cytosceleton

Dr. I Wayan Sugiritama

QUESTIONS: How cell maintain their shape ?

How cell organize its organelles?

How cell transport vesicles?

How the segregation of chromosomes into daughter cells at mitosis ?

How epithelial cell can withstand to the mechanical stress?

How spermatozoa can reach the eggs ?

How leucoyte can move to the extracelluler space ?

Cytoskeleton: the skeleton of a cell

Cells need a (cyto)skeleton to:

•create shape

•change shape

•allow movement

=

dynamic!

CYTOSKELETON

Complex network of :

Microtubules

Intermediate filaments

And actin filaments

Provide for :

The shaping of the cells

Movement of organelles and intracytoplasmic vesicles

Movement of entire cells

General properties of cytoskeleton elements

All are protein polymers

Dynamic structures withfilaments able to grow and shrink rapidly

Accessory proteinsRegulate polymerization and depolymerization

Regulate function

Structure of actin filaments

Polymerization of actin filaments

Organization of actin filaments

Actin binding protein

Function of actin filaments

Structure of actin filaments Composed of two chains of globular

subunit (G-actin), coiled each other to form a filamentous prot. (F-actin)

Thinnest class of fibers (6 nm thick)

Has stuctural polarity

Associated with a large number actin-binding protein variety of organization and function

Depending on isoelectric point :

α-actin of muscle

β-actin & γ-actin of non muscle

Actins polymerization Actin filaments can grow by

addition of actin monomer at either end

When filament reach desire length, capping proteins attach to the plus end and terminating polymerization

8

Actin monomer binding proteins

Control pool of unpolymerized actin

Two proteins

Profilin Inhibits addition of

monomers to pointed (slowgrowing) end

Thymosin β4 If a filament is capped at both

ends it is effectively stabilized

Actin binding proteinActin bundling protein : hold actin filaments together in parallel bundle (microvilli)

Cross-linking protein : hold actin filaments in a gel-like meshwork (cell cortex)

Actin binding proteinFilament-seve ring protein : convert actin gel to a more fluid state (gelsolin)

Motor protein

Organization of microfilamentsMicrofilaments can organized in many forms :

Skeletal muscle : paracrystalline array integrated with myosin filaments

Non muscle cells :

Cell cortex : form a thin sheath beneath the plasmallema

Associated with myosin form a purse string ring result in cleavage of mitotic cells

Organization of microfilaments

microvilli contractile bundles

in the cytoplasm

lamellipodia

filopodia

contractile ring

during

cell division

Actin and cell locomotion Three steps :

The cell pushes out protrution at its front (lamellipodia & filopodia) Actin polymerization

These protrution adhere to the surface Integrins adhere to the actin

filaments and the extracellular matrix on the surface

The rest of the cell drags itself forward Interaction actin filaments with

myosin

Structure of IF

Types of IF

Function of IF

IF binding protein

Structure of Intermediate filaments

• Ropelike with many long strands twisted together

• The subunit are elongated fibrous proteins (many types)

• Intermediate in size 8-12nm

• Form a network troughout the cytoplasm and surrounding nucleus

Polymerization of Subunit structure

•The subunit :•N-terminal globular head

•C-terminal globular tail

•Central elongated rod domain

•The subunit form stable dimer

•Two dimer form tetramer

•Tetramer bind to one another and-to-end generate ropelike

According to protein subunit, Intermediate filaments in the cytoplasm can be

grouped into:

Types of intermediate filaments

Intermediate filament binding protein

Link, stabilized and reinforced the intermediate filaments into three-dimensional network :

Fillagrin : binds keratin filaments into bundles

Synamin & Plectin : binds desmin & vimentin, links intermediates filaments to microtubules, actin and desmosome

Plakins : maintenance of contact between keratin and hemidesmosomes of epithelial cells

Function of intermediate filament Tensile strength cells enable to withstand the

mechanical stress (streched)

Provide stuctural support for the cell

Function of intermediate filament Form a deformable three-dimensional structural

framework for the cell

Rreinforce cell shape & fix organelle location

The nuclear envelope is supported by a meshwork of intermediate filaments

The structure of

microtubules

Assembly of mirotubules

Microtubule function

Microtubule association with

motor protein

Structure and function of

cilia and flagella

Structure of Microtubules Hollow tube about 25 nm in diameter

The subunit is heterodimer α and βtubulin

Polarized : having plus end & minus end

Dynamic structure : grow or shrink as more tubulin molecules are added or removed

Polymerization of microtubules Microtubules are form by

outgrowth from MOC (exp. the centrosome)

Centrosome contains γ-tubulin ring; serve as starting point for growth

Αβ-tubulin dimers add to the γ-tubulin form hollow tube

Polymerization more rapid in plus end

Function of microtubules Microtubules participate in the intracellular transport

of organelles and vesicles

Axoplasmic transport of neuron

Melanin transport

Chromosome movement by mitotic spindle

Vesicle movement among different cell compartments

Under control by motor protein

Molecular motors

microtubules actin filaments microtubules

Motility of the Cell and Its PartsMotor Molecules – requires ATP

Intracellular transport

microtubules

kinesins

dyneins

actin filaments

myosins

Function of microtubules

Pair of centriolesorganize microtubules guiding chromosomes in cell division

Cilia & Flagella Motile processes, with higly

organized microtubule core

Core consist of 9 pairs of microtubules arround 2 central microtubule (axoneme)

bending of cilia & flagella is driven by motor protein(Dynein)

At the base is basal body, that control the assembly of the axoneme

Cilia Cilia = numerous & short (hair-like)

Oar-like movement

alternating power & recovery strokes

generate force perpendicular to cilia’s axis

flagella Flagella = 1-2/cell & longer (whip-like)

move unicellular & small multicellular organisms by propelling water past them

undulatory movement , force generated parallel to flagellum’s axis

cilia sweep mucus & debris from lungs

flagellum of sperm cells

How does it work?

Dynein Arms

So….

Summary Microtubules thickest

cell structure & cell motility

tubulin

Microfilaments thinnest

internal movements within cell

actin, myosin

Intermediate filaments intermediate

more permanent fixtures

keratin

Distribution of different cytoskeletal elementsin the same cell

actin filaments (F-actin)

(rhodoamin-phaloidin)

intermediate filaments (IF)

(anti-vimentin)

microtubules MT)

(anti-tubulin)

Cytoskeletal elements in eukaryotes

Rapid changes in cell morphology associated with a dynamic cytoskeleton

Inactive platellet Active (spread) Active (contract)

Without the cytoskeleton ?

Wounds would never heal !

Muscle would be uselless !

Sperm never reach the egg !