copy of cytoskeleton

The cytoskeleton

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

Fig. 6-20

Microtubule

Microfilaments0.25 µm

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


Fig. 6-21

VesicleATP

Receptor for motor protein

Microtubuleof cytoskeleton

Motor protein (ATP powered)

(a)

Microtubule Vesicles

(b)

0.25 µm

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


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

Table 6-1a10 µm

Column of tubulin dimers

Tubulin dimer

25 nm

Table 6-1b

Actin subunit

10 µm

7 nm

Table 6-1c

5 µm

Keratin proteins

Fibrous subunit (keratinscoiled together)

8–12 nm

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

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

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


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


Fig. 6-UN3

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.

13 protofilaments

• 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!

Nucleation

• Gamma tubulin in MTOC/centriole- MT’s grow from there

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).

Dynamic instability:predominant in microtubules

GTP hydrolysis “catch up”

Treadmilling: predominant in actin filaments

Lateral bonds force GDP-containingprotofilaments into a linear conformation

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

The time course of actin polymerization in a test tube

GTP

GTP!

The structure of a microtubule and its subunits

13 parallel protofilaments

hollow and cylindrica and polar

heterodimer

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

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

Fig. 6-22Centrosome

Microtubule

Centrioles

0.25 µm

Longitudinal section of one centriole

Microtubules Cross sectionof the other centriole

GTP hydrolysis causes filament to curve

MT drugs

• Colchicine- prevents MT formation- arrests cells at metaphase

• Useful in determining role of MT’s in a process

Effect of the drug taxol on microtubule organization

treatment of cancers

Actin and tubulin are highly conserved: they have to bind tomany proteins directly and indirectly

Accessory proteins and intermediate filament proteinsare not as conserved

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

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

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

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


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

• 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


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

• 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


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


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


• 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


Fig. 6-28

Secondary cell wall

Primary cell wall

Middle lamella

Central vacuoleCytosol

Plasma membrane

Plant cell walls

Plasmodesmata

1 µm

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

Nucleation

• Gamma tubulin in MTOC/centriole- MT’s grow from there

Like MT

OC

/ cenriole!

MT’s are a highway- bringing things out and back from the center of the cell.

/

Cilia action

• Cilia= short, many• Flagella= long, few; NOT the same as the

bacterial flagellum!!

9+2; nexin, radial spokes, dynein

A,B

The different sides of the cilium may slide, depending on the direction of sliding.

These sliding more

These sliding more

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).

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)

Octamers of Tetramers makeup the structure. No polarity!Subunits are filamentous, rather thanglobular.

• Keratin- epithelial cells, hair, nails• Neurofilaments- in, well, nerves• Lamins- lines the nucleus

When they are mutant

• Smaller nerve fibers- a natural mutant quail!• Fragile skin• Sometimes muscle weakness• Sometimes nothing!

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


Fig. 6-26

Microvillus

Plasma membrane

Microfilaments (actin filaments)

Intermediate filaments

0.25 µm

• 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


• 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


• 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


Microfilaments (Actin)

• Where we’re going: • Basic structure, polarity, treadmilling• Muscle contraction• Amoeboid movement

Domains 1-4

Minus endATP binding cleft

Subunits= G actin-bound w/ATP; F-actin= microfilaments

Looks like a double helix!

S1 is a myosin fragment that binds to actin- the points point to the minus end

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.

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.

The treadmilling of an actin filament

D form polymer leans towards disassembly

Structural difference between the two ends

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.

Multinucleated cell, arises from fusion; great big thing- 100mm X 100 um!

2.5 uM length

Muscle contractility

These are myofibrils

Electron micrograph of sarcomere with bands lettered

The functional anatomy of muscle fibere

Myosin I, hauling a vesicle

Microtubules= interstate; actin= side roads

Light band Dark band Light bandhttp://www.youtube.com/watch?v=0kFmbrRJq4w

Troponin binds Ca++, moves the tropomyosin 1.5 nm- myosin binds

Actin accessory proteins

ARP

Filamin

gelsolin

Fimbrin

Profilin

(thymosins)

copy of cytoskeleton

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