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Lecture 14 Cytoskeleton: components. Cytoskeleton proteins revealed by Commassie staining. Cytoskeleton: filament system. Three types of filaments and accessory proteins (assembly of cytoskeleton, motor proteins that move organelles or filaments). Internal order Shape and remodel surface

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lecture 14 cytoskeleton components
Lecture 14

Cytoskeleton: components

slide2

Cytoskeleton proteins revealed by Commassie staining

Cytoskeleton: filament system

Three types of filaments

and accessory proteins

(assembly of cytoskeleton, motor

proteins that move organelles

or filaments)

Internal order

Shape and remodel surface

Move organelles

Movement

Cell division

slide3

Dynamic and adaptable

Actin filaments:

shape of the cell’s surface

and whole cell locomotion

5-9 nm diameter

Microtubules:

positions of membrane-

enclosed organelles,

intracellular transport

25 nm diameter

Intermediate filaments:

mechanical strength and

resistance to shear stress

10 nm diameter

slide4

Cytoskeletal filaments are all constructed from

smaller protein subunits

Intermediate filaments: smaller

elongated and fibrous subunits

Actin and microtubule filaments:

compact and globular subunits

All form as helical assemblies

of subunits

Noncovalent interactions:

rapid assembly and disassembly

slide8

The structure of a microtubule and its subunits

hollow and cylindrica and polar

GTP!

GTP

heterodimer

13 parallel protofilaments

slide9

The structure of an actin monomer and actin filament

two parallel protofilaments

that twist around each other

in a right-handed helix

polar

ATP

monomer

Flexible but cross-linked and

bundled together by accessory

proteins in a living cell

slide10

The preferential growth of microtubules at the plus end

Plus end: polymerize and

depolymerize faster than

minus end

Microtubules:

Plus end- b subunit

Minus end- a subunit

Actin filaments

Plus end- barbed end

Minus end- pointed end

slide11

The treadmilling of an actin filament

Structural difference between the two ends

D form polymer leans towards disassembly

slide12

Treadmilling behavior of a microtubule

as in a living cells

The extent of treadmilling inside the cell

Is uncertain.

Actin treadmilling is observed in vitro.

A treadmilling-like phenomenon is seen in

living cells for microtubules

Tubulin conjugated with fluorescent dye

1/20 subunit is fluorescent

“Microtubule lattice”

slide13

Dynamic instability:predominant in microtubules

GTP hydrolysis “catch up”

Treadmilling: predominant in actin filaments

slide14

GTP hydrolysis causes

filament to curve

slide15

Lateral bonds force GDP-containing

protofilaments into a linear conformation

slide16

Direct observation of the dynamic instability of

microtubules in a living cell

Dynamic instability of individual actin filaments cannot be

observed readily-difference between two ends are not so extreme

However the actin filament turn over is rapid: individual filament

persists for a only few minutes

slide17

The dynamic behavior of filaments allows cells to

change structures rapidly and

Giant multinuclear

cell of a fly early

embryo

Actin filaments:red

Mircotubules:green

One division

per 10 minutes

slide18

Actin and tubulin are highly conserved: they have to bind to

many proteins directly and indirectly

Accessory proteins and intermediate filament proteins

are not as conserved

slide19

A model of intermediate filament construction

Intermediate filaments

are only found in some

metazoans:vertebrates,

nematodes,molluscs

Parallel

Antiparrel

Not required in

every cell type

Ancesters: nuclear lamins

“subunit”

No polarity!

Easily bent

Hard to break

8 parallel protofilaments

slide20

Mechanical properties of actin, tubulin and

Intermediate filament polymers

viscometer

Microtubules: easily

deformed and then rupture

Actin filaments are more

rigid and also rupture

easily

Intermediate filaments:

easily deformed and don’t

rupture--maintain cell integrety

slide21

Intermediate filaments impart mechanical stability

to animal cells

Keratin filaments in epithelial cells

The most diverse family

20 in human epithelial cells

10 more in hair and nails

Diagnosis of epithelial

cancers (carcinomas)

“desmosomes”

slide22

Blistering of the skin caused by mutant keratin genes

Epidermolysis bullosa simplex:the skin blisters in response to

very slight mechanical stress

Truncated keratin (missing both the N- C- domains) Tg mice

Other blistering diseases:

mouth, esophageal lining and cornea of the eye--

mutations of different keratins

slide23

Two types of intermdiate filaments in cells of the nervous system

Neurofilaments:axons

Regular spacing

NF-L, NF-M, NF-H proteins coassemble

glia

axon

NF-M and NF-H have long C-terminal tails

That bind to neighboring filaments:uniform spacing

When axons grow, subunits are added at the filament ends

and along the filament length; axon diameter increase 5 fold

In ALS (Lou Gehrig’s Disease), there is an accumulation and abnormal assembly of

Neurofilaments in motor neuron cell bodies and axon--interfere with normal axon transport

slide24

Actin filaments and microtubules are targets of

many plant toxins

Amanita Phalloids (death cap)

(Eat raw meat)

slide26

Summary

Three types of cytoskeletal filaments, protofilaments;

Subunits, polymerization, treadmilling, dynamic

instability;

Intermediate filaments, cell integrity, diseases caused

by mutations in the intermediate filament genes

4. Natural toxins and cytoskeleton