q

What are the three stages in which cells move across the surface?

A
  1. Extension of leading edge
  2. Attachment of leading edge to the substratum
  3. Retraction of the rear of the cell into the cell body
93
Q

What does extension of the leading edge involve?

A
  1. branching

2. polymerization of actin filaments

94
Q

Rho proteins

A

regulate Formation of cell surface protrusions in response to extracellular stimuli
-activate WASP proteins

95
Q

WASP proteins

A

stimulate the Arp2/3 complex and initiate growth of branched actin filaments

96
Q

Myosin

A

a molecular motor

  • proteins that converts chemical energy (ATP) to mechanical energy
  • generates muscle force and movement
97
Q

muscle fibers

A

skeletal muscles are arranged in muscle fibers

-large cells formed by fusion of many cells during development

98
Q

myofibrils

A

bundles of thick myosin filaments and thin actin filaments

-each individual myofibril contains a chain of contractile units called sarcomere

99
Q

sarcomere

A

makes up a chain of contractile units. gives skeletal and cardiac muscles their striated appearance

100
Q

Sarcomere structure

A

-bands correspond to presence or absence of myosin filaments

101
Q

Z disc

A

-where actin filaments are attached

102
Q

M line

A

myosin filaments are anchored to the m line

103
Q

I band

A

-contains only actin filaments

104
Q

A band

A

myosin filaments overlab in the peripheral region of the A band

105
Q

Sliding filament model of muscle contraction

A
  • sarcomere shortens, z discs are brought closer together.
  • there is no change in width of A band, but I band and H zone almost disappear
  • myosin filaments slide past one another so that actin filaments move into the A band and H zone
  • due to myosin binding to actin filaments
106
Q

Myosin 2

A
  • type of myosin in muscle
  • has 2 heavy chains & 2 pairs of light chains
  • heavy chains have globular head region & long alpha-helical tail
  • tails twist around each other in a coiled coil
107
Q

Organization of actin & myosin heads during muscle contraction

A
  • have Thick filaments which are several hundred myosin molecules in parallel staggered array
  • globular heads bind actin & form cross-bridges between thick and thin filaments
  • orientation of filaments reverses at the M line
108
Q

What are other proteins present in sarcomeres?

A

Titin & Nebulin

109
Q

TItin

A
  • extremely large; one titin molecule extends from M line to Z disc
  • titan acts like a spring to keep myosin filaments centered in the sarcomere & maintain the resting tension that allows a muscle to snap back if overextended
110
Q

Nebulin

A
  • filaments are associated with actin

- help organize & maintain orientation & structure of thin filaments

111
Q

What happens to initiate the process of myosin head movement along actin filaments?

A
  • Myosin heads bind & hydrolyze ATP, this provides energy to drive filament sliding
  • Myosin changes shape during repeated cycles of interaction between myosin heads and actin
  • conformational changes in myosin result in movement of myosin heads along actin filaments
112
Q

How does ATP binding and dissociation affect myosin heads?

A
  • Binding of ATP dissociates myosin from actin

- ATP hydrolysis induces a conformational change that displaces the myosin head groups

113
Q

What happens during the power stroke?

A
  • myosin head binds to a new position on the actin filaments, Pi is released
  • myosin heads return to its original conformation while attached to actin. THis drives actin filament sliding
  • ADP is released
114
Q

How is contraction of skeletal muscle triggered?

A
  • triggered by nerve pulses that stimulate release of Ca2+ ions from the sarcoplasmic reticulum
  • increased Ca2+ concentration in the cytosol affects tropomyosin and troponin
115
Q

What are actin filament binding proteins? How are they structured?

A
  • tropomyosin & troponin.

- tropomyosin binds lengthwise along actin filaments & is also bound to troponin

116
Q

How does Ca2+ interact with tropomyosin and troponin

A
  • As Ca2+ is absent, the tropomyosin-troponin complex blocks binding of myosin to actin.
  • binding of Ca2+ to troponin C shifts complex & allows contraction to proceed
117
Q

Give examples of contractile assemblies that are not muscles

A

-stress fibers & adhesion belts

118
Q

Cytokinesis

A

Division of a cell folloying mitosis

119
Q

Contractile ring

A
  • a ring of actin and myosin 2 that is assembled by membrane-bound myosin just beneath the plasma membrane
  • contraction of the ring pinches cell into 2 sections
120
Q

How is contraction regulated in nonmuscle cells and smooth muscle?

A
  • phosphorylation of a myosin light chain

- catalyzed by MLCK

121
Q

MLCK

A

myosin light chain kinase

  • catalyses phosphorylation in nonmuscle cells and smooth muscle
  • regulated by Ca2+ binding protein calmodulin
122
Q

Calmodulin

A

a binding protein that regulated MLCK (myosin light chain kinase)
-Ca2+ binds to calmodulin,

123
Q

What are Unconventional myosins? What are their functions?

A
  • myosins not in muscle don’t form filaments and are not involved in contration.
  • THey function in cell movement, such as transport of vesicles and organelles
124
Q

Describe the structure & function of Myosin 1

A
  1. Have globular head groups that act as molecular motors
  2. Short tails bind to other structures. Movement of Myosin 1 along an actin filament can transport its attached cargo, such as a vesicle.
125
Q

Myosin V

A

a) two headed dimer that transports vesicles and other cargo along actin filaments

126
Q

What is the role of unconventional actins?

A

Some unconventional actins are involved in actin filament reorganization & anchor actin filaments to the plasma membrane

127
Q

What are intermediate filaments? What are they composed of?

A

a) intermediate between actin filaments and microtubules
b) two types: keratin and vimentin
c) composed of many types of proteins expressed in different types

128
Q

What are functions of intermediate filaments?

A

a) provide mechanical strength

b) provide a scaffold for localization of cell processes

129
Q

Keratin

A
  • an intermediate filament
  • in epithelial cells
  • type 1
130
Q

Vementin

A
  • an intermediate filament
  • forms a network extending out from the nucleus toward cell periphery
  • type 2
  • expressed in fibroblasts, WBC’s and other cells
131
Q

What is a neurofilament NF protein?

A

a) a major intermediate filament of many neurons

b) provides support for long axons

132
Q

What is a nestin

A

-expressed during embryonic development in some stem cells.

133
Q

Type V lamins

A
  • nuclear lamins

- form a meshwork underlying the nuclear membrane

134
Q

Describe structure of intermediate filaments

A

a) central alpha-helical rod domain which plays a central role in filament assembly
b) head and tail domains determine specific function

135
Q

Intermediate filament assembly

A

a) central rod domains of 2 polypeptides form a coiled coil.
b) dimers associate in a staggered antiparallel fashion to form tetramers, which assemble end-to-end to form protofilaments
c) eight protofilaments wind together to form a filament

136
Q

Desmosomes

A

-junctions between adjacent cells in epithelial tissue

137
Q

Keratin filaments

A
  • attach to dense protein plaques on the cytoplasmic side

- attachment mediated by desmoplakin of the plakin protein family

138
Q

Hemidesmosomes

A
  • junctions between epithelial cells and underlying connective tissue
  • have keratin filaments that are attached to different plakins (plectin)
  • transmembrane integrins link to the extracellular matrix
139
Q

What are the main roles of intermediate filaments?

A
  1. strengthen cytoskeleton of cells in the tissues of multicellular organisms
140
Q

Give 2 manipulation examples that can change the role of intermediate filaments

A
  1. injection of cultured cells with antibody against vimentin disrupts intermediate filament networks without affecting cell growth or movement
  2. mutation of keratin: disrupts normal keratin cytoskeleton resulting in severe skin abnormalities

Molecular Cell Biology (16 decks)

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