Cytoskeleton
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The cytoskeleton
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The cytoskeleton is a network of fibers that organizes structures and activities
in the cell• The cytoskeleton is a network of fibers
extending throughout the cytoplasm• It organizes the cell’s structures and activities,
anchoring many organelles• It is composed of three types of molecular
structures:– Microtubules– Microfilaments– Intermediate filaments
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 6-20
Microtubule
Microfilaments0.25 µm
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Roles of the Cytoskeleton: Support, Motility, and Regulation
• The cytoskeleton helps to support the cell and maintain its shape
• It interacts with motor proteins to produce motility
• Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton
• Recent evidence suggests that the cytoskeleton may help regulate biochemical activities
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 6-21
VesicleATP
Receptor for motor protein
Microtubuleof cytoskeleton
Motor protein (ATP powered)
(a)
Microtubule Vesicles
(b)
0.25 µm
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Components of the Cytoskeleton
• Three main types of fibers make up the cytoskeleton:– Microtubules are the thickest of the three
components of the cytoskeleton– Microfilaments, also called actin filaments, are
the thinnest components– Intermediate filaments are fibers with diameters
in a middle range
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Table 6-1
10 µm 10 µm 10 µm
Column of tubulin dimers
Tubulin dimer
Actin subunit
25 nm
7 nm
Keratin proteins
Fibrous subunit (keratins coiled together)
8–12 nm
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Table 6-1a10 µm
Column of tubulin dimers
Tubulin dimer
25 nm
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Table 6-1b
Actin subunit
10 µm
7 nm
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Table 6-1c
5 µm
Keratin proteins
Fibrous subunit (keratinscoiled together)
8–12 nm
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Cytoskeleton proteins revealed by Commassie staining
Three types of filamentsand accessory proteins(assembly of cytoskeleton, motorproteins that move organellesor filaments)
Internal orderShape and remodel surfaceMove organellesMovementCell division
Cytoskeleton: filament system
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Intermediate filaments:mechanical strength and resistance to shear stress
Microtubules: positions of membrane-enclosed organelles, intracellular transport
Actin filaments:shape of the cell’s surfaceand whole cell locomotion
Dynamic and adaptable
5-9 nm diameter
25 nm diameter
10 nm diameter
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Microtubules
• Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long
• Functions of microtubules:– Shaping the cell– Guiding movement of organelles– Separating chromosomes during cell division
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Centrosomes and Centrioles• In many cells, microtubules grow out from a
centrosome near the nucleus• The centrosome is a “microtubule-
organizing center”• In animal cells, the centrosome has a pair of
centrioles, each with nine triplets of microtubules arranged in a ring
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Fig. 6-UN3
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Microtubules• Microtubules• I. Introduction: Long, hollow cylinders, 25 nm in diameter, made of
tubulin. The basic subunit is a heterodimer of α and β tubulin (9.8); 13 protofilaments in a typical cylinder. See below about GTP binding, treadmilling, growth and dynamic instability (9.26). There is a + end, fast growing, w/β tubulin at its end, and a – end, slow growing, w/α tubulin at its end. The GTP’s are important in assembly (9.8)
• A. They have MAPs, that influence their use- linking them together, stabilizing them, or destabilizing them.
• B. They form a network, coming from the microtubule organizing center, which is usually the centrosome or centriole, w/ the – end anchored there. (9.10-13, 19)
• C. Also form cilia and flagella, and spindle fibers in mitosis.
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13 protofilaments
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• F. MICROTUBULE DYNAMICS: 9.25
• Key points- the cap means that subunits are added easily- loss of GTP = harder to add subunits, need higher subunit conc. to add.
• Produces microtubule catastrophes!
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Nucleation
• Gamma tubulin in MTOC/centriole- MT’s grow from there
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FIGURE 9.25 The structural cap model of dynamic instability. Accordingto the model, the growth or shrinkage of a microtubule dependson the state of the tubulin dimers at the plus end of the microtubule.Tubulin-GTP dimers are depicted in red. Tubulin-GDP dimers are depictedin blue. In a growing microtubule (step 1), the tip consists of anopen sheet containing tubulin-GTP subunits. In step 2, the tube has begunto close, forcing the hydrolysis of the bound GTP. In step 3, the tubehas closed to its end, leaving only tubulin-GDP subunits. GDP-tubulinsubunits have a curved conformation compared to their GTP-boundcounterparts, which makes them less able to fit into a straight protofilament.The strain resulting from the presence of GDP-tubulin subunitsat the plus end of the microtubule is released as the protofilaments curloutward from the tubule and undergo catastrophic shrinkage (step 4).
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Dynamic instability:predominant in microtubules
GTP hydrolysis “catch up”
Treadmilling: predominant in actin filaments
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Lateral bonds force GDP-containingprotofilaments into a linear conformation
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Remodeling
• The fact that MT’s aren’t fixed means that cells can remodel their shape- plant cells, our cells in mitosis- round up, as MT’s used to make spindle fibers
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The time course of actin polymerization in a test tube
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GTP
GTP!
The structure of a microtubule and its subunits
13 parallel protofilaments
hollow and cylindrica and polar
heterodimer
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monomer
ATP
polar
two parallel protofilamentsthat twist around each otherin a right-handed helix
The structure of an actin monomer and actin filament
Flexible but cross-linked andbundled together by accessoryproteins in a living cell
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The preferential growth of microtubules at the plus end
Plus end: polymerize and depolymerize faster thanminus end
Actin filamentsPlus end- barbed endMinus end- pointed end
Microtubules:Plus end- subunitMinus end- subunit
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Fig. 6-22Centrosome
Microtubule
Centrioles
0.25 µm
Longitudinal section of one centriole
Microtubules Cross sectionof the other centriole
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GTP hydrolysis causes filament to curve
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MT drugs
• Colchicine- prevents MT formation- arrests cells at metaphase
• Useful in determining role of MT’s in a process
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Effect of the drug taxol on microtubule organization
treatment of cancers
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Actin and tubulin are highly conserved: they have to bind tomany proteins directly and indirectly
Accessory proteins and intermediate filament proteinsare not as conserved
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Intermediate filamentsare only found in some metazoans:vertebrates,nematodes,molluscs
Not required inevery cell type
Ancesters: nuclear lamins
Parallel
Antiparrel
“subunit”No polarity!
8 parallel protofilaments
Easily bentHard to break
A model of intermediate filament construction
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Two types of intermdiate filaments in cells of the nervous system
Neurofilaments:axonsNF-L, NF-M, NF-H proteins coassemble
NF-M and NF-H have long C-terminal tailsThat bind to neighboring filaments:uniform spacing
When axons grow, subunits are added at the filament endsand along the filament length; axon diameter increase 5 fold
In ALS (Lou Gehrig’s Disease), there is an accumulation and abnormal assembly ofNeurofilaments in motor neuron cell bodies and axon--interfere with normal axon transport
Regular spacing
axon glia
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Summary
1. Three types of cytoskeletal filaments, protofilaments;2. Subunits, polymerization, treadmilling, dynamic
instability;3. Intermediate filaments, cell integrity, diseases caused
by mutations in the intermediate filament genes4. Natural toxins and cytoskeleton
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Cilia and Flagella• Microtubules control the beating of cilia and
flagella, locomotor appendages of some cells• Cilia and flagella differ in their beating patterns
Video: Video: ChlamydomonasChlamydomonas Video: Video: Paramecium Paramecium CiliaCilia
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 6-23
5 µm
Direction of swimming
(a) Motion of flagella
Direction of organism’s movement
Power stroke Recovery stroke
(b) Motion of cilia15 µm
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• Cilia and flagella share a common ultrastructure:– A core of microtubules sheathed by the plasma
membrane– A basal body that anchors the cilium or
flagellum– A motor protein called dynein, which drives the
bending movements of a cilium or flagellum
Animation: Cilia and FlagellaAnimation: Cilia and Flagella
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Fig. 6-24
0.1 µm
Triplet
(c) Cross section of basal body
(a) Longitudinal section of cilium
0.5 µm
Plasma membrane
Basal body
Microtubules
(b) Cross section of cilium
Plasma membrane
Outer microtubule doublet
Dynein proteins
Central microtubuleRadial spoke
Protein cross-linking outer doublets
0.1 µm
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• How dynein “walking” moves flagella and cilia:
− Dynein arms alternately grab, move, and release the outer microtubules
– Protein cross-links limit sliding– Forces exerted by dynein arms cause doublets
to curve, bending the cilium or flagellum
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Intermediate Filaments
• Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules
• They support cell shape and fix organelles in place
• Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Cell Walls of Plants
• The cell wall is an extracellular structure that distinguishes plant cells from animal cells
• Prokaryotes, fungi, and some protists also have cell walls
• The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water
• Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein
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• Plant cell walls may have multiple layers:– Primary cell wall: relatively thin and flexible– Middle lamella: thin layer between primary
walls of adjacent cells– Secondary cell wall (in some cells): added
between the plasma membrane and the primary cell wall
• Plasmodesmata are channels between adjacent plant cells
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Fig. 6-28
Secondary cell wall
Primary cell wall
Middle lamella
Central vacuoleCytosol
Plasma membrane
Plant cell walls
Plasmodesmata
1 µm
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Cytoskeletal filaments are all constructed from smaller protein subunits
Intermediate filaments: smallerelongated and fibrous subunits
Actin and microtubule filaments:compact and globular subunits
All form as helical assembliesof subunits
Noncovalent interactions:rapid assembly and disassembly
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Nucleation
• Gamma tubulin in MTOC/centriole- MT’s grow from there
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Like MT
OC
/ cenriole!
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MT’s are a highway- bringing things out and back from the center of the cell.
/
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Cilia action
• Cilia= short, many• Flagella= long, few; NOT the same as the
bacterial flagellum!!
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9+2; nexin, radial spokes, dynein
A,B
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The different sides of the cilium may slide, depending on the direction of sliding.
These sliding more
These sliding more
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Intermediate filaments
• 10 nm in diameter• Only in animals! (??plant/fungal nucleus??)• Variety of types- 60 genes!• Seem to be involved in providing strength
to cells.• Able to interact with both MT's and
microfilaments (actin filaments).
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Keratin filaments in epithelial cells
“desmosomes”
The most diverse family20 in human epithelial cells10 more in hair and nails
Intermediate filaments impart mechanical stability to animal cells
Diagnosis of epithelialcancers (carcinomas)
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Octamers of Tetramers makeup the structure. No polarity!Subunits are filamentous, rather thanglobular.
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• Keratin- epithelial cells, hair, nails• Neurofilaments- in, well, nerves• Lamins- lines the nucleus
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When they are mutant
• Smaller nerve fibers- a natural mutant quail!• Fragile skin• Sometimes muscle weakness• Sometimes nothing!
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Microfilaments (Actin Filaments)
• Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits
• The structural role of microfilaments is to bear tension, resisting pulling forces within the cell
• They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape
• Bundles of microfilaments make up the core of microvilli of intestinal cells
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Fig. 6-26
Microvillus
Plasma membrane
Microfilaments (actin filaments)
Intermediate filaments
0.25 µm
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• Microfilaments that function in cellular motility contain the protein myosin in addition to actin
• In muscle cells, thousands of actin filaments are arranged parallel to one another
• Thicker filaments composed of myosin interdigitate with the thinner actin fibers
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• Localized contraction brought about by actin and myosin also drives amoeboid movement
• Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments
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• Cytoplasmic streaming is a circular flow of cytoplasm within cells
• This streaming speeds distribution of materials within the cell
• In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming
Video: Cytoplasmic StreamingVideo: Cytoplasmic Streaming
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Microfilaments (Actin)
• Where we’re going: • Basic structure, polarity, treadmilling• Muscle contraction• Amoeboid movement
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Domains 1-4
Minus endATP binding cleft
Subunits= G actin-bound w/ATP; F-actin= microfilaments
Looks like a double helix!
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S1 is a myosin fragment that binds to actin- the points point to the minus end
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Treadmilling of actin filaments
actin sub units can flow through the filaments by attaching preferentially to the(+) end and dissociating preferentially from the (-) end of the filament. This treadmilling phenomenon occur in some moving cells.The oldest subunits In treadmilling filament lie at the (-) end.
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Treadmilling-it’s easier to add to the + than – end at any concentration, and at some concentrations it’s adding at the + end at the rate it’s coming off the – end= treadmilling.
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The treadmilling of an actin filament
D form polymer leans towards disassembly
Structural difference between the two ends
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Muscle Contraction
• Three types of muscle fibers:• Skeletal, striated, voluntary• Heart- more like skeletal, but not
multinucleated. Its structure allows the propagation of an action potential (the heart beats by itself, w/o outside signals)
• Involuntary, smooth muscle- gut, uterus, etc.
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Multinucleated cell, arises from fusion; great big thing- 100mm X 100 um!
2.5 uM length
Muscle contractility
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These are myofibrils
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Electron micrograph of sarcomere with bands lettered
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The functional anatomy of muscle fibere
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Myosin I, hauling a vesicle
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Microtubules= interstate; actin= side roads
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Light band Dark band Light bandhttp://www.youtube.com/watch?v=0kFmbrRJq4w
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Troponin binds Ca++, moves the tropomyosin 1.5 nm- myosin binds
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Actin accessory proteins
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ARP
Filamin
gelsolin
Fimbrin
Profilin
(thymosins)
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