cytoskeltons

1. SURYA C S
1ST YEAR
MSC MICROBIOLOGY

2. INTRODUCTION
• A network of protein filaments and tubules, extends from the
nucleus to the plasma membrane
• Structural framework
• cell shape, localize organelles, general organization of the
cytoplasm
• Movement
• Cell movement, internal transport of organelles, muscle
contraction
• Dynamic structures, continually reorganized

3. CYTOSKELETAL FILAMENTS
1. Microfilaments (7 nm)
2. Microtubules ( 25 nm)
3. Intermediate filaments (8-11 nm)
Composed of three main types of protein filaments

4. Structure and Organization of MicroFilaments
• Microfilaments are Polymer of
Actin, flexible fibers 7 nm in
diameter, several µm in length
• Actin first isolated from muscle cells
in 1942
• Abundant in all types of eukaryotic
cells
• Mammals have 6 actin genes: 4 are
expressed in muscle cells and 2 in
nonmuscle cells
• Highly conserved
• Prokaryotic ancestor is MreB
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5. Actin and Actin Filament
• 3-D structure determined in 1990
• Actin: globular (G-actin), barbed and pointed ends, binds
head-tail to nucleate a trimer
• Filamentous (F-actin): monomers added to both end
• Filament is polar pointed end vs barbed end
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6. G-Actin and Actin Filament
• Polymerization is reversible
• The rate at which monomers are added to filaments is
proportional to their concentration
• ATP bound actin binds to barbed end with high affinity
• ADP-actin has low affinity to the pointed ends
• when ATP hydrolyses to ADP
• ADP-actin dissociates from filaments more readily than
ATP-actin
• Therefore, the critical concentration of actin monomers
is higher for addition to the pointed end than to the
barbed end of actin filaments
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7. Treadmilling: polarity of F-actin growth
• At cellular actin concentrations
• Barbed end of a filament grows 5–10 times faster than
the pointed end
• ADP-actin dissociates from pointed end
• Exchange of ATP for ADP added to barbed-end
• Process is called
Treadmilling
• Dynamic growth
• Direction?
PointedBarbed
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8. Actin Binding Proteins
• Actin Binding Proteins (ABP): modulate the Assembly
and disassembly of actin filaments
• ABD/Actin interaction has diverse functionality
• Contribute to the cellular role of actin filaments
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9. Actin Remodeling
• Some actin-binding proteins bind along the length of actin
filaments, stabilizing them or cross-linking them to one another
• Others stablize by capping the ends and preventing dissociation
• Others promote dissociation, while others regulate the exchange
of ATP for ADP.
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10. Initiation of Actin Filament polymerization
• Nucleation is the rate-limiting step
• Formin and the Arp2/3 complex determine where
filaments are formed by facilitating nucleation
• Formins nucleate long unbranched actin filaments
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11. Initiation of Actin Filament polymerization
• actin filaments actively turn over and branch
extensively
• These filaments are nucleated by the Arp2/3 (Actin
Related Protein) complex, which binds actin/ATP near
the barbed ends
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12. Actin Filament Depolymerization
• The ADF/cofilin (Actin Depolymerizing Factor) family modifies
existing filaments
• enhance the rate of dissociation of actin/ADP monomers
from the pointed end, and remain bound to the monomers,
preventing their reincorporation
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13. Actin Filament Severing
• ADF/cofilin can also bind to and sever actin filaments
• Profilin reverses the ADF/Cofilin effect
• Stimulate exchange of bound ADP for ATP and dissociating
the actin/ATP monomers from cofilin
• Become available for reassembly in two filaments
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14. Organization Of Actin Filaments
• Actin bundles—cross-linked
into closely packed parallel
arrays
• Actin networks—cross-linked in
arrays that form 3-D meshworks
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15. Actin Bundles
• Parallel filaments cross linked
by actin-bundling proteins
• Have two domains to bind
actin and align the filaments
Two types of actin bundles:
1. Non contractile
– filaments (14 nm apart)
aligned in parallel, same
polarity, barbed ends
adjacent to the plasma
membrane
• Fimbrin: a 68 kD proteins, cross
links by its two actin binding
domains (ABD)
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16. Actin Bundles: Contractile
2. Contractile bundles: more
widely-spaced filaments (40
nm), cross-linked by α-actinin
• α-actinin: a 102 kD protein with
single ABD and an α-helical
spacer
• Interacts with actin as a dimer
• Increased spacing allows actin
interaction with motor protein
myosin II
• Important in muscle fiber
contraction
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17. Higher order Actin assembly: Actin Network
• Filamin (280 kD) form flexible
cross-links
• Filamin dimer: flexible V-shaped
molecule
• actin-binding domains at the end
of each arm
• dimerization domain
• Β-sheet spacer
• Binds actin orthogonally, form 3-D
network beneath the plasma
membrane
• network (cell cortex) determines
cell shape, and cell movement
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18. Actin Network and cell cortex
• Red blood cells as mode
• lack other cytoskeletal components, so the cortical
cytoskeleton is the principal determinant of cell shape
• Spectrin, major actin-binding cortex protein
• tetramer of two polypeptide chains, α and β
• ends of the spectrin tetramers bind actin filaments,
resulting in the spectrin-actin network
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19. Actin Network and Cell Cortex
• Ankyrin links the spectrin-actin network and the plasma
membrane
• Protein 4.1 is another link that binds spectrin-actin
junctions and the transmembrane protein glycophorin
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20. Actin bundles: Intenstinal Microvilli
• Actin bundles take part in
avariety of cell surface
protrusions
• cell movement
• phagocytosis
• absorption of nutrients
• Intenstinal microvilli:
• Membrane projections,
increase absorption surface
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21. Actin bundles: Intenstinal Microvilli
• closely packed parallel
bundles of 20 to 30 actin
filaments
• Relatively permanent
• The filaments are cross-linked
in part by fimbrin and villlin
• The actin bundles are
attached to the plasma
membrane by the calcium-
binding protein calmodulin in
association with myosin I
• At the base attach to actin
cortex
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22. Actin bundles: membrane protrusions
• Other surface protrusions are transient and form in
response to environmental stimuli
• Pseudopodia- responsible for phagocytosis and the
movement of amoebas
• Lamellipodia- broad, sheetlike extensions at the leading
edge of fibroblasts
• Filopodia- thin projections of the plasma membrane in
migrating cells
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23. Association of Actin filaments with Motor proteins
• Brings higher level of functional complexity to cells
• Cellular or organismal movement
• Intracellular cargo transportation, cell division
Association with motor protein myosin
• Myosin is a molecular motor: converts chemical energy
(ATP) to mechanical energy  force and movement.
• Muscle contraction: model for understanding actin-myosin
interactions and the motor activity of myosin molecules
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24. Actin-Myosin and Muscle Contraction
• muscle fibers, large cells
(50 µm in diameter and up
to several centimeters in
length)
• Cytoplasm consists of
myofibrilsmyosin
filaments and thin actin
filaments
• sarcomeres, myofibril units
of skeletal and cardiac
muscle
• actin filaments attached at
their barbed ends to the Z
disc 24

25. Sacromere: a structural and contractile unit
• Titin is extremely large protein; extend from the M line to
the Z disc
• keep myosin II filaments centered in the sarcomere
• maintain the resting tension that allows a muscle to snap back
if overextended
• Nebulin, associated with actin, regulate assembly of actin
filaments
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26. Sliding Filament Model
• was proposed in 1954
• Myosin slides on actin filament
• Sarcomere shortens, bringing the Z discs closer
• There is no change in the width of the A band, but the I
bands and H zone almost disappear
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27. Sliding Filament Model
• Tropomyosin binds along
actin filaments, also
bound to troponin
• No Ca2+, tropomyosin-
troponin block binding of
myosin to actin
• nerve impulses, stimulate
release of Ca2+ from the
sarcoplasmic reticulum
• Ca2+ binds troponin C,
shifts the complex
• Allows myosin binding to
actin
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28. Sliding Filament Model
• Myosin II (the type in muscle), large protein with two heavy chains
and two pairs of light chains
• heavy chains have a globular head region and a long α-helical tail
• Tails twist around in a coiled-coil
• globular heads bind actin
• myosin moves the head
groups along the actin
filament in the direction
of the barbed end
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29. Sliding Filament Model
•ATP hydrolysis is
required
•Binding of ATP
dissociates myosin from
actin
•ATP hydrolysis induces a
conformational change
that displaces the
myosin head group
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30. Sliding Filament Model
•myosin head binds to a
new position on the actin
filament and Pi is released
•The “power stroke”:
Myosin head returns to its
original conformation,
which drives actin filament
sliding, and ADP is released
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31. Actin and myosin in cell divison
• Cytokinesis—division of a
cell following mitosis
• A contractile ring of actin
and myosin II is assembled
underneath the plasma
membrane
• Contraction of the ring
pinches the cell in two
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32. Actin and myosin: vesicular transport
• Myosin I: much smaller
than myosin II,contains a
globular head group, acts
as a molecular motor
• Short tails bind to other
structures
• Movement of myosin I
along actin filament
• transport cargo, such as a
vesicle
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